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The planktonic foraminifera genus Globigerinoides provides a prime example of a species-rich genus in which genetic and morphological divergence are uncorrelated. To shed light on the evolutionary processes that lead to the present-day diversity of Globigerinoides, we investigated the genetic, ecological and morphological divergence of its constituent species. We assembled a global collection of single-cell barcode sequences and show that the genus consists of eight distinct genetic types organized in five extant morphospecies. Based on morphological evidence, we reassign the species Globoturborotalita tenella to Globigerinoides and amend Globigerinoides ruber by formally proposing two new subspe-cies, G. ruber albus n.subsp. and G. ruber ruber in order to express their subspecies level distinction and to replace the informal G. ruber "white" and G. ruber "pink", respectively. The genetic types within G. ruber and Globigerinoides elongatus show a combination of ende-mism and coexistence, with little evidence for ecological differentiation. CT-scanning and ontogeny analysis reveal that the diagnostic differences in adult morphologies could be explained by alterations of the ontogenetic trajectories towards final (reproductive) size. This indicates that heterochrony may have caused the observed decoupling between genetic and morphological diversification within the genus. We find little evidence for environmental forcing of either the genetic or the morphological diversification, which allude to biotic interactions such as symbiosis, as the driver of speciation in Globigerinoides. PLOS ONE | https://doi.org/10.1371/journal.pone.
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RESEARCH ARTICLE
Genetic and morphological divergence in the
warm-water planktonic foraminifera genus
Globigerinoides
Raphae
¨l MorardID
1
*, Angelina Fu¨llbergID
1
, Geert-Jan A. Brummer
2,3
, Mattia GrecoID
1
,
Lukas JonkersID
1
, Andre
´Wizemann
4
, Agnes K. M. Weiner
1,5
, Kate Darling
6,7
,
Michael Siccha
1
, Ronan Ledevin
8
, Hiroshi Kitazato
9
, Thibault de Garidel-ThoronID
10
,
Colomban de Vargas
11,12
, Michal Kucera
1
1MARUM Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, Bremen,
Germany, 2NIOZ Royal Netherlands Institute for Sea Research, Department of Ocean Systems, and Utrecht
University, Den Burg, and Utrecht University, The Netherlands, 3Vrije Universiteit Amsterdam, Department
of Earth Sciences, Faculty of Science, Amsterdam, The Netherlands, 4Leibniz Centre for Tropical Marine
Research, Bremen, Germany, 5Department of Biological Sciences, Smith College, Northampton,
Massachusetts, United States of America, 6School of GeoSciences, University of Edinburgh, Edinburgh,
Scotland, United Kingdom, 7School of Geography and Sustainable Development, University of St Andrews,
St Andrews, Scotland, United Kingdom, 8UMR5199 PACEA, Universite
´de Bordeaux, Alle
´e Geoffroy Saint
Hilaire, Pessac, France, 9Japan Agency for Marine Earth Science and Technology (JAMSTEC), Yokosuka,
Kanagawa, Japan, 10 Aix-Marseille Universite
´, CNRS, IRD, Collège de France, INRA, CEREGE, Aix-en-
Provence, France, 11 Sorbonne Universite
´, CNRS, Station Biologique de Roscoff, UMR 7144, ECOMAP,
Roscoff, France, 12 Research Federation for the Study of Global Ocean Systems Ecology and Evolution,
FR2022/Tara GOSEE, Paris, France
*rmorard@marum.de
Abstract
The planktonic foraminifera genus Globigerinoides provides a prime example of a species-
rich genus in which genetic and morphological divergence are uncorrelated. To shed light
on the evolutionary processes that lead to the present-day diversity of Globigerinoides, we
investigated the genetic, ecological and morphological divergence of its constituent species.
We assembled a global collection of single-cell barcode sequences and show that the
genus consists of eight distinct genetic types organized in five extant morphospecies.
Based on morphological evidence, we reassign the species Globoturborotalita tenella to
Globigerinoides and amend Globigerinoides ruber by formally proposing two new subspe-
cies, G.ruber albus n.subsp. and G.ruber ruber in order to express their subspecies level
distinction and to replace the informal G.ruber “white” and G.ruber “pink”, respectively. The
genetic types within G.ruber and Globigerinoides elongatus show a combination of ende-
mism and coexistence, with little evidence for ecological differentiation. CT-scanning and
ontogeny analysis reveal that the diagnostic differences in adult morphologies could be
explained by alterations of the ontogenetic trajectories towards final (reproductive) size.
This indicates that heterochrony may have caused the observed decoupling between
genetic and morphological diversification within the genus. We find little evidence for envi-
ronmental forcing of either the genetic or the morphological diversification, which allude to
biotic interactions such as symbiosis, as the driver of speciation in Globigerinoides.
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 1 / 30
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OPEN ACCESS
Citation: Morard R, Fu¨llberg A, Brummer G-JA,
Greco M, Jonkers L, Wizemann A, et al. (2019)
Genetic and morphological divergence in the
warm-water planktonic foraminifera genus
Globigerinoides. PLoS ONE 14(12): e0225246.
https://doi.org/10.1371/journal.pone.0225246
Editor: Fabrizio Frontalini, Universita degli Studi di
Urbino Carlo Bo, ITALY
Received: June 23, 2019
Accepted: October 31, 2019
Published: December 5, 2019
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0225246
Copyright: ©2019 Morard et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All newly generated
Sanger sequences are accessible on NCBI under
the accession numbers MN383323-MN384218.
Introduction
Species of the genus Globigerinoides are the dominant constituent of tropical-subtropical
planktonic foraminifera assemblages throughout the Neogene and represent a cornerstone
for paleoceanography. The extant members of the genus feature one of the most iconic spe-
cies of planktonic foraminifera that was formally described from the Atlantic by d’Orbigny in
1839 as Globigerina rubra, after the reddish coloration of its test. The species definition was
later widened to include colorless specimens as variants with the same morphology because
shell color was not considered taxonomically relevant at the species level [1,2]. It was further
broadened by Parker [3] to include the morphologically similar Globigerinoides elongatus
(d’Orbigny) and Globigerinoides pyramidalis (van den Broeck) that were originally distin-
guished using characteristics such as the compression of the last chamber and a higher tro-
chospire. Parker [3] considered that the three species formed a morphological continuum
with G.ruber and this broad definition was endorsed by Kennett and Srinivasan in 1983 [4],
who interpreted G.elongatus,G.pyramidalis and also G.cyclostomus (Galloway and Wissler)
as ecophenotypic variants of G.ruber. This broad species definition has remained stable
since, but most researchers continued to distinguish the two “chromotypes” as G.ruber
“white” and G.ruber “pink”, because of differences in biogeography, seasonality and isotopic
composition [5]. Their distinction is particularly highlighted by the extinction of G.ruber
“pink” in the Indian and Pacific Oceans 120,000 years ago, while persisting in the Atlantic to
the present day [6].
The lumping of G.elongatus,G.pyramidalis,G.cyclostomus with G.ruber was questioned
by Robbins and Healy-Williams [7], who identified stable isotopic differences among morpho-
logical variants. This motivated Wang [8] to further test for isotopic differences between mor-
phological variants of G.ruber “white”. Wang [8] informally re-created the split between
G.ruber and G.elongatus, that had already been identified by d’Orbigny and referred to the
original G.ruber as G.ruber sensu stricto (s.s.) and lumped the specimens matching the
description of G.elongatus,G.pyramidalis and G.cyclostomus into G.ruber sensu lato (s.l.).
Wang [8] showed subtle but statistically significant differences of 0.21 ±0.21‰ for δ
18
O and
0.28±0.29‰ for δ
13
C between the two informal taxonomic units in the South China Sea and
suggested that G.ruber s.s. lived in the upper 30 meters of the water column and G.ruber s.l.
lived below 30 meters. Wang used this feature to reconstruct the variation of the thermal struc-
ture of the water column during the last glacial cycle. The work of Wang [8] triggered a series
of studies during the last two decades that examined chemical/compositional, morphological
and ecological differences between G.ruber s.s. and G.ruber s.l. [821] to assess their useful-
ness for paleoceanography.
In parallel to the investigation of the ecology of G.ruber s.l. and s.s., sequencing of the
small sub-unit of the ribosomal RNA gene (SSU rDNA) shed new light on the diversity
within the genus Globigerinoides. The earliest molecular phylogenies by Darling et al. [22,23]
demonstrated that the two chromotypes of G.ruber are genetically distinct, in line with the
well-established biogeographical and ecological differences [5]. Later, Darling and Wade [24]
described further genetic diversity within G.ruber “white” and Kuroyanagi et al. [25] sug-
gested that the genetic discontinuity observed within G.ruber “white” mirrored the sensu
stricto/sensu lato division of Wang [8]. These observations were confirmed by Aurahs et al.
[26] who identified four genotypes in G.ruber “white” (Ia, Ib, IIa and IIb) and in a second
study [27], these authors analysed images of the barcoded specimens to show that genotypes
Ia and Ib matched the diagnosis of G.ruber s.s., whilst genotype IIa matched the diagnosis of
G.ruber s.l. As a result, they proposed to reinstate Globigerinoides elongatus as a valid name
for genotype IIa. Irrespective of the complicated taxonomy, all genetic studies consistently
Molecular and morphological diversity in Globigerinoides
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 2 / 30
Funding: This work was supported by grants from
ANR-09-BLAN-0348 POSEIDON, ANR-JCJC06-
0142-PALEO-CTD, from Natural Environment
Research Council of the United Kingdom (NER/J/
S2000/00860 and NE/D009707/1), the Leverhulme
Trust and the Carnegie Trust for the Universities of
Scotland, from DFG-Research Center/Cluster of
Excellence ‘The Ocean in the Earth System’, from
the Deutsche Forschungsgemeinschaft KU2259/19
and through the Cluster of Excellence “The Ocean
Floor – Earth’s Uncharted Interface”.
Competing interests: The authors have declared
that no competing interests exist.
identified G.elongatus (or G.ruber s.l.) as a sister to the morphologically distinct species
G.conglobatus. The contrast between the genetic divergence and morphological similarity
of G.elongatus and G.ruber implies a disconnection between genetic and morphological
evolution in the genus. Thus, next to the need to clarify and stabilize its nomenclature, the
complex diversification pattern in the genus also calls for a comprehensive study of the pat-
tern of speciation and morphological diversification leading to the present-day diversity in
Globigerinoides.
To this end, we assembled a global dataset of single-cell SSU rDNA sequences covering all
morphospecies of the genus, applied an objective molecular nomenclature system [28] to parse
the genetic variability and used the shell morphology of the barcoded specimens to map the
genetic units onto a morphological taxonomic framework. To explore patterns of morphologi-
cal evolution within the genus, we used CT scanning to quantify the ontogenetic trajectory of
the five morphospecies [29,30]. This allowed us to investigate whether the diagnostic differ-
ences in adult morphology between closely related species in the genus could be the result of
heterochrony, with slight alteration in the developmental sequence leading to large differences
in adult shape and size. Finally, we use our collection of globally distributed samples to analyze
the ecology of the morphological and cryptic species in the genus and discuss the potential
drivers of their evolution.
Material
Living planktonic foraminifera of the morphospecies Globoturborotalita rubescens,Globigeri-
noides ruber,Globigerinoides conglobatus,Globigerinoides elongatus and Globigerinoides
tenellus were sampled between 1993 and 2015 during 23 research cruises and 6 near shore
sampling campaigns (Fig 1) in all oceans. No sampling permit was needed for planktonic fora-
minifera. G.rubescens was included in the analysis to serve as outgroup in phylogenetic analy-
ses. The specimens were sampled using different open-closing plankton net systems, simple
plankton nets or ship pump systems between 0 and 700 m water depth and mesh sizes from
63 to 200 μm. The specimens were separated from other plankton, cleaned with brushes and
either transferred onto cardboard slides and air-dried or directly transferred into DNA extrac-
tion buffer and stored at -20˚C or -80˚C. The specimens stored on cardboard slides were trans-
ferred into DNA extraction buffer later in the laboratory.
Methods
Molecular analyses
DNA extraction was performed using either the DOC protocol, the GITCprotocol or the
Urea Protocol [31]. A fragment located at the 3’end of the of the SSU rDNA between the prim-
ers S14F1 or S14p and 1528R [32] was amplified and the PCR products obtained were purified
and sequenced directly with Sanger sequencing by several service providers (LGC Genomics
Berlin, University of Edinburgh Gene Pool, AGOWA and Station Biologique de Roscoff). In
addition, we randomly selected eight specimens for cloning in order to quantify potential
intragenomic variability and used the TOPO TA cloning kit (Invitrogen) according to manu-
facturer instructions. Between 2 and 13 clones were sequenced per individual. All chromato-
grams were carefully checked to ensure sequence quality and were deposited on NCBI under
the accession numbers MN383323 to MN384218. The methodologies used for sampling, DNA
extraction, amplification and cloning of single planktonic foraminifera cells are described in
Weiner et al. [31].
Molecular and morphological diversity in Globigerinoides
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Public databases
We completed our dataset with sequences already made available by earlier studies. First, we
retrieved all 359 SSU rDNA sequences of the six morphospecies that were stored in the PFR
2
database v 1.0 [33]. We then manually queried the NCBI portal (last accession: 15.11.2018)
and retrieved seven additional sequences of G.ruber (Accession numbers KY397454-
KY397460).
Detailed information on handling procedures, sequences and associated metadata of the
newly generated data and those retrieved from public databases are provided in S1 Table.
Molecular nomenclature
The genetic diversity within the six morphospecies was classified into a three-tier hierarchical
scheme of Molecular Taxonomic Units following the system described in Morard et al [28].
The system uses the amplified ~1000 bp long sequence fragment located at the 3’end of the
SSU rDNA between stems 32 and 50 as molecular marker [32], which is the barcode selected
for benthic foraminifera [34] that covers six variable regions, three of which are foraminifera-
specific. To exclude potential sequencing errors when constructing the nomenclature, we
retained only sequences for which the individual sequence pattern was observed at least three
times across our dataset. All distinct sequences in the resulting trimmed dataset were consid-
ered as basetypes.Basetypes co-occurring within one or several individuals (because of intra-
individual variability among tandem copies of the gene) were assembled into basegroups, and
constitute the lowest level of the nomenclature (MOTUs lvl-3). The variability observed
between the basetypes represents at least the intragenomic (intra-individual) variability and
the variability observed among different basegroups is considered to represent at least the level
of population variability. If a unique basetype is observed within a single specimen, which is
the majority of cases in our dataset (see Results), the resulting basegroup contains a single
Fig 1. Samples collection. (A) Locations of the samples analyzed in the study. Each symbol corresponds to a scientific cruise or near shore collection
site. Cruise names are indicated in the legend. The background color represent the annual sea surface temperature extracted from the World Ocean
Atlas [105]. (B) Sampling coverage of the five species of the genus Globigerinoides. The colors in the background represent the relative abundance in
sediments extracted from the FORCENS database [106]. Note that G.ruber albus n.subsp. and G.elongatus have the same map because they usually
were not be discriminated in micropaleontological studies. The maps were generated using Ocean Data View [107].
https://doi.org/10.1371/journal.pone.0225246.g001
Molecular and morphological diversity in Globigerinoides
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basetype. The levels 1 and 2 of the nomenclature (following Morard et al. [28]) were con-
structed using a combination of two automated delimitation methods, the Automated Barcode
Gap Discovery method (ABGD; [35]) and the Poisson Tree Process (PTP; [36]). The sequences
were aligned with MAFFT v.7 [37] and a phylogenetic inference was calculated with 1000 non-
parametric bootstrapping pseudo replicates based on a BioNJ starting tree using PhyML [38].
The best substitution models were selected using the Smart Model Selection [39] under Akaike
Information Criterion and the model GTR+I+G was selected. The resulting trees were submit-
ted to the PTP server (http://species.h-its.org/) under default settings. The same alignment that
served to generate the tree was submitted to the online ABGD server (http://wwwabi.snv.
jussieu.fr/public/abgd/abgdweb.html) using the Kimura K80 distance and default options. We
retained the initial (coarsest delimitation) and recursive partition (finest delimitation) pro-
vided with the lowest prior intraspecific divergence. We defined the MOTU lvl-2 as the finest
delimitation proposed by either ABGD or PTP and the MOTU lvl-1 as the coarsest. The pro-
posed delimitations are retained as working hypotheses provided that two clones belonging to
the same basegroup were not attributed to different partitions (oversplit) and that sequences
belonging to different morphospecies were not grouped in the same partition (lumping). The
delimitation proposed by ABGD and PTP as well as the retained delimitation are reported in
Fig 2. As multiple, but partly overlapping, nomenclatural schemes were proposed by successive
studies [2124,40,41], we reported the correspondence between these schemes and their
equivalent in our system (Fig 3 and S2 Table).
A significant part of the sequences had insufficient quality and/or coverage to be included
in the assessment of the diversity within the Globigerinoides plexus, but carried enough infor-
mation to be attributed to at least one MOTU level of our nomenclatural system. The Sanger
Fig 2. Molecular taxonomy of the genus Globigerinoides.Each branch represents a unique basetype, the symbol next to the branch represent the
individual basegroup and the colors represent unique morphospecies. The first set of rectangles represent the three automated delimitation proposed by
ABGD and PTP. The coarsest partition is retained as Lineage (MOTUs level-1) and encircled with a solid line, the finest partition is retained as
Genotype (MOTUs level-2) and encircled in dotted line. The resulting 3-rank molecular taxonomy is showed in the second set of rectangles.
https://doi.org/10.1371/journal.pone.0225246.g002
Molecular and morphological diversity in Globigerinoides
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sequences not meeting the quality criteria were compared to the basetype sequences and
received the finest taxonomic attribution possible based on the availability of diagnostic sites
in the region they covered (See S1 Table). Biogeography and temporal occurrences of the
genotypes and basegroups are shown in Fig 4.
Sample coverage and environmental parameters
We calculated rarefaction curves at MOTUs lvl-2 and lvl-3 (Fig 5) and complemented the
approach with a first order Jackknifing to evaluate the coverage of our dataset (Table 1).
Because G.tenellus and G.conglobatus were under-sampled (60 sequences in a dataset of 1251
sequences), we calculated the rarefaction curves to include all species and selectively only for
G.ruber and G.elongatus separately (Fig 5). Likewise, the Jackknifing was applied to G.ruber
and G.elongatus combined at the MOTUs lvl-2 and on each species separately at the lvl-3
(Table 1). We then applied the analyses to the global dataset and separately on three main bio-
geographic regions: North Atlantic Ocean, Indian Ocean and Pacific Ocean.
The dataset constituted for this study is the result of the efforts by multiple research teams
and re-exploitation of public data, therefore it was difficult to recover and harmonize the envi-
ronmental parameters measured during each sampling campaign. In order to analyze the
Fig 3. Development and consistency across the nomenclatural scheme proposed for the genus Globigerinoides.The Sankey diagram indicates the
change in the names, addition of new taxa, lumping and splitting of existing units across the successive studies. The change of colors indicates when
formal taxonomic revisions were made.
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Molecular and morphological diversity in Globigerinoides
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Fig 4. Biogeographic distribution of constitutive genotypes (MOTUs lvl-2) and basegroups (MOTUs lvl-3) of the genus Globigerinoides in the
sample set. (A) The circles indicate where the genotypes have been collected and are filled when the basegroup has been identified in the sample. Note
that the coverage for G.conglobatus and G.tenellus is insufficient for robust interpretation. The maps were generated using Ocean Data View [107]. (B)
Windrose diagram showing the month of collection of each genotype and basegroup. The month of collection have been normalized in regard to
hemisphere.
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Molecular and morphological diversity in Globigerinoides
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ecological preferences of the sampled genotypes and basegroups, we chose to use geographic
coordinates and collection date to extract the monthly average values of the following environ-
mental parameters from public databases: Sea Surface Temperature (SST), Mixed Layer
Salinity (MLS), Chlorophyll concentration (CHL), Particulate Organic Carbon (POC) and
Productivity (PROD). The SST, CHL and POC parameters were extracted from the MODI-
S-Aqua (NASA, Greenbelt, MD, USA) database [4244], the MLS was extracted from the Iso-
pycnal/Mixed-layer Ocean Climatology (MIMOC) database [45] and PROD was calculated
following the Vertically Generalized Production Model from Behrenfeld and Falkowski [46].
In this way, we could gather a homogeneous environmental dataset although it is less precise
than in-situ measurements. We display the environmental parameter values at the morphospe-
cies, genotype and basegroup levels in Fig 6 and tested if the distribution of values of sister taxa
at each taxonomic level was the same (null hypothesis) with a simple non-parametric Wil-
coxon-Mann-Whitney U-test using the Bonferroni correction (Table 2). All statistical analyses
were performed in PAST 3.21 [47].
Fig 5. Assessment of species richness. Rarefaction curves for the different basins and the entire dataset at the genotype (MOTUs lvl-2) and basegroup
(MOTUs lvl-3) levels, and for all morphospecies together and for the better sampled G.ruber and G.elongatus only.
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Molecular and morphological diversity in Globigerinoides
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Phylogeny and molecular clock
To reconstruct the evolutionary history of the genus Globigerinoides, we applied a molecular
clock estimation using the same alignment as for the maximum likelihood tree inference (Fig
2). We used the divergence between G.rubescens and the genus Globigerinoides (23.8 Ma [48]),
the First Appearance Datum (FAD) of G.conglobatus (8–8.6 Ma) and G.tenellus (2.5 Ma),
which are known from the fossil record [49], as minimum ages to constrain the phylogeny.
We used a relaxed clock model implemented in BEAST v.1.8.4 [50]. Model parameters were
set using BEAUti v1.8.4. The distribution of the fixed node age prior was considered normal
and the speciation rate was assumed constant under the Yule-Process. The GTR (Generalised
Time Reversible) model was selected as substitution model and an UPGMA (Unweighted Pair
Group method with arithmetic mean) tree was calculated as starting tree. Markov-Chain-
Monte Carlo (MCMC) analyses were conducted for 10,000,000 generations, with a burn-in of
1000 generations and saving each 1000th generation. The maximum clade credibility tree with
median node heights was calculated in TREEAnnotator from the BEAST package, with a
burn-in of 100 trees and a posterior probability limit of 0. The resulting tree was then visual-
ized in FigTree v. 1.3.1 [51] and is shown in Fig 7.
3D morphology
We produced CT-scans of G.rubescens,G.ruber albus n.subsp., G.conglobatus,G.elongatus
and G.tenellus to assess the ontogenetic development of each species. To ensure that the speci-
mens had completed their life cycle, which usually is not the case for the living specimens col-
lected in the water column, we used specimens recently deposited on the seafloor from a core
top sample retrieved south of Barbados at station GeoB3935 (12˚36.8 N, 59˚23.2 W; bottom
depth 1554 meters) [52]. We chose this sample because of the exceptional preservation of the
tests, which were free of fine-grained sediment. Moreover, its provenance is close to the sam-
pling localitions where Globigerinoides spp. were previously analysed for their ontogeny [53].
From this sample, we selected one specimen per morphospecies, choosing specimens with
Table 1. Results of the Jackknifing analyses that provide the comparison between the observed diversity (S
o
) and the estimated basegroup diversity (S
e
) for G.ruber
and G.elongatus basegroup at global and basins scales. Note that the entire diversity of G.ruber and G.elongatus may not have been entirely captured in the Atlantic
Ocean and the Indian Ocean respectively because S
o
does not fall into the 95% confidence interval (CI
95
).
Global North Atlantic Ocean Indian Ocean Pacific Ocean
G.ruber (albus + ruber) +G.elongatus (GENOTYPE) S
o
5 4 4 4
S
e
5 4 4 4
CI
95
0 0 0 0
So 2Se ±CI95 TRUE TRUE TRUE TRUE
G.ruber (albus + ruber) + G.elongatus (BASEGROUP) S
o
9 6 6 8
S
e
9 7.97183 6.97872 8
CI
95
0 2.713228 1.9182971 0
So 2Se ±CI95 TRUE FALSE FALSE TRUE
G.ruber (albus + ruber) (BASEGROUP) S
o
6 4 3 5
S
e
6 5.97183 3 5
CI
95
0 2.713228 0 0
So 2Se ±CI95 TRUE FALSE TRUE TRUE
G.elongatus (BASEGROUP) S
o
3 2 3 3
S
e
3 2 3.97872 3
CI
95
0 0 1.9182971 0
So 2Se ±CI95 TRUE TRUE FALSE TRUE
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Molecular and morphological diversity in Globigerinoides
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well-developed characteristic features. We choose specimens that were of large size and had a
thick test, which indicates maturity and facilitates CT-scanning at good resolution. Indeed,
four of the five species have a diminutive final chamber indicative of reproduction by gameto-
genesis (the reproductive terminal stage sensu Brummer et al. [53]), while G.ruber is
Fig 6. Environmental parameters. Distribution of the monthly values of Sea Surface Temperature (SST), Mixed Layer
Salinity (MLS), Chlorophyll (CHL), Particulate Organic Carbon (POC) and Productivity (Prod), observed for the
morphospecies, genotypes and basegroups of G.elongatus,G.ruber albus n.subsp. and G.ruber ruber. The statistical
tests to compare the distribution are provided in Table 2. The box plot were generated with R [108] using the ggplot2
package [109].
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Molecular and morphological diversity in Globigerinoides
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normalform. We realize that planktonic foraminifera are morphologically variable, not only in
their adult shape but also throughout their ontogeny [53], so the decision to analyze only a sin-
gle specimen per morphospecies was made in order to achieve a first rough assessment of the
main differences of ontogenetic trajectories among the morphospecies. Such trajectories are
known to differ between species but are stable within species, with much variability correlated
with proloculus size [29,30,53]. The selected specimens were individually mounted on a stub
and scanned at a cubic resolution of 1.2 μm with a General Electrics V/Tome/x micro-scanner
(PACEA, Bordeaux University). Each scan was performed at 80 kV and 180 μA without filter
as the shell had a low X-ray absorption rate. The smaller specimen of G.rubescens was analyzed
with a cubic resolution of 0.68 μm with a Zeiss Versa 500 at 80kV, 7W and with a filter LE1.
Table 2. Results of Mann-Whitney tests for environmental parameters comparisons. The significant values are shown in bold.
Morphospecies SST MLS CHL POC Prod
G.elongatus vs G.ruber albus 1.18E-04 0.02 0.82 1.00 0.18
G.elongatus vs G.ruber ruber 0.72 0.10 0.87 1.00 1.00
G.ruber albus vs G.ruber ruber 2.32E-07 3.82E-07 0.05 0.20 0.01
Genotype
G.ruber albus Ia vs G.ruber albus Ib 1.00 1.00 1.00 1.00 0.02
G.ruber albus Ia vs G.ruber albus Ic 1.00 1.00 1.00 1.00 0.29
G.ruber albus Ib vs G.ruber albus Ic 1.00 1.00 1.00 1.00 1.00
Basegroup
G.elongatus Ia1 vs G.elongatus Ia2 2.00E-03 0.02 1.00 1.00 0.01
G.elongatus Ia1 vs G.elongatus Ia3 0.10 0.43 1.00 1.00 0.25
G.elongatus Ia2 vs G.elongatus Ia3 0.56 1.00 1.00 1.00 0.37
G.ruber albus Ia1 vs G.ruber albus Ia2 1.00 1.00 1.00 1.00 1.00
G.ruber albus Ib1 vs G.ruber albus Ib2 0.28 0.82 1.00 1.00 1.00
https://doi.org/10.1371/journal.pone.0225246.t002
Fig 7. Molecular clock estimates of the diversification of the Globigerinoides genus rooted on Globoturborotalita rubescens.The grey bars indicate
the uncertainties in the dating of the node and the stars indicate the nodes used for calibration (See text for details).
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Semi-automated segmentation was used to reconstruct three-dimensional (3D) virtual surfaces
of the calcite volume (external morphology) of each specimen (Fig 8), and the inner volume of
individual chambers (Fig 9) were produced by manual segmentation with the ITK-SNAP v 3.6
software [54] to reconstruct the ontogenetic trajectory of each morphospecies. We automati-
cally extracted the volume, centroid position and major axis of individual chambers using a
custom script in MATLAB R2017b to calculate growth parameters of the trochospire, follow-
ing the model of Raup [55]. We calculated the whorl expansion rate W, the relative distance
between the generating curve and the axis of coiling D, the translation rate Tand the shape of
the generating curve S. The calculated growth parameters of each species are displayed in Fig
10 and the numerical values are provided in S3 Table.
Results
Genetic diversity within Globigerinoides
Our dataset on the molecular diversity within the genus Globigerinoides and its sister species
G.rubescens includes 1251 Sanger sequences, of which 893 are new. All 1251 sequences cover
the same rDNA barcode region and originated from a total of 1159 individuals collected at 179
sampling stations (Fig 1). Among the 1251 sequences, 147 met the quality criteria to derive
molecular taxonomy and served to define a total of 17 basetypes (unique, replicable sequence
motifs). We observed three basetypes that co-occurred within two single individuals of
G.rubescens that were consequently grouped into a single basegroup. Additionally, we identi-
fied the co-occurrence of two basetypes within three clones from a single individual of G.
ruber, published by Kuroyanagi et al. [25]. Since this is the only observation of intragenomic
variability within the SSU rRNA gene in G.ruber, we consider it likely that it resulted from
contamination or PCR/sequencing error and we thus reject this single observation as evidence
Fig 8. 3D morphology. CT- scans of external morphology of representative specimensof the five species in four standard views for (1) G.conglobatus,
(2) G.ruber, (3) G.elongatus, (4) G.tenellus and (5) G.rubescens. The scaling of the species respects the difference in sizes.
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for intragenomic variability in the species. As a result, we retained 15 basegroups (Fig 2), 14 of
these consisting of a single basetype, which provided a basis for the construction of a molecular
nomenclature of the group. The automated taxa partitions proposed by ABGD and PTP did
not violate any of the conditions of the taxonomic system (lumping of sequences belonging to
different morphotaxa or splitting of basetypes belonging to the same basegroup) and were thus
retained. Partitions by ABGD reflected the morphological species concept of the group. The
PTP analysis identified three partitions within G.ruber albus n.subsp. and two within G.con-
globatus, which were retained as distinct genotypes. No partitions were identified within the
morphospecies G.rubescens,G.elongatus,G.tenellus and G.ruber ruber indicating that these
morphospecies consist of only a single genotype.
Molecular and morphological revision of existing taxonomic concepts
The first DNA sequences of members of the genus Globigerinoides were made available in the
earliest publications on the genetic diversity of planktonic foraminifera [23,5658], but
nomenclatural schemes to describe the cryptic diversity in the genus were presented only a
decade later in parallel and independently by Darling and Wade [24] and Kuroyanagi et al.
[25], who both identified five cryptic species within G.ruber (Fig 3). The complexity of naming
cryptic species further increased in the following year when Ujiie
´and Lipps [40] produced a
distinct nomenclatural scheme with only four cryptic species within G.ruber, whereas Aurahs
et al. [26] further developed the scheme initially proposed by Darling and Wade [24], but
Fig 9. Ontogenetic development of the five selected morphospecies. (A) The addition of individual chambers is shown with segmentation of the
inner volume from the proloculus to the final chamber. To accommodate the differencein size during the ontogeny and between the species, we have
decreased the relative size of the successive stage by 10% and provide scale bars at the beginning, middle and end of their ontogeny for reference. (B)
Relative proportions of the total inner volume occupied by each chamber. Color coding of the chamber is the same as in (A) with indication of the
transition between the successive ontogenetic stages marked colored lines (See main text for details). The dotted lines indicate when the exact transition
between stages is uncertain.
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Molecular and morphological diversity in Globigerinoides
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chose to interpret all subtle sequence differences across their dataset and produced a scheme
with 14 different cryptic species. Two years later, Aurahs et al. [27] reduced the diversity to
only eight cryptic species by considering only the most repeatable sequence pattern in their
dataset. Furthermore, they split the genetic diversity between G.ruber s. s. (G.ruber Ia, Ib, Ib2
and pink) and G.ruber s. l. (G.ruber IIa, IIa1, IIa2 and IIb) with the genotype IIa being consid-
ered as G.elongatus. Andre
´et al. [59] proposed a revision of the available nomenclature based
on automated methods for species delimitation to define cryptic diversity in planktonic fora-
minifera, which reduced the diversity to five cryptic species only.
In our study, we quadrupled the size of the dataset compared to previous studies and placed
all the formally described cryptic species into a new framework, and we identified only one
new basegroup in G.ruber albus n.subsp. (Ib2). We have also generated a SSU rDNA sequence
from a specimen identified on collection and by later observations as G.tenellus (S1 Fig) that
was identical to the sequences obtained from specimens of G.ruber Type IIb of Aurahs et al.
[27]. This allowed us to recognize this type as G.tenellus and thus return the species to Globi-
gerinoides, as a sister to G.elongatus. The extended and strictly curated dataset allowed for
Fig 10. Raup’s parameters. The scheme on the left represents the position of the centroids of the chambers in G.conglobatus in 3D space. The z-axis is
given by the coiling axis of the specimen. The radius r(distance between the coiling axis and the centroid of a given chamber), the height z(distance
between the centroids of the proloculus and a given chamber along the coiling axis) and the angle α(measured between the radii of two successive
chambers) are illustrated on the scheme. The segmentation of the inner volume of the last chamber is given in the right bottom corner of the scheme
together with the biometric measures H (Height of the chamber) and L (Lengthof the chamber). The equations of the parameters of the Raup model are
provided next to the graph (See explanation in the main text). The six panels on the right show the results for the Raup parameters for each chamber of
each specimen together with the cumulative volume and the whorl number. The results of the measurements and calculation of the Raup parameters
are provided in the S3 Table.
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identifying one new basegroup in G.conglobatus as well as in G.tenellus and reducing the
number of basetypes to three within G.elongatus.
Since the genetic distance separating the “pink” and “white” chromospecies of G.ruber is
greater than the distance separating G.elongatus and G.tenellus (Fig 2), we feel compelled to
express the genetic and phenotypic distinction between the two lineages in formal taxonomy.
However, the phenotypic distinction reflects the color of the shell, not shell morphology, and
this character fades with age, rendering it impossible to distinguish the lineages in fossil mate-
rial older than ~750 kyr [6]. Therefore, we propose to use the subspecies names G.ruber albus
n.subsp. and G.ruber ruber, facilitating continuity by allowing the use of the nominotype
G.ruber” at the species level in situations where the chromospecies cannot be differentiated.
Moreover, the physical holotype designated here combines the shell morphology with the SSU
rDNA drawn from the same individual, the first for a planktonic foraminifer. The physical
specimens have been deposited at the Naturalis Biodiversity Center, Leiden, the Netherlands.
Systematics
Phylum Foraminifera d’Orbigny, 1826
Class Globothalamea Pawlowski, Holzmann & Tyszka, 2013
Order Rotaliida Delage and He
´rouard, 1896
Superfamily Globigerinoidea Carpenter, Parker & Jones, 1862
Family Globigerinidae Carpenter, Parker & Jones, 1862
Genus Globigerinoides Cushman, 1927, amended by Spezzaferri et al., 2015
Type species Globigerina rubra d’Orbigny, 1839
Species Globigerinoides ruber (d’Orbigny, 1839)
Subspecies Globigerinoides ruber albus n. subsp.
Type material: Holotype: Voucher C319 collected at 7.409˚S, 165.274˚E on 12.03.2013
between 0–20 meters water depth (Museum number: RGM.1332320). Paratypes: Voucher
C208 collected at 6.414˚N, 143.024˚E on 18.03.2013 between 80–100 meters water depth
(Museum Number: RGM.1332321), Voucher C281 collected at 22.719˚S, 170.918˚E on
08.03.2013 between 60–80 meters water depth (Museum Number: RGM.1332322) and Vouch-
ers C329 collected at 7.409˚S, 165.274˚E on 12.03.2013 between 0–20 meters water depth
(Museum number: RGM.133233). Light microscopy images of the type specimens are pro-
vided in S2 Fig.
Diagnosis: Differs from G.ruber ruber by the absence of reddish color of the shell, by the
presence of a distinct sequence motive in the SSU rDNA gene, by its seasonality and depth
habitat in the modern Atlantic and its presence in the Indopacific throughout the last 120 ka.
The two subspecies cannot be distinguished prior to 750 ka due to the fading of the color with
time and both are then captured as G.ruber well into the Neogene.
Description. The new subspecies largely overlaps with G.ruber ruber in test morphology,
but differs in the color of the test, which develops during the neanic stage [53]. The morphol-
ogy of the species and its changes during the ontogeny have been described in detail by Brum-
mer et al. [53] and is formalized accordingly below. The holotype has been selected such that
the test shows all key features of the species, but lacks color and because it yielded a SSU rDNA
sequence of genetic type G.ruber albus n.subsp. Ia (Voucher C319).
Prolocular stage. Proloculus small, 12.5 ±1.5 μm (10–16 μm), wall imperforate, smooth and
non-spinose; aperture interiomarginal, circular with thickened rim, in multi-chambered tests
larger than deuteroconch and truncated by flat wall shared with deuteroconch.
Juvenile stage. Starting with deuteroconch, test lobate, umbilico-convex, umbilicus open,
wide, narrowing after completion of initial whorl; chambers hemispherical, 7–12 (9.3 ±1.2)
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added in ±1.5 whorls of near planispire, with 5–6 in initial whorl, totaling 8–13 (9.7 ±1.2)
chambers in tests 54–76 (65.3 ±5.8) μm in diameter. Aperture interiomarginal-marginal, a
small, low arch with marked rim. Spines sparse, thin, flexible; microspines present; pores
sparse, exclusively along sutures on spiral side; wall texture spinose, non-cancellate. No prefer-
ential shell coiling direction; algal symbionts acquired.
Neanic stage. Test rapidly changing towards adult morphology, becoming sphaeroidal with
umbilicus closing; chambers globose, 3–4 in half to complete whorl of low trochospire,
decreasing to 3 in last whorl, totaling 12–16 (14 ±1.3) chambers in tests 120–190 (140 ±25)
μm in diameter. Aperture widening to a wide, high arch and migrating to the umbilicus.
Spines and pores becoming numerous and evenly distributed; spines becoming thicker and
more rigid; spine bases, inter-spine ridges and pore pits develop; wall becoming coarsely perfo-
rate and cancellate.
Adult stage. Test sphaeroidal to elongate with reddish color, chambers globose in a low-
medium trochospire, at least 1, usually 2 to 3, up to 4 chambers are added, totaling 14–18
chambers in test >180, up to 510 μm in diameter, until reproduction (gametogenesis). Second-
ary aperture(s) develop. Wall texture cancellate-spinose and macroperforate.
Terminal stage. Usually one, occasionally two normalform and/or diminutive (kummer-
form) chambers are added, rarely one or two bullate chambers capping the secondary aper-
tures. Spines progressively shed, wall coarsely perforate, smooth to coarsely cancellate. Loss of
algal symbionts, loss of buoyancy. Terminal shells 230–560 μm in diameter with 15–19 cham-
bers in 3–4 whorls of low to medium trochospire.
Distribution and ecological preferences of Globigerinoides MOTUs
Although our study benefits from a globally distributed sampling, we unfortunately lack sam-
pling points in the Southern Atlantic. The rarefaction curves, however, confirm that the geno-
type diversity within Globigerinoides likely has been entirely captured by our global dataset as
well as in the individual ocean basins when considering all morphospecies and the better sam-
pled G.ruber ruber,G.ruber albus n.subsp. and G.elongatus respectively (Fig 5). We are confi-
dent that all existing genotypes and the majority of basegroups have been detected, so that we
are able to interpret their biogeographic patterns (Fig 4A). We observe that the genotypes G.
ruber albus n.subsp. Ia and Ib are cosmopolitan whilst the genotype G.ruber albus n.subsp. Ic
was not found in the North Atlantic. A similar pattern could hold for the basegroup G.ruber
albus n.subsp. Ib2 as well, as it has not been found in the North Atlantic. This may be a sam-
pling bias because its genotype has been encountered only at two stations in the Caribbean
Sea. Also, G.elongatus basegroups Ia1 and Ia3 have a cosmopolitan distribution whilst base-
group Ia2 was not found in the North Atlantic. The unique basetype detected in G.ruber ruber
Ia1 was only found in the North Atlantic in our dataset. Unfortunately, the biogeography of
the MOTUs of G.conglobatus and G.tenellus remains unknown due to the low number of
observations.
While saturation is also reached at the basegroup level in the global dataset for the three
morphospecies, it is not reached for the Indian and North Atlantic oceans, indicating that our
sampling was not sufficient in these two basins. Jackknifing analysis indicates that it is likely
that two basegroups of G.ruber have not been sampled in the North Atlantic, while it is possi-
ble that one basegroup of G.elongatus may still be discovered in the Pacific Ocean. However,
this seems unlikely for G.elongatus because the diversity in the Indian Ocean would thus be
higher (three observed and four estimated genotypes) than in the global dataset (three
observed and estimated genotypes). These results may be the consequence of our unevenly dis-
tributed sampling and the fact that the detection of basegroups depends on the fragment of
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SSU rDNA covered, which depends of the primer used in each study. Therefore, it is impossi-
ble to say whether we failed to capture the diversity in G.ruber or G.elongatus in every basin,
also given the lack of data from the South Atlantic, or if these results reflect an existing bias in
our sample set.
We observe a significant difference in the sea surface temperature and mixed layer salin-
ity at which G.ruber albus n.subsp., G.ruber ruber and G.elongatus were collected (Fig 6,
Table 2). However, the apparent preference of G.ruber ruber for higher salinity may be arti-
ficial because most of our sampling for this species originates from the Caribbean and Medi-
terranean Seas (characterized by higher salinity) and the central Atlantic has not been
sampled yet precluding a robust assessment of the true preferences of this taxa. Our sam-
pling suggests differences between the basegroups G.elongatus Ia1 and Ia2, which occupy
the lower and upper end of the thermal range of the morphological species. We also find
G.ruber albus n.subsp., G.ruber albus n.subsp. Ib and G.elongatus Ia2 in more productive
waters compared to G.ruber ruber,G.ruber albus n.subsp. Ia and G.elongatus Ia1, but do
not observe differences with respect to chlorophyll content or particulate organic carbon.
Our dataset does not reveal any seasonality in the occurrence of either the genotypes or
basegroups (Fig 4B), but we stress that the sample set may not be suited to reveal such
patterns.
Phylogeny of Globigerinoides
The topology and timing of diversification between members of the genus Globigerinoides
(Figs 2and 7) is largely congruent with the phylogeny proposed by Aurahs et al. [27]. The
deepest split in the molecular clock phylogeny (Fig 7) separates G.ruber from G.conglobatus,
G.elongatus and G.tenellus and is dated at 17.59 Ma but with a large credible interval on the
age of the split (23.25 to 12.58 Ma). The Maximum-likelihood inference (Fig 2) does not sup-
port the monophyly of this clade and it is not possible to conclude from the molecular perspec-
tive alone if G.conglobatus is more closely related to G.elongatus and G.tenellus or to the G.
ruber clade. The next diversification event in each lineage occurred in the late Miocene, when
G.conglobatus diverged from the ancestor of G.elongatus and G.tenellus (ca. 8.29 Ma) and G.
ruber albus n.subsp. and G.ruber ruber separated (ca. 6.74 Ma). Further diversification
occurred between the late Pliocene and early Quaternary, when G.elongatus and G.tenellus
separated concomitantly with the deepest split among the constitutive genotypes of G.ruber
albus n.subsp. and G.conglobatus. A further divergence occurred in the course of the Quater-
nary between the genotypes Ib and Ic of G.ruber albus n.subsp., but all the remaining six
divergences at the level of basetypes emerged into the Pleistocene, estimated between ~9 and
224 ka.
3D ontogenetic morphology
The largest shell diameter of the analyzed specimens ranges from 250 μm in G.rubescens and
G.tenellus, to 700 μm for G.conglobatus (Fig 8), and the CT scans revealed that the specimens
consist of 15 to 18 chambers (Fig 9A). The number of chambers is not fixed within a species
and specimens with smaller proloculus seem to have more chambers [53]. For example, the
chamber number can vary from 15 to 19 chambers in G.ruber albus, and the onset of the onto-
genetic stage is not tied to the development of a particular chamber [53]. In this study, we use
the chamber number as a descriptive term for convenience to explore only our results, and
do not mean to imply a fixed boundary between the ontogenetic stages. In all five morphospe-
cies the proloculus is consistently larger than the deuteroconch. Proloculus diameters differ
among species, ranging from 9 μm in G.elongatus to 17 μm in G.conglobatus (Fig 10) and the
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ontogenetic development is accompanied by marked differences in the pattern of chamber
addition among the species (Figs 9and 10).
The ontogenetic trajectory of G.rubescens is the most stable. It begins with a steady loga-
rithmic increase of chamber size from chambers two to thirteen, then levels off towards cham-
ber 16 and ends with a diminutive final chamber 17 after 3.5 whorls (Figs 9and 10). While the
chamber shape (S) remains the same throughout its ontogeny, the whorl expansion rate (W)
first drops steeply to chamber 5, then decreases slowly until chamber 13, slightly increases
until chamber 15 and then decreases again to the final chamber. Inversely, the translation rate
(T) increases slowly until chamber 13, then drops until chamber 15, to rise sharply over the
two last chambers, while the relative distance between the coiling axis and the chamber cen-
troid (D) decreases steadily throughout the ontogeny except in chambers 14–15.
Ontogenetic trajectories of G.tenellus and G.elongatus are initially similar and only diverge
in the last stages. The analyzed specimen of G.tenellus produced slightly larger chambers but
terminated its growth with two chambers less than G.elongatus (Fig 10). The chambers of G.
elongatus gradually flatten between chambers 14–18, resulting in a decreasing S, whilst in G.
tenellus they become rounder between chambers 14–16, which results in the divergent final
shape that distinguishes between the sister species. As for G.rubescens, the final chamber of
the scanned specimens of G.tenellus and G.elongatus is smaller than the penultimate chamber,
which is indicative of the terminal reproductive stage.
Largest shells are typically found in G.conglobatus and G.ruber, but shell size is clearly not
associated with the growth of more chambers: G.ruber has only 15 chambers in our dataset.
The ontogenetic trajectory of G.ruber differs from all other species in its whorl number, which
increases more steeply from chamber 9 onwards (Fig 10) in line with the higher angular incre-
ment between successive chambers. However, its expansion rate Wis close to all other species
except for G.conglobatus (Fig 10). G.ruber and G.conglobatus show a higher rate of size
increase in consecutive chambers that the other species, such that for G.ruber the three last
chambers occupy 94% of the total chamber volume (Fig 9B). The rates Dand Tare mirrored
in their unevenness due to the abrupt decrease of the radius during the ontogeny (see S3
Table), the elevation of the trochospire and the tighter coiling axis. Finally, G.conglobatus has
the largest test but its most distinctive feature is the increase of the sphericity between cham-
bers 1 to 10 that is followed by compression between chambers 11 to 18. Its whorl expansion
rate (W) is the highest throughout its ontogeny, but the formation of its high trochospire
occurs over the last two chambers with an increase of Tand a decrease of D.
Our Raupian analysis of the 3D ontogenetic trajectory of the five species could be used to
determine changes in the position in the growth sequence when the juvenile, near-planispiral,
many-chambered stage ends (onset of neanic stage sensu Brummer et al. [53]) and when the
diagnostic, reproductive morphology is established (onset of adult stage sensu Brummer et al.
[53]). The distinction of the ontogenetic stages in the CT reconstructions is based mainly on
the parameters of chamber addition, but in several cases, the observed transitions could also be
correlated with the emergence of further indicative traits, such as supplementary apertures.
The analysis of the ontogenetic trajectories reveals that the allocation of chamber number and
chamber volume to the ontogenetic trajectory remained similar between G.rubescens and G.
ruber (Fig 9), but the other species show distinct differences in allocation. G.conglobatus dif-
fers most from the other species, exhibiting distinct juvenile-neanic stage with radially elon-
gated chambers. G.elongatus shows a morphologically normal juvenile stage with 10 chambers
and becomes trochospiral late in its ontogeny. Both species develop compressed chambers but
the compression starts during the neanic stage at chamber 11 for G.conglobatus and at the
onset of adult stage at chambers 14–15 for G.elongatus. By comparison, G.tenellus is much
smaller, does not develop chamber compression and has fewer chambers (16).
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Discussion
Strict dataset curation of the genetic dataset associated with the application of our nomencla-
ture system confirms recent metabarcoding results which indicate that the biological diversity
in planktonic foraminifera is limited [60,61]. We identified only eight genotypes and 14 base-
groups within the five sequenced morphospecies of Globigerinoides, which likely covers the
entire genotypic diversity in the genus. At the basegroup level, Globigerinoides conglobatus and
Globigerinoides tenellus remain undersampled, but for the Globigerinoides ruber plexus and
Globigerinoides elongatus, the sampling effort is sufficient to analyze the distribution of genetic
diversity at all hierarchical levels (Fig 5,Table 2).
Our data confirm earlier work [26] in their conclusions that G.ruber ruber occurs only in
the Atlantic, is the only type with test color and constitutes a single basegroup. We observe no
other basegroup or genotype restricted to the Atlantic within the genus (Fig 4), but instead
note the apparent absence of the basegroups G.elongatus Ia2 as well as G.ruber albus n.subsp.
Ic1 and potentially Ib2 from the North Atlantic. Despite the fact that our first order Jackknif-
ing (Table 1) and rarefaction analyses (Fig 4) suggest that the diversity in the North Atlantic
may not have been captured entirely for G.ruber albus n.subsp. at the basegroup level, it does
seem to be the case for G.elongatus at the basegroup level and for G.ruber albus n.subsp. at the
genotype level. Therefore, the observed distribution pattern likely highlights an isolation of the
tropical Atlantic from the Indian and Pacific Oceans.
Because of the equatorial position of the continents, the subtropical-tropical waters of the
world oceans are only connected to a limited degree. At present, transport of tropical/subtropi-
cal marine plankton is largely unidirectional, from the Pacific to the Indian Ocean via the
Indonesian throughflow, and from the Indian Ocean into the Atlantic via the Agulhas leakage.
During glacial times, these connections likely became even more restricted [62]. Indeed, the
disappearance of G.ruber ruber from the Indian and Pacific Oceans 120 kyrs ago [6] and its
persistence in the Atlantic indicate a reduced ability to re-invade the Indian Ocean from the
Atlantic. Dispersal from the Indian and Pacific Oceans into the Atlantic via Agulhas leakage is
evidenced by the existence of a number of cosmopolitan basetypes (G.elongatus Ia1/Ia3 and
G.ruber albus n.subsp. Ia1/Ia2/Ib2). In this scenario, the absence of G.elongatus Ia2 and G.
ruber albus n.subsp. Ib2/Ic1 in the North Atlantic cannot be the result of dispersal limitation.
Instead, the apparent accumulation of recently diverged endemic basegroups in the Pacific
rather than the Atlantic (Figs 4and 5) is reminiscent of the pattern observed in the hyperdi-
verse Globigerinella [63], where it has been ascribed to incumbency (expansion of a species
into a new environment being prevented by an incumbent species with similar ecological pref-
erences [64]). In our case, it might be that the Atlantic residents G.ruber ruber Ia1, G.ruber
albus n.subsp. Ia1/Ia2/Ib1 and G.elongatus Ia1/Ia3 impede the establishment of invading
genotypes recently diverged in the Indian and Pacific Oceans. The lack of diversity in the
Atlantic endemic G.ruber ruber, compared to the cosmopolitan sister clade (Figs 2and 4) sug-
gests that no diversification occurs in the North Atlantic. Therefore, the Indian and Pacific
Oceans seem to act as the primary source for biodiversity and the North Atlantic as a sink
within the Globigerinoides genus.
Notwithstanding the pattern of limited connectivity between the Atlantic and the Indian
and Pacific Oceans, the majority of the MOTUs has a cosmopolitan distribution within the
(sub)tropical habitat of Globigerinoides, with co-occurrences at all taxonomic levels at the
same stations (Figs 1and 4), consistent with their apparently similar ecological niches (Fig 6).
Although we did not sample G.ruber ruber in the South Atlantic, the distribution of the bet-
ter-covered taxa is associated with higher SST in G.ruber albus n.subsp. compared to G.elon-
gatus (Fig 6,Table 2). We acknowledge that our sampling of G.ruber ruber, with more
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sampling stations in the Caribbean and Mediterranean Seas compared to the central Atlantic,
may have produced a biased view on the ecological preferences of this morphospecies. How-
ever, we are confident that our dataset of G.ruber albus and G.elongatus does not suffer from
this limitation (Fig 4). The difference in thermal niches between G.ruber albus n.subsp. and G.
elongatus has been a matter of debate since the seminal work of Wang [8]. Several studies repli-
cated the observation of the preference of G.elongatus for colder waters compared to G.ruber
albus n.subsp. akin to our observations [9,1416,18,19,65,66], but observations of the absence
of such differences have also been made. Indeed, a global synthesis of seasonally and depth-
resolved sediment trap and plankton net observations [11] showed no statistically significant
difference between G.ruber albus n.subsp. and G.elongatus in Mg/Ca composition of the shell.
Studies conducted in the Gulf of Mexico [10,21] and in the central North Atlantic [67] showed
similar absence of oxygen isotopic offsets between the morphospecies and argued that the dif-
ference in habitat, seasonal and calcifying depth is not systematic. Downcore analyses of Mg/
Ca ratios from the southwest Pacific [15,20] showed that the difference between the two mor-
phospecies was not stable though time and varied between 0 and 2˚C in temperature space.
This is consistent with the findings of Numberger et al. [18] in Mediterranean sediments, who
noted oxygen isotopic offsets between the species, but the value and direction of the offset
changed during the last 400 kyrs. Altogether, the niches of the two morphospecies may differ,
but temperature sensitivity alone is unlikely to be the sole factor explaining the niche
difference.
The conflicting observations on the degree of overlap between the ecological niches of G.
ruber albus n.subsp. and G.elongatus raise the question of whether the degree of the overlap
could be driven by ongoing diversification at the genotype and basegroup levels. In our analy-
sis, we observe little to no ecological differences between the genotypes and basetypes of G.
ruber albus n.subsp. and G.elongatus, except for (small) differences in temperature, salinity
and productivity niches between G.elongatus basegroups Ia1 and Ia2 (Fig 6 and Table 2).
Therefore, the regionally and temporally varying overlap between the ecological niches of the
two morphospecies is unlikely to be the result of ecological differentiation among the constitu-
ent MOTUs. There is no evidence for the existence of ecological or biogeographic differentia-
tion between the genotypes of G.ruber albus n.subsp. nor G.elongatus such as those that were
discovered in morphospecies like Orbulina universa [6870], Globorotalia inflata [71,72], Glo-
borotalia truncatulinoides [7375], Globigerina bulloides [7679], Neogloboquadrina pachy-
derma [8083] and Pulleniatina obliquiloculata [84,85]. An explanation invoking a vertical
niche separation as observed in Hastigerina pelagica [86] is unlikely, because G.ruber albus n.
subsp. and G.elongatus are both symbiont-bearing taxa limited to the photic zone and a con-
sistent separation with depth or season would result in a constant isotopic offset, which con-
trasts general observations (see above).
Although abiotic factors, such as temperature, are important drivers of plankton commu-
nity structure [87,88], recent studies have shown that biotic interactions may be even more
important drivers of plankton diversification. Analyses of plankton metacommunity structure
showed that abiotic factors alone explained only 18% of the variability in the distribution of
environmental OTUs [89], leaving biotic interactions as the main driver of ecological and bio-
logical diversification in the open ocean. Photosymbiosis is the biotic interaction that has been
most studied in foraminifera [90] and is of interest to paleoceanographers, not only because it
ties photosymbiotic species to photic depths, but also because it impacts the incorporation of
stable carbon isotopes and trace elements in the calcareous shell [9193]. Photophysiology
[92,9498] investigations have documented the dynamic relationship between the foraminif-
era and their photosymbionts, but the diversity of these interactions, including other interac-
tions such as parasitism or commensalism, has not yet been systematically resolved. Indeed,
Molecular and morphological diversity in Globigerinoides
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Shaked and de Vargas [99] found 21 phylotypes of the dinoflagellate Symbiodinium hosted by
four morphospecies of tropical planktonic foraminifera, including G.ruber and G.conglobatus,
and suggested that this number most likely represents the lower bound of the true symbiotic
diversity, leaving ample space for differentiation due to preference for different symbiont
strains.
Planktonic foraminifera, like many protists living in the oligotrophic ocean, are capable of
mixotrophy (capable of autotrophy by symbiosis and heterotrophy) and the type of mixotro-
phy influences the biogeography and seasonality of the mixotrophs hosting the symbionts
[100]. We hypothesize that the position in the trophic network occupied by planktonic forami-
nifera may control when and where they calcify their shell. The control of temperature on
planktonic foraminifera individual species abundance and occurrence could be indirect and
the physico-chemical condition of the water column that the planktonic foraminifera record
may reflect their relationships with other organisms rather than a mere thermal response. In
this scenario, temperature alone would not explain evolution in planktonic foraminifera [101]
and vital effects impacting the incorporation of carbon isotopes could have varied through
time as a function of varying symbiotic association and mixotrophy level [93]. Indeed, a prom-
inent role of biotic factors in the diversification of Globigerinoides species is consistent with the
lack of physical niche differentiation at the level of genotypes and basegroups. The large num-
ber of apparently recently diverging basegroups could result from a high turnover driven by
biotic interactions which rarely leads to persistent separation of lineages, resulting in a contin-
uous diversification in the genus throughout the late Neogene and Quaternary (Fig 7), without
a clear partitioning of the ecological space along abiotic factors.
Diversification at the cryptic level in the genus likely reflects biotic interactions, but it
remains to be explained why and how the morphological evolution and genetic divergence are
disconnected at the morphospecies level. For instance, G.ruber ruber and G.ruber albus n.
subsp. diverged around ~6.7 Ma and remained morphologically identical, whereas G.elonga-
tus and G.conglobatus diverged around 8.3 Ma (Fig 7) but are morphologically distinct from
juvenile to adult. Similarly, G.tenellus and G.elongatus, which are morphologically dissimilar
diverged around 2.4 Ma and this event could be concomitant with the divergence time of the
constitutive genotype of G.conglobatus and G.ruber albus n.subsp (Fig 7). Because of a simi-
larity in shape, G.tenellus was previously considered a sister species of G.rubescens. The appar-
ent similarity motivated us to analyze the ontogeny of this species as well. Our strategy was to
recover the potential phylogenetic information contained in the ontogenetic development of
the five extant morphospecies of Globigerinoides and to use Globoturborotalita rubescens as an
outgroup. Because of the time-consuming nature of 3D analysis, we limited our approach to a
single representative specimen per species to obtain the main differences in the ontogenetic
development between species. We acknowledge that intra-species variability in the ontogenetic
development exists [53] and that our study design prevents assessing the magnitude of this var-
iability. Nevertheless, the observed contrasting patterns of growth allocation to ontogenetic
stages are substantial and associated with systematic changes in chamber shape and growth
pattern (Fig 9), in a manner that can be best described in the light of heterochrony [102]. Het-
erochrony is defined as evolutionary change in the rate and timing of ontogenetic develop-
ment. Although heterochrony is a concept developed to understand the connection between
evolution and development in multicellular organisms, we apply it in a broad sense to plank-
tonic foraminifera because the sequential growth of their tests preserves the sequence of shapes
during individual growth. Also, we stress that heterochrony as a concept does not explain the
mechanistic cause for evolutionary change, but provides a framework in which the emergence
of the divergent adult shapes can be described through changes in the ontogenetic trajectory
[102].
Molecular and morphological diversity in Globigerinoides
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 21 / 30
In this heterochronic framework, we observe that the G.rubescens specimen displays the
most stable development with relatively little change in the shape of its chambers during
ontogeny compared to the other species (Figs 9and 10). Considering this morphospecies as
outgroup (given its phylogenetic position; Figs 2and 7), we explore the divergence of adult
morphologies of the individual species in terms of Raupian alterations in the ontogenetic
trajectory and the successive emergence of new characters. Compared to G.rubescens, the
morphological innovations in Globigerinoides are the emergence of elongate chambers, com-
pressed chambers and supplementary apertures. Chamber elongation is restricted to the juve-
nile stage of the G.conglobatus specimen and it is followed by compression in the neanic-adult
stages of large G.conglobatus. Chamber compression also occurs in the adult stage of G.elon-
gatus and its absence in small G.tenellus hints at heterochrony by dwarfing. Supplementary
apertures are lacking in the small ancestral G.rubescens but are typically found in in the sister
clade and their reduction to the last 1–2 chambers in G.tenellus is consistent with hetero-
chrony by dwarfing. In G.tenellus a single secondary aperture is typically present in the final
chamber, whereas all other species of the genus develop at least in the final chambers two sup-
plementary apertures per chamber.
The ratio Sdescribing the evolution of the roundness of the chambers is more stable during
the ontogeny of G.ruber compared to the four other species (Fig 10). The analyzed specimen
is large (400 μm) for the few (15) chambers it has, and lacks chamber compression in compari-
son to G.elongatus and G.conglobatus, indicating that G.ruber may have a neotenic ontoge-
netic trajectory. Neoteny is characterized by a conservation of juvenile features during the
adult stage, reduced compression of the last chamber in the case of G.ruber, without a change
of size. It is associated with a steeper increase of chamber size at a higher angular increment
towards the end of the growth. This scenario would be consistent with the hypothesis that
G.ruber evolved from G.obliquus (which has more compressed chambers) as proposed by
Aurahs [27]. In contrast, the ontogenetic trajectory of G.conglobatus appears hypermorphic,
which is characterized by larger final size. Finally, G.elongatus and G.tenellus seem to follow
similar ontogenetic paths and to differ in the last three chambers, with the compression of the
chambers of G.elongatus and the increase of the roundness of G.tenellus chambers. Also,
G.tenellus has larger chambers through its ontogeny and its final size is smaller than G.elonga-
tus, suggesting progenesis. Progenesis is defined as a loss of an adult feature, the final com-
pressed chamber akin to what we hypothesize for G.ruber, but in this case associated with a
reduction in size due to a premature interruption of the growth. In terms of size, G.tenellus is
one of the few known examples of dwarfing in planktonic foraminifera, but unlike the fossil
species Globorotalia exilis,Globorotalia miocenica and Morozovelloides crassatus the dwarfing
in G.tenellus does not (yet) seem to be associated with a reduction of abundance preceding
extinction [103].
Evolution through heterochrony could provide an explanation for the erroneous taxonomic
placement of G.elongatus as a sister to G.ruber that led to the informal delimitation G.ruber
s.l. and s.s. by Wang [8]. Indeed, we hypothesize that G.elongatus may not attain the size and
shape of G.conglobatus because it has smaller chambers, which are less compressed, and could
consequently converge towards the size and shape of G.ruber. Similarly, G.tenellus may create
a morphological convergence with G.rubescens despite having markedly different pre-adult
ontogenetic trajectories (Fig 9). The presence of supplementary apertures in G.tenellus is thus
an apomorphy of Globigerinoides. Based on our observations, we proposed several interpreta-
tions of the molecular phylogeny topology that would be in agreement with the morphology,
taking into account the heterochronic development within Globigerinoides genus (Fig 11).
Similar to previous studies [29,30] our results show that CT-scanning offers a promising
avenue for ontogenetic analysis and resolve phylogenetic relationships among extinct species
Molecular and morphological diversity in Globigerinoides
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 22 / 30
of planktonic foraminifera [104]. We recognize that we cannot draw firm conclusions from
our analysis because of the limited amount of specimen analyzed, and stress the need for repli-
cate analysis to confirm our results. Even though ontogenetic analysis may not explain what
triggered the divergence and convergence of juvenile and adult morphologies, it could provide
a viable explanation for the apparent disconnection between morphological and genetic diver-
gence. Heterochrony is a process through which large changes in adult morphology could be
achieved at genetically low cost [102], creating an impression of large change not matched by
the degree of genetic kinship.
Supporting information
S1 Fig. Light microscopy images of the specimen CA1261 identified as Globigerinoides
tenellus and from which sequence match the type IIb of Aurahs et al [26,27]. (a) Umbilical
(b) spiral (c) lateral views. The scale bar represents 100 μm.
(TIF)
S2 Fig. Light microscopy images of the holotype of (C319) and paratypes (C208, C281,
C329) of G.ruber albus n.subsp. The archiving museum numbers at the Naturalis
Fig 11. Cladogram representing the morphological evolution of the Genus Globigerinoides.The cladogram (A) represents the retained scenario and
the cladograms (B) and (C) possible but rejected alternatives. (A) The presence of supplementary apertures and compressed last chambers are
synapomorphies of the genus. The last compressed chamber is lost in G.ruber and G.tenellus through neoteny and progenesis respectively. The pink
coloration in G.rubescens and G.ruber ruber is a homoplasic character that appear independently during the evolution of the two species. (B)
Alternative scenario where the pink coloration is a synapormophic character of the Globoturborotalita and Globigerinoides genus but lost in G.ruber
albus n.subsp. and by the common ancestor of G.conglobatus,G.elongatus and G.tenellus. Although we cannot with certainty choose between the
scenario (A) and (B) regarding the pink coloration because the character is not preserved in sediments before 750 ka [6], we prefer the scenario (A) due
to its higher parsimony. (C) Alternative scenario where the last compressed chamber is not a synapomorphic character but acquired only in the
monophylum G.conglobatus,G.elongatus and G.tenellus and lost by G.tenellus. We do not retain this scenario because Globigerinoides obliquus, the
likely common ancestor of the modern species shows high compression in its last chamber [27].
https://doi.org/10.1371/journal.pone.0225246.g011
Molecular and morphological diversity in Globigerinoides
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 23 / 30
Biodiversity Center, Leiden, The Netherlands are provided below the voucher of the speci-
mens. The scale bar represents 100 μm.
(TIF)
S1 Table. Metadata and taxonomy of the Sanger sequences used in the study.
(XLSX)
S2 Table. Taxonomic equivalence between the existing taxonomic nomenclatures proposed
in the literature and our updated molecular taxonomy.
(XLSX)
S3 Table. Volume, Cartesian coordinates and parameters of the Raup’s model measured
on individual chambers of the five selected morphological species (Figs 9and 10).
(XLSX)
Acknowledgments
We thank all crew members and scientist for their help in the collection of planktonic forami-
nifera. We are thankful to Dr. Yurika Ujiie
´for her help collecting planktonic foraminifera and
for producing genetic data, Dr. Barbara Donner for providing access to the sediment material
to produce the CT-scans. We also thank Dr. Julie Meilland for imaging the specimen of G.
tenellus and the holotype and paratypes of G.ruber albus n.subsp. Dr. Willem Renema and Dr.
Martina de Freitas Prazeres are acknowledged for their help submitting the holotype and para-
types specimens to the Naturalis Biodiversity Center. We are thankful to Prof. Ralf Schiebel
and two anonymous reviewers who provided constructive comments that helped us to
improve the present manuscript.
Author Contributions
Conceptualization: Raphae¨l Morard, Colomban de Vargas, Michal Kucera.
Data curation: Raphae¨l Morard, Mattia Greco, Kate Darling.
Formal analysis: Raphae¨l Morard, Geert-Jan A. Brummer, Mattia Greco, Lukas Jonkers,
Andre
´Wizemann, Agnes K. M. Weiner, Kate Darling, Michael Siccha, Ronan Ledevin.
Funding acquisition: Kate Darling, Hiroshi Kitazato, Thibault de Garidel-Thoron, Colomban
de Vargas, Michal Kucera.
Investigation: Raphae¨l Morard, Angelina Fu¨llberg, Geert-Jan A. Brummer, Mattia Greco,
Michael Siccha, Michal Kucera.
Methodology: Raphae¨l Morard, Geert-Jan A. Brummer, Michael Siccha.
Project administration: Colomban de Vargas, Michal Kucera.
Resources: Kate Darling, Hiroshi Kitazato, Michal Kucera.
Software: Michael Siccha.
Supervision: Raphae¨l Morard, Michal Kucera.
Visualization: Raphae¨l Morard, Angelina Fu¨llberg.
Writing – original draft: Raphae¨l Morard, Geert-Jan A. Brummer, Mattia Greco, Michal
Kucera.
Molecular and morphological diversity in Globigerinoides
PLOS ONE | https://doi.org/10.1371/journal.pone.0225246 December 5, 2019 24 / 30
Writing – review & editing: Raphae¨l Morard, Geert-Jan A. Brummer, Mattia Greco, Lukas
Jonkers, Andre
´Wizemann, Agnes K. M. Weiner, Kate Darling, Michael Siccha, Ronan
Ledevin, Thibault de Garidel-Thoron, Michal Kucera.
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Molecular and morphological diversity in Globigerinoides
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... This new and independent information had two consequences for taxonomy. First, it largely confirmed the choice and interpretation of traits used in species concepts based solely on shell morphology, resulting only in minor amendments (Darling et al., 2006;Aurahs et al., 2011;Morard et al., 2019a). Second, it led to the discovery of extensive genetic diversity within most morphologically defined species, which likely signifies the presence of biological (reproductively isolated) species that are morphologically similar or even indistinguishable, i.e. cryptic species (Darling and Wade, 2008). ...
... The scheme builds on the classification as presented in Hemleben et al. (1989) who recognized 43 holoplanktonic species, with Neogloboquadrina incompta, Globigerinoides elongatus and Globigerinella radians being added later based on genetic confirmation of their different morphologies (Darling et al., 2006;Aurahs et al., 2011; and accepted by Schiebel and Hemleben (2017). Recently, Globigerinoides ruber albus was established as a subspecies name necessary to differentiate the two genetically distinct lineages within G. ruber (Morard et al., 2019a). Finally, we reviewed all species names known to us that were established for living foraminifera collected from the plankton. ...
... The genus Hastigerinella has been subject to formal ICZN rulings that were necessary to stabilize the nomenclature of fossil taxa (Coxall, 2003: ICZN Case 3245;ICZN Opinion 2105, 2005. Similarly, the application of a phylogenetically consistent taxon concept at the level of genera leads us to accept the genus Trilobatus, which is necessary to avoid a polyphyletic Globigerinoides (Spezzaferri et al., 2015), and following Morard et al. (2019a), we return the species "tenella" to Globigerinoides, where it was placed originally by Parker (1962). In addition, we note that the name Tenuitella, commonly applied to three distinct extant small microperforate taxa with extraumbilical apertures, is typified by the latest Eocene to late Oligocene species Globorotalia gemma, and there is at present no consensus on how the extant taxa are related to the Paleogene and early Neogene representatives of this group . ...
Article
Full-text available
Applications of fossil shells of planktonic foraminifera to decipher past environmental change and plankton evolution require a robust operational taxonomy. In this respect, extant planktonic foraminifera provide an opportunity for benchmarking the dominantly morphological species concepts and classification of the group by considering ecological, physiological and genetic characters. Although the basic framework of the taxonomy of extant planktonic foraminifera has been stable for half a century, many details have changed, not the least in light of genetic evidence. In this contribution, we review the current taxonomy of living planktonic foraminifera, presenting a comprehensive standard that emerged from the meetings and consultations of the SCOR/IGBP Working Group 138 “Planktonic foraminifera and ocean changes”. We present a comprehensive annotated list of 50 species and subspecies recognized among living planktonic foraminifera and evaluate their generic and suprageneric classification. As a result, we recommend replacing the commonly used names Globorotalia menardii by G. cultrata and Globorotalia theyeri by G. eastropacia, recognize Globorotaloides oveyi as a neglected but valid living species, and propose transferring the three extant species previously assigned to Tenuitella into a separate genus, Tenuitellita. We review the status of types and designate lectotypes for Globoturborotalita rubescens and Globigerinita uvula. We further provide an annotated list of synonyms and other names that have been applied previously to living planktonic foraminifera and outline the reasons for their exclusion. Finally, we provide recommendations on how the presented classification scheme should be used in operational taxonomy for the benefit of producing replicable and interoperable census counts.
... Whilst these fine scale morphological traits can be used for species identification, as I show in Chapter 4, caution should be taken when doing so as advances in genetic analysis have shown that cryptic variation is commonplace in planktonic foraminifera (de Vargas et al., 1999;Darling and Wade, 2008;Aurahs et al., 2011). Whilst the morphological based species concept sometimes doesn't correspond to genetics in planktonic foraminifera (Aurahs et al., 2011), gross morphology is consistent within genera indicating morphology does reflect a broader metric of evolutionary relatedness (Jablonski and Finarelli, 2009;Morard et al., 2019). ...
... Many palaeoceanographic studies rely on the accurate identification of foraminifera and these organisms form a key archive of oceanic conditions. Despite genetic advances (Morard et al., 2015(Morard et al., , 2019André et al., 2014), foraminiferal taxonomy remains reliant on morphological comparisons. Palaeoceanographic studies ubiquitously use geochemical proxies on morphologically identified species to infer the ambient environment (Zachos et al., 2001). ...
Thesis
Humans are changing the Earth. What is unknown is how biotic communities and ecosystems will react to this change on both short and long timescales. The fossil record can provide us with a means of investigating ecosystem responses to long-term climatic fluctuations which can act as baselines for future anthropogenic induced change. How we utilize the fossil record is therefore of critical Importance. The high spatial and temporal resolution of the planktonic foraminifera fossil record provides an ideal system to investigate ecosystem responses to climatic fluctuations at multiple scales and levels. The primary objective of this thesis is to measure and understand the relationship between planktonic foraminifera and their environment, to enable a more biologically informative assessment of the fossil record. I created a diversity record of planktonic foraminifera through the Middle Eocene Climatic Optimum comprising of 22,800 individuals classified to three taxonomic levels and investigated the responses of these assemblages using effective diversity: a novel approach for Palaeogene and deep-time systems (Chapter 2). The results from this study show that analytical size fraction choice is a key determinant of diversity signals in deep-time and furthermore it is small species that maintain ecological function during transient climatic events. I then investigated a key component of these assemblages, Subbotina, using individual morphological and geochemical measurements to link their traits to the environment and assess their persistence through the climatic fluctuations of the Middle Eocene (Chapter 3). I found that longevity of Subbotina is a result of morphological and geochemical trait plasticity resulting in a wide ecological niche which in turn allowed for continued persistence and dominance through the Middle Eocene whilst other groups faltered. Next, I explored the relationship between geochemistry and morphology within a relatively recent system to understand the relationship between geochemistry, size, and genetically identified species (Chapter 4). The results showed that fine resolution geochemical analyses can be used to unpick the drivers of intraindividual variability. However, more work is needed to understand the drivers of geochemistry at the individual level which is possible using the methods I advocate and explore in this thesis. Together, these discoveries expand our understanding of how planktonic foraminifera communities are linked to their environment and demonstrate that by using the appropriate analytical approaches we can investigate this relationship in a more biologically meaningful way. Future studies on planktonic foraminifera will require the application of traitbased approaches through the integration of geochemistry, morphology, and diversity measurements to further our understanding of how past communities responded to climatic perturbations with an aim to inform our understanding of biotic responses to current and future anthropogenic change
... However, foraminifera have fast evolving ribosomal genes (Pawlowski and Holzmann, 2002) and a wide occurrence of intragenomic variability (Weber and Pawlowski, 2014). Genetic markers do not always yield satisfying results, and recent studies tend to combine molecular with morphological approaches (Macher et al., 2021;Morard et al., 2019;Pawlowski et al., 2013). Yet, the phylogenetic relationships within one of the most prominent and abundant foraminiferal genera, Amphistegina, have not been sufficiently revisited using modern methodologies, so uncertainties still exist (Renema, 2018;Langer and Hottinger, 2000). ...
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Foraminifera are highly diverse and have a long evolutionary history. As key bioindicators, their phylogenetic schemes are of great importance for paleogeographic applications, but may be hard to recognize correctly. The phylogenetic relationships within the prominent genus Amphistegina are still uncertain. Molecular studies on Amphistegina have so far only focused on genetic diversity within single species and suggested a cryptic diversity that demands for further investigations. Besides molecular sequencing-based approaches, different mass spectrometry-based proteomics approaches are increasingly used to give insights into the relationship between samples and organisms, especially as these do not require reference databases. To better understand the relationship of amphisteginids and test different proteomics-based approaches we applied de novo peptide sequencing and similarity clustering to several populations of Amphistegina lobifera, A. lessonii and A. gibbosa. We also analyzed the dominant photosymbiont community to study their influence on holobiont proteomes. Our analyses indicate that especially de novo peptide sequencing allows to reconstruct the relationship among foraminiferal holobionts, although the detected separation of A. gibbosa from A. lessonii and A. lobifera may be partly influenced by their different photosymbiont types. The resulting dendrograms reflect the separation in two lineages previously suggested and provide a basis for future studies.
... Each sample was washed over a 63 µm sieve and oven dried at temperatures below 60°C. The taxonomical identification of the planktonic Foraminifera species, from subsamples of at least 300 specimens larger than 150 µm split with a microsplitter, followed Bé (1967), Bé et al. (1977), Bolli and Saunders (1989), Hemleben et al. (1989), Kemle-von Mücke and Hemleben (1999), Schiebel and Hemleben (2017), and Morard et al. (2019). ...
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Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased NE summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.
... Individuals bearing cytoplasm were assumed living and counted separately from what was considered empty shells. Following Morard et al. (2019) we use Globigerinoides ruber ruber instead of the commonly used Globigerinoides ruber pink. ...
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It has long been assumed that the population dynamics of planktonic foraminifera is characterised by synchronous reproduction associated with ontogenetic vertical migration. However, due to contradictory observations, this concept became controversial, and subsequent studies provided evidence both in favour and against these phenomena. Here we present new observations from replicated vertically resolved profiles of abundance and shell size variation in four species of planktonic foraminifera from the tropical Atlantic to test for the presence, pattern, and extent of synchronised reproduction and ontogenetic vertical migration in this oceanic region. Specimens of Globigerinita glutinata, Globigerinoides ruber ruber, Globorotalia menardii and Orbulina universa were collected over the first 700 m resolved at nine depth intervals at nine stations over a period of 14 d. Dead specimens were systematically observed irrespective of the depth interval, sampling day and size. Conversely, specimens in the smaller size fractions dominated the sampled populations at all times and were recorded at all depths, indicating that reproduction might have occurred continuously and throughout the occupied part of the water column. However, a closer look at the vertical and temporal size distribution of specimens within each species revealed an overrepresentation of large specimens in depths at the beginning of the sampling (shortly after the full moon) and an overrepresentation of small individuals at the surface and subsurface by the end of the sampling (around new moon). These observations imply that a disproportionately large portion of the population followed for each species a canonical reproductive trajectory, which involved synchronised reproduction and ontogenetic vertical migration with the descent of progressively maturing individuals. This concept is consistent with the initial observations from the Red Sea, on which the reproductive dynamics of planktonic foraminifera has been modelled. Our data extend this model to non-spinose and microperforate symbiont-bearing species, but contrary to the extension of the initial observations on other species of foraminifera, we cannot provide evidence for ontogenetic vertical migration with ascent during maturation. We also show that more than half of the population does not follow the canonical trajectory, which helps to reconcile the existing contrasting observations. Our results imply that the flux of empty shells of planktonic foraminifera in the open ocean should be pulsed, with disproportionately large amounts of disproportionately large specimens being delivered in pulses caused by synchronised reproduction. The presence of a large population reproducing outside of the canonical trajectory implies that individual foraminifera in a fossil sample will record in the calcite of their shells a range of habitat trajectories, with the canonical trajectory emerging statistically from a substantial background range.
... Both time series yielded perfectly preserved planktonic foraminifera shells without any signs of dissolution. The identification is based on the taxonomic concept of Schiebel and Hemleben (2017) as amended by Morard et al. (2019) and Spezzaferri et al. (2015). Following Spezzaferri et al. (2015), we grouped specimens of Trilobatus sacculifer with and without the terminal sac-shaped chamber into one category. ...
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Planktonic foraminifera precipitate calcite shells, which after the death of the organisms are exported to the seafloor. Globally, the resulting calcite flux constitutes up to half of the pelagic calcite flux. Given their importance for the marine calcite budget and for the carbonate counter pump, which counteracts the biological pump in terms of oceanic capacity for CO2 uptake, it is crucial to understand the mechanisms regulating the planktonic foraminifera calcite flux. In principle, variability in individual species calcite flux could be caused by changes in (a) shell flux, (b) shell size, and (c) calcification intensity. Where size and calcification intensity differ among species, variations can be caused by changes in species composition. To assess the importance of these factors in regulating the planktonic foraminifera calcite flux, we investigated two sediment trap time series from the Cape Blanc upwelling area. On intra‐annual timescales, 82% of the variability in the calcite flux can be explained by shell flux alone. Since the intra‐annual flux variability at the study site covers the global range of mean annual shell fluxes, our results indicate that a global prediction of steady‐state planktonic foraminifera calcite flux can be predicted by shell flux variability in combination with species‐specific average shell mass. However, our results show that on inter‐annual timescales, shell mass variability can be as important as shell flux variability. Therefore, this implies that in order to predict temporal changes in the planktonic foraminifera calcite flux variability in shell size and calcification intensity also require considered.
... The level of intraspecific plasticity remains poorly quantified in studies of ontogeny, because tomographic scanning of microfossils is routinely performed for a small number of specimens per sample due to the time-consuming nature of ontogenetic reconstructions. So far, chamber-by-chamber growth trajectories have been reconstructed to compare extant species of foraminifera (Caromel et al. , 2017Burke et al. 2019;Morard et al. 2019), but the ontogenetic change throughout the history of a single lineage has yet to be investigated. ...
Article
Studies in extant populations have shown that plasticity in developmental trajectories can contribute to the origin of novel traits and species divergence via the expression of previously cryptic variation in response to environmental change. Finding evidence for plasticity-led evolution in the fossil record remains challenging due to the poor preservation of developmental stages in many organisms. Planktic foraminifera are ideally suited for addressing this knowledge gap, because adult organisms in species in which development has been studied retain information about all the ontogenetic stages they have undergone. Here we map changes in the developmental trajectories of 68 specimens in the Globorotalia plesiotumida–tumida lineage of planktic foraminifera from the late Miocene until Recent using high-resolution computer tomography techniques. Our unique dataset shows that the transition from the ancestral G. plesiotumida to the descendant G. tumida is preceded by an increased variability in total cumulative volume—an important indicator of reproductive success in this taxon. We also find that the transition interval is marked by a distinct shift in developmental trajectory, which supports a rapid lineage division rather than gradual change. We suggest that high levels of plasticity—particularly in the early stages of development—have contributed to divergence in the ancestral morphology when subjected to a global cooling trend in the late Miocene. The large variation in developmental trajectories that we uncover within our samples emphasizes the need for high-throughput approaches in studies of ontogenetic change in the fossil record.
... One of the most pressing and fundamental issues for palaeoceanography, is whether fossil organisms identified via their external morphology which are used for the inference of paleoclimatic data maintain ecological uniformitarianism for the entirety of their stratigraphic range. Our study, alongside other novel research on modern and fossil populations 18,[54][55][56][57][58][59][60][61][62][63] suggests not, and as such deriving environmental interpretations from fossil taxa, particularly during intervals of climate variability, should be treated with caution. Whether the documentation of these behaviours indicate failed efforts at stress mitigation via water-depth associated parapatric anagenesis is currently undetermined, but further high-resolution comparable investigations through speciation events may help to understand the fundamental mechanisms driving evolution and extinction in an ecosystem with limited vicariance potential such as the open ocean. ...
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Extinction rates in the modern world are currently at their highest in 66 million years and are likely to increase with projections of future climate change. Our knowledge of modern-day extinction risk is largely limited to decadal-centennial terrestrial records, while data from the marine realm is typically applied to high-order (> 1 million year) timescales. At present, it is unclear whether fossil organisms with common ancestry and ecological niche exhibit consistent indicators of ecological stress prior to extinction. The marine microfossil record, specifically that of the planktonic foraminifera, allows for high-resolution analyses of large numbers of fossil individuals with incredibly well-established ecological and phylogenetic history. Here, analysis of the isochronous extinction of two members of the planktonic foraminiferal genus Dentoglobigerina shows disruptive selection differentially compounded by permanent ecological niche migration, “pre-extinction gigantism”, and photosymbiont bleaching prior to extinction. Despite shared ecological and phylogenetic affinity, and timing of extinction, the marked discrepancies observed within the pre-extinction phenotypic responses are species-specific. These behaviours may provide insights into the nature of evolution and extinction in the open ocean and can potentially assist in the recognition and understanding of marine extinction risk in response to global climate change.
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Time-series sediment trap experiments were carried out in the western to central equatorial Pacific (Sites MT3, MT4, MT5, and MT7) from 1999 to 2002, over the transitional time period from La Niña to El Niño conditions, in order to evaluate temporal variability in planktic foraminiferal fluxes and their assemblages. The foraminiferal test flux follows a trophic gradient from higher fluxes in the Equatorial Upwelling Region (EUR) and lower fluxes in the Western Pacific Warm Pool (WPWP) region. Globigerinoides ruber commonly dominates in all sites through the experimental period. Trilobatus sacculifer, Globigerinita glutinata, and Neogloboquadrina dutertrei occurred especially in the WPWP. In contrast, Globigerina bulloides and Pulleniatina obliquiloculata characterized the fauna in the EUR. A change in hydrologic conditions from La Niña to El Niño was documented along the sites during the sampled time interval. Simultaneous with an eastward advancement of the WPWP, the EUR retreated to the east. Rapid decreases in the fluxes of G. ruber, G. bulloides, and P. obliquiloculata were recognized immediately after the more oligotrophic WPWP conditions prevailed at Sites MT4, MT5, and MT7. Fluxes of the total planktic foraminifer assemblage increased both in the EUR and in the western side of the WPWP under full El Niño conditions during the second half of 2002. Such increases in test fluxes in the western side of the WPWP were mainly attributed to G. ruber and N. dutertrei, suggesting inputs of eutrophic waters from the northern coastal area of Papua New Guinea. Test fluxes under El Niño conditions in the WPWP were at the same level as those in the EUR, giving rise to strongly increased carbonate fluxes in the western equatorial Pacific.
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The morphological evolution was investigated in the tropical Neogene planktonic foraminiferal lineage Globorotalia menardii, G. limbata and G. multicamerata during the past 8 million years at ODP Hole 806C (Ontong-Java Plateau, western equatorial Pacific). This research is an extension of several similar studies since 2007 from the Caribbean Sea, the tropical Atlantic and the Eastern Equatorial Pacific. The goal is to find empirical and quantitative confirmation for morphological speciation – splitting or phyletic gradualism or accelerated evolution – in planktonic foraminifera with the above lineage as model objects. The present study from ODP Hole 806C serves as a test to discriminate between these evolutionary scenarios. In the western equatorial Pacific warm and stable environments prevailed back to Pliocene times, and potential influences of Northern Hemisphere Glaciation are thought to bear less severely on shell size evolution than in the Atlantic Ocean. A slow and gradual pattern of shell size increase is therefore expected in the western tropical Pacific, in contrast to the intermittent rapid menardiform shell size increase during periods of intensified formation of warm water eddies in the southern to tropical Atlantic. For this study a total of 11,101 specimens from 37 stratigraphic levels extending over the past 8 million years were morphometrically investigated thanks to the AMOR robot for imaging and microfossil orientation. Of those, 6080 specimens comprise the G. menardii–limbata-multicamerata plexus. Special attention was given to trends of spiral height (δX) versus axial length (δY) in keel view, for which bivariate contour- and volume-density diagrams were constructed to visualize speciation and evolutionary trends. The investigation at Hole 806C showed, that G. menardii evolved in a more gradual manner than in the Atlantic. Contour plots of δX versus δY reveal modest bimodality between 3.18 Ma – 2.55 Ma with a dominant branch consisting of smaller G. menardii (δX<∼300 μm) persisting until the Late Quaternary, and a less dominant branch of larger G. menardii (δX>∼300 μm) until 2.63 Ma. There is evidence for cladogenesis – splitting with subsequent morphological divergence in the Late Pliocene G. menardii-limbata-multicamerata lineage, and which may be linked to changes in the thermocline. Due to the general scarcity of G. multicamerata at Site 806, the divergence was less clearly expressed than originally expected. Up-section, bimodality vanished but G. menardii populations shifted towards extra large shells between 2.19-1.95 Ma. The morphological evolution of Pacific menardiform globorotalids contrasts the one observed in the Atlantic. This inter-oceanic asymmetry may indicate possible long-distance dispersal of G. menardii, at least during intermittent phases. For plankton biostratigraphy this implies, that correlating events represent more often than previously thought large scale environmental perturbations with local morphological ecophenotypic adaptations and nuances.
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Photosymbiosis has played a key role in the diversification of foraminifera and their carbonate production throughout geologic history. However, identification of photosymbiosis in extinct taxa remains challenging, and even among the extant species the occurrence and functional relevance of photosymbiosis remain poorly constrained. Here, we investigate photosymbiosis in living planktonic foraminifera by measuring active chlorophyll fluorescence with fast repetition rate fluorometry. This method provides unequivocal evidence for the presence of photosynthetic capacity in individual foraminifera, and it allows us to characterize multiple features of symbiont photosynthesis including chlorophyll a (Chl a) content, potential photosynthetic activity (Fv∕Fm), and light-absorption efficiency (σPSII). To obtain robust evidence for the occurrence and importance of photosymbiosis in modern planktonic foraminifera, we conducted measurements on 1266 individuals from 30 species of the families Globigerinidae, Hastigerinidae, Globorotaliidae, and Candeinidae. Among the studied species, 19 were recognized as symbiotic and 11 as non-symbiotic. Of these, six species were newly confirmed as symbiotic and five as non-symbiotic. Photosymbiotic species have been identified in all families except the Hastigerinidae. A significant positive correlation between test size and Chl a content, found in 16 species, is interpreted as symbiont abundance scaled to the growth of the host and is consistent with persistent possession of symbionts through the lifetime of the foraminifera. The remaining three symbiont-bearing species did not show such a relationship, and their Fv∕Fm values were comparatively low, indicating that their symbionts do not grow once acquired from the environment. The objectively quantified photosymbiotic characteristics have been used to design a metric of photosymbiosis, which allows the studied species to be classified along a gradient of photosynthetic activity, providing a framework for future ecological and physiological investigations of planktonic foraminifera.
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Photosymbiosis has played a key role in the diversification of foraminifera and their carbonate production through geologic history. However, identification of photosymbiosis in extinct taxa remains challenging and even among the extant species the occurrence and functional relevance of photosymbiosis remains poorly constrained. Here, we investigate photosymbiosis in living planktonic foraminifera by measuring active chlorophyll fluorescence with fast repetition rate fluorometry. This method provides unequivocal evidence for the presence of photosynthetic capacity in individual foraminifera and it allows us to characterize multiple features of symbiont photosynthesis including chlorophyll a (Chl a) content, potential photosynthetic activity (Fv / Fm), and light absorption efficiency (σPSII). To obtain robust evidence for the occurrence and importance of photosymbiosis in modern planktonic foraminifera, we conducted measurements on 1266 individuals from 30 species of the families Globigerinidae, Hastigerinidae, Globorotaliidae, and Candeinidae. Among the studied species, 19 were recognized as symbiotic and 11 as non-symbiotic. Of these, six species were newly confirmed as symbiotic and five as non-symbiotic. Photosymbiotic species have been identified in all families except the Hastigerinidae. A significant positive correlation between test size and Chl a content, found in 16 species, is interpreted as symbiont growth scaled to the growth of the host, consistent with persistent possession of symbionts through the lifetime of the foraminifera. The remaining three symbiont-bearing species did not show such a relationship, and their Fv / Fm values were comparatively low, indicating that their symbionts do not grow once acquired from the environment. The objectively quantified photosymbiotic characteristics have been used to design a metric of photosymbiosis, which allows the studied species to be classified along a gradient of photosynthetic activity, providing a framework for future ecological and physiological investigations of planktonic foraminifera.
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Most research on extant planktonic foraminifera has been directed towards larger species (>0.150 mm) which can be easily manipulated, counted and yield enough calcite for geo-chemical analyses. This has drawn attention towards the macroperforate clade and created an impression of their numerical and ecological dominance. Drawing such conclusions from the study of such "giants" is a dangerous path. There were times in the evolutionary history of planktonic foraminifera when all species were smaller than 0.1 mm and indeed numerous small taxa, mainly from the microperforate clade, have been formally described from the modern plankton. The significance of these small, obscure and neglected species is poorly characterized and their relationship to the newly discovered hyperabundant but uncharac-terized lineages of planktonic foraminifera in metabarcoding datasets is unknown. To determine , who is hiding in the metabarcoding datasets, we carried out an extensive sequencing of 18S rDNA targeted at small and obscure species. The sequences of the newly characterized small and obscure taxa match many of the previously uncharacterized lineages found in metabarcoding data. This indicates that most of the modern diversity in planktonic forami-nifera has been taxonomically captured, but the role of the small and neglected taxa has been severely underestimated.
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We present a detailed analysis of the seasonal distribution, size, morphological variability, and geochemistry of co-occurring pink and white chromotypes of Globigerinoides ruber from a high-resolution (1–2 weeks) and long-running sediment trap time series in the northern Gulf of Mexico. We find no difference in the seasonal flux of the two chromotypes. Although flux of G. ruber is consistently lowest in winter, the flux-weighted signal exported to marine sediments represents mean annual conditions in the surface mixed layer. We observe the same morphological diversity among pink specimens of G. ruber as white. Comparison of the oxygen and carbon isotopic composition (δ ¹⁸ O and δ ¹³ C) of two morphotypes (sensu stricto and sensu lato) of pink G. ruber reveals the isotopes to be indistinguishable. The test size distribution within the population varies seasonally, with the abundance of large individuals increasing (decreasing) with increasing (decreasing) sea surface temperature. We find no systematic offsets in the Mg/Ca and δ ¹⁸ O of co-occurring pink and white G. ruber. The sediment trap data set shows that the Mg/Ca-temperature sensitivity for both chromotypes is much lower than the canonical 9%/°C, which can likely be attributed to the secondary influence of both salinity and pH on foraminiferal Mg/Ca. Using paired Mg/Ca and δ ¹⁸ O, we evaluate the performance of a suite of published equations for calculating sea surface temperature, sea surface salinity, and isotopic composition of seawater (δ ¹⁸ O sw ), including a new salinity-δ ¹⁸ O sw relationship for the northern Gulf of Mexico from water column observations.
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The unique macroevolutionary dataset of Aze & others has been transferred onto the TimeScale Creator visualisation platform while, as much as practicable, preserving the original unrevised content of its morphospecies and lineage evolutionary trees. This is a “Corrected Version” (not a revision), which can serve as an on-going historical case example because it is now updatable with future time scales. Both macroevolutionary and biostratigraphic communities are now equipped with an enduring phylogenetic database of Cenozoic macroperforate planktonic foraminiferal morphospecies and lineages for which both graphics and content can be visualised together. Key to maintaining the currency of the trees has been specification of time scales for sources of stratigraphic ranges; these scales then locate the range dates within the calibration series. Some ranges or their sources have undergone mostly minor corrections or amendments. Links between lineage and morphospecies trees have been introduced to improve consistency and transparency in timing within the trees. Also, Aze & others’ dual employment of morphospecies and lineage concepts is further elaborated here, given misunderstandings that have ensued. Features displayed on the trees include options for line styles for additional categories for range extensions or degrees of support for ancestor–descendant proposals; these have been applied to a small number of instances as an encouragement to capture more nuanced data in the future. In addition to labeling of eco- and morpho-groups on both trees, genus labels can be attached to the morphospecies tree to warn of polyphyletic morphogenera, and the lineage codes have been decoded to ease their recognition. However, it is the mouse-over pop-ups that provide the greatest opportunity to embed supporting information in the trees. They include details for stratigraphic ranges and their recalibration steps, positions relative to the standard planktonic-foraminiferal zonation, and applications as datums, as well as mutual listings between morphospecies and lineages which ease the tracing of their interrelated contents. The elaboration of the original dataset has been captured in a relational database, which can be considered a resource in itself, and, through queries and programming, serves to generate the TimeScale Creator datapacks.
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Changes in biodiversity at all levels from molecules to ecosystems are often linked to climate change, which is widely represented univariately by temperature. A global environmental driving mechanism of biodiversity dynamics is thus implied by the strong correlation between temperature proxies and diversity patterns in a wide variety of fauna and flora. Yet climate consists of many interacting variables. Species probably respond to the entire climate system as opposed to its individual facets. Here, we examine ecological and morphological traits of 12 633 individuals of two species of planktonic foraminifera with similar ecologies but contrasting evolutionary outcomes. Our results show that morphological and ecological changes are correlated to the interactions between multiple environmental factors. Models including interactions between climate variables explain at least twice as much variation in size, shape and abundance changes as models assuming that climate parameters operate independently. No dominant climatic driver can be ident- ified: temperature alone explains remarkably little variation through our highly resolved temporal sequences, implying that a multivariate approach is required to understand evolutionary response to abiotic forcing. Our results caution against the use of a ‘silver bullet’ environmental parameter to represent global climate while studying evolutionary responses to abiotic change, and show that more comprehensive reconstruction of palaeobiological dynamics requires multiple biotic and abiotic dimensions.
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Some species of planktic foraminifera inhabiting oligotrophic surface water environments are in an obligate symbiotic relationship with dinoflagellate microalgae, which can assimilate carbon (C) through photosynthesis. However, the mechanism and dynamics of C photosynthate translocation to the foraminiferal host, and related benefits for the dinoflagellates in this symbiotic association, are poorly constrained. As a consequence, the role of planktic foraminifera as autotroph organisms in ocean surface ecosystems is not well understood. Here, we performed pulse-chase experiments with ¹³C-enriched dissolved inorganic carbon, followed by TEM and quantitative NanoSIMS isotopic imaging to visualize photosynthetic C assimilation by individual symbiotic dinoflagellates and subsequent translocation to their Orbulina universa host. Although most of the dinoflagellate population migrates out of the host endoplasm onto external spines during the day, our observations show that a small fraction remains inside the host cell during daytime. All symbionts, whether outside or inside the foraminifera cell, effectively assimilate C into starch nodules during daytime photosynthesis. At the onset of night, all dinoflagellates from the exterior spine–ectoplasm region migrate back into the foraminiferal cell. During the night, respiration by dinoflagellates and carbon translocation to the host, likely in the form of lipids, greatly reduces the abundance of starch in dinoflagellates. Dinoflagellate mitosis is only observed at night, with a substantial contribution of carbon fixed during the previous day contributing to the production of new biomass.
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The ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells is commonly used as a proxy for past ocean temperature. Recent advances in elemental analyses now enable single-specimen measurements of planktic foraminifera and thus, can provide information on past seasonal and interannual variability, owing to the near-monthly lifespan of foraminifera. In this study, we explore the temperature variance recorded by Mg/Ca in tests of foraminifera Globigerinoides ruber, a planktic species that occurs throughout the year in tropical waters. Using LA-ICP-MS, we characterize Mg/Ca variability in single specimens of two morphotypes of G. ruber picked from a sediment core retrieved offshore New Caledonia. We provide an estimate of the range of calcification temperatures for these morphotypes during five interglacial-glacial cycles over the last 1.55 Ma. First, we find significant and systematic differences between the morphotypes and second, the temperature difference between morphotypes does not remain constant through time. Our results highlight a progressive increase in surface-water temperatures during interglacials and a progressive decrease in glacial subsurface water temperatures. These changes in surface and subsurface temperatures potentially highlight a change in the stratification of the water column over the Mid-Pleistocene Transition. We conclude that single-specimen Mg/Ca on foraminiferal morphotypes can offer unique perspectives on paleoenvironmental reconstructions.
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Eight SSU rDNA genetic types have been described in the planktonic foraminifera Neogloboquadrina pachyderma, but the level of correlation between genetic diversity and morphological variation remains unknown in this morphospecies. In this study, we combine molecular and morphometric analyses of specimens of N. pachyderma sampled during two consecutive years across a latitudinal gradient in the Indian sector of the Southern Ocean. We observe that three genetic types of N. pachyderma inhabit the (sub-)polar waters of the southern Indian Ocean where they have equivalent regional distributions to those previously observed in the South Atlantic. The geographic ranges of these genetic types are largely overlapping. Our morphometric data show that contrary to other planktonic foraminiferal morphospecies, there is no relationship between genetic diversity and morphological differentiation in at least two of the austral representatives of N. pachyderma (Type III and Type IV) despite a high morphological variability and large genetic distance between these types. These genetic types of N. pachyderma in the southern Indian Ocean thus constitute true cryptic species of planktonic foraminifera.
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Habitat patterns of subtropical and tropical planktic foraminifers in the Caribbean Sea were obtained from plankton samples collected in spring 2009 and 2013. The spatial distribution in surface waters (3.5 m water depth) and depth habitat patterns (surface to 400 m) of 33 species were compared with prevailing water-mass conditions (temperature, salinity, and chlorophyll-a concentration) and planktic foraminiferal test assemblages in surface sediments. Distribution patterns indicate a significant relationship with seawater temperature and trophic conditions. A reduction in standing stocks was observed close to the Orinoco River plume and in the Gulf of Paria, associated with high turbidity and concomitant low surface-water salinity. In contrast, a transient mesoscale patch of high chlorophyll concentration in the eastern Caribbean Sea was associated with higher standing stocks in near surface waters, including high abundances of Globigerinita glutinata and Neogloboquadrina dutertrei. Globorotalia truncatulinoides mainly lives close to the seasonal pycnocline and can be linked to winter conditions indicated by lower sea-surface temperatures (SSTs) of ∼20°C. Globigerinoides sacculifer and Globoturborotalita rubescens were associated with oligotrophic conditions in the pelagic Caribbean Sea during early spring and showed a synodic lunar reproduction cycle. The live assemblages in the water column from 2009 and 2013 were similar to those reported in earlier studies from the 1960s and 1990s and to assemblages of tests in the surface sediments. Minor differences in faunal proportions were attributed to seasonal variability and environmental differences at the local scale. An exception was the low relative abundance of Globigerinoides ruber in the Caribbean Sea in 2009 compared to surface sediment samples and plankton net samples collected in the 1960s and 1990s. Decreasing abundance of Gs. ruber white in the Caribbean Sea may be associated with increasing SSTs over past decades and changes in nutrient flux and primary production.