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Diversity and ecology of Radiolaria in modern oceans

April 2022
Environmental Microbiology 24(15)

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Among the many inhabitants of planktonic communities, several lineages have biomineralized intricate skeletons. These have existed for millions of years and include the Radiolaria, a group of marine protists, many of which bear delicate mineral skeletons of different natures. Radiolaria are well known for their paleontological signatures, but little is known about the ecology of modern assemblages. They are found from polar to tropical regions, in the sunlit layers of the ocean down to the deep and cold bathypelagic. They are closely involved in the biogeochemical cycles of silica, carbon and strontium sulfate, carrying important amounts of such elements to the deep ocean. However, relatively little is known on the actual extent of genetic diversity or biogeographic patterns. The rapid emergence and acceptance of molecular approaches have nevertheless led to major advances in our understanding of diversity within and evolutionary relationships between major radiolarian groups. Here, we review the state of knowledge relating to the classification, diversity and ecology of extant radiolarian orders, highlighting the substantial gaps in our understanding of the extent of their contribution to marine biodiversity and their role in marine food‐webs. This article is protected by copyright. All rights reserved.

Diversity of the different extant Radiolaria orders. A-B. Acantharia: Lithoptera fenestra and Amphilonche elongata. C-D. Taxopodida: Sticholonche zanclea (C: live specimen; D: scanning electron microscopy image). E-F. Polycystine -Spumellaria: Diplosphaera spinosa and Euchitonia elegans. G-H. Polycystine -Nassellaria: Pteroscenium sp. and Plectaniscus sp. I-J. Polycystine -Orodaria: Oroscena regalis and Cytocladus tricladus. K-L. Polycystine -Collodaria: colonial (Collozoum sp.) and solitary (Thalassicolla nucleata) specimens. All scale bars are 50 μm. Images A-B and E-H courtesy of John Dolan (CNRS). Images C-D courtesy of Karine Leblanc (CNRS). Monographs I-J from Haecker (1908). Many more images of Radiolaria and plankton can be found online at Aquaparadox and MIO Plankton images.

…

Schematic illustration of prey capture among different radiolarian orders. A. Nassellaria: the 'terminal cone (TC)' comprises several 'terminal projections' (TP) and the 'axial projection' (AxP). Prey capture (here a ciliate) involves the different pseudopodia of the 'terminal cone'. B. Spumellaria: prey capture (here a copepod) involves many axopodia (Axo) radiating in all directions and forming a dense pseudopodial network. C. Collodaria: prey capture (here some tintinnids) is similar to Spumellaria and involves a dense axopodial network surrounding the specimen.

…

Schematic morpho-molecular classification of Acantharia based on ribosomal genes (18S and 28S rDNA) from Decelle et al. (2012a). A. Phylogeny showing molecular clades, taxonomic suborders and environmental clades. Non-supported nodes are reported by a surrounded question mark. B. Corresponding extent of the morpho-molecular framework and main morphological features (including the presence and type of symbionts reported). C. Scanning electron microscopy illustrations of typical strontium sulfate structures found in three acantharian clades. All scale bars are 50 μm. Images courtesy of Karine Leblanc (CNRS).

…

Schematic morpho-molecular classification of Nassellaria based on ribosomal genes (18S and 28S rDNA) from Sandin et al. (2019). A. Phylogeny showing molecular clades, taxonomic super-families. Group with an asterisk denotes changes in taxonomic nomenclature from Sandin et al. (2019) following the latest classification scheme for Radiolaria (Suzuki et al., 2021). Non-supported nodes are reported by a surrounded question mark. Colours correspond to molecular lineages (ML): blue = ML I, red = ML II, orange = ML III and green = ML IV. B. Corresponding extent of the morpho-molecular framework and main morphological features (including the presence and type of symbionts reported). C. Scanning electron microscopy illustrations of typical silicified structures found in the four main nasellarian molecular lineages. All scale bars are 50 μm. Images courtesy of Miguel Sandin (Uppsala University).

…

Summary of known radiolarian predators and occurrence of radiolarians in gut contents.

…

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Diversity and ecology of Radiolaria in modern oceans

Tristan Biard

To cite this version:

Tristan Biard. Diversity and ecology of Radiolaria in modern oceans. Environmental Microbiology,

Society for Applied Microbiology and Wiley-Blackwell, 2022, 24, pp.2179 - 2200. �10.1111/1462-

2920.16004�. �hal-03716815�

Minireview

Diversity and ecology of Radiolaria in modern oceans

Tristan Biard *

Laboratoire d’Océanologie et de Géosciences, Univ.

Littoral Côte d’opale/Univ. Lille/CNRS/IRD, Wimereux,

France.

Summary

Among the many inhabitants of planktonic communi-

ties, several lineages have biomineralized intricate

skeletons. These have existed for millions of years

and include the Radiolaria, a group of marine pro-

tists, many of which bear delicate mineral skeletons

of different natures. Radiolaria are well known for

their paleontological signatures, but little is known

about the ecology of modern assemblages. They are

found from polar to tropical regions, in the sunlit

layers of the ocean down to the deep and cold bathy-

pelagic. They are closely involved in the biogeo-

chemical cycles of silica, carbon and strontium

sulfate, carrying important amounts of such elements

to the deep ocean. However, relatively little is known

on the actual extent of genetic diversity or biogeo-

graphic patterns. The rapid emergence and accep-

tance of molecular approaches have nevertheless led

to major advances in our understanding of diversity

within and evolutionary relationships between major

radiolarian groups. Here, we review the state of

knowledge relating to the classiﬁcation, diversity and

ecology of extant radiolarian orders, highlighting the

substantial gaps in our understanding of the extent

of their contribution to marine biodiversity and their

role in marine food webs.

Introduction

Planktonic realms have attracted attraction since the early

ages of modern ocean exploration in the 19th century.

Among the diverse publications from these early days, sci-

entists like Christian Gottfried Ehrenberg (e.g. Ehrenberg,

1854), Johannes Müller (e.g. Müller, 1859)orlater

Ernst Haeckel depicted the beauty of plankton in their

monographs. Haeckel’s major taxonomic works on the

Radiolaria were of particular impact. The ﬁrst was his

1862 monograph ‘Die Radiolarian’; it earned him many

accolades and is said to have astonished Charles Darwin

(Richards, 2009). Die Radiolarian was followed by his

renowned ‘The Voyage of H.M.S. Challenger’report dedi-

cated to Radiolaria (Haeckel, 1887). The massive work

inspired not only several generations of researchers but

also the general public via the presentation of hundreds of

aesthetic drawings of radiolarians in 128 plates, which

still inspire today a broad audience, from the new gene-

ration of scientists to modern designers (Williams

et al., 2015).

But what exactly are Radiolaria? While the scientiﬁc

deﬁnition has varied through time, and in particular during

the last two decades, Radiolaria can be said to be a

group of diverse unicellular eukaryotes (also known as

protists) often bearing delicate and elegant mineral skele-

tons. The closest relative to Radiolaria are Foraminifera

(Cavalier-Smith et al., 2018), another group of skeleton-

bearing protist (calcium carbonate), also classiﬁed among

the eukaryotic supergroup Rhizaria with the Radiolaria

(Biard, in press). It is their skeletons that popularized

Radiolaria to a broader scientiﬁc community. Indeed,

upon the death of a radiolarian, only its skeleton remains

and eventually sinks to the bottom of the ocean, where it

will be slowly incorporated into sediments and rocks

through diagenesis. The radiolarian skeleton, now fossil-

ized, will record the imprint of its surrounding environ-

ment and save it for millennia, providing excellent tools to

reconstruct past environments (De Wever et al., 2002).

Along with Foraminifera, Radiolaria have the most exten-

sive fossil record (starting as old as 521 Mya) of any

other extant protistan lineage (Suzuki and Oba, 2015). At

present though, knowledge of radiolarian fossil records is

by far inversely proportional to the knowledge of extant

radiolarians. We lack very basic knowledge concerning

their ecology, this point being hampered by the lack of

reliable estimates of their abundances across the various

size classes (Biard et al., 2016). Knowledge on their con-

tribution to contemporary biogeochemical cycles is more

advanced. Radiolaria are active and important

Received 4 February, 2022; revised 4 April, 2022; accepted 5 April,

2022. *For correspondence. E-mail: tristan.biard@univ-littoral.fr.

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use,

distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Environmental Microbiology (2022) 00(00), 00–00 doi:10.1111/1462-2920.16004

contributors to the carbon (Lampitt et al., 2009) and sili-

con cycles (Takahashi, 1987). Speciﬁcally, Acantharia

exert a major control in the ocean’s strontium budget

(Bernstein et al., 1987). However, we still lack for Radio-

laria full characterization of their contribution to global

biogeochemical cycles, particularly with respect to their

broad range of sizes. This imbalance is further exempli-

ﬁed by the current state of radiolarian classiﬁcation, rap-

idly evolving for now more than 130 years. Haeckel’s

original radiolarian classiﬁcation included four legions

(Acantharia, Spumellaria, Nassellaria and Phaeodaria)

subdivided into two subclasses (Porulosa and Osculosa)

based on distribution patterns of openings (or pores) at

the surface of their mineral skeleton. While this classiﬁca-

tion has inﬂuenced modern schemes through most of the

20th century, new classiﬁcations have separated Radio-

laria into different groups, in particular, based on the min-

eral nature of their skeleton. Such recent developments,

also based on molecular evidence, have separated Pha-

eodaria, long considered as radiolarians, from the core

Radiolaria (Polet et al., 2004) but still classiﬁed among

Rhizaria, within the phylum Cercozoa (Biard, in press).

Therefore, Phaeodaria will not be considered in this

review. For each core radiolarian order, taxonomic classi-

ﬁcations are presented below, along with existing knowl-

edge of their ecology and core information on

environmental diversity signatures.

General morphology, ecology and classiﬁcation

Morphology

According to the latest classiﬁcation schemes

(Sandin, 2019; Nakamura et al., 2021), Radiolaria are

now divided into six orders: Acantharia, Taxopodida,

Spumellaria, Nassellaria, Collodaria and Orodaria

(Fig. 1). The last four orders are further grouped within

the Class Polycystinea and hereafter refer as to poly-

cystine radiolarians (Suzuki and Not, 2015). Two key

morphological criteria help subdivide Radiolaria into the

six orders: (i) the main skeletal chemical component, and

(ii) the type/structure of skeleton. Acantharia (Fig. 2A and

B) are unique with respect to their peculiar skeletons

made out of strontium sulfate (SrSO

), which makes

them the only known protist biomineralizing this element

(Decelle and Not, 2015). All remaining radiolarians con-

sistently biomineralize opaline silica (SiO

O) in a var-

ious range of skeleton types, mainly being shells.

Taxopodida, however, possess oar-like spines (referred

to as axopodia) containing silica and are unique (Fig. 2C

and D). Spumellaria (Fig. 2E and F) and Nassellaria

(Fig. 2G and H) are likely the two best-known radiolarian

orders (due to their extensive fossil records), with their

distinctive spherical and conical siliciﬁed skeletons

respectively. The two remaining Polycystine orders,

Orodaria and Collodaria, possess several unique mor-

phological characters. Both often reach cell sizes of a

thousand micrometres, while typical cell sizes for other

radiolarians are a few hundred micrometres. Orodaria

(Fig. 2I and J) possess the largest known siliciﬁed skele-

ton in any other extant radiolarian order (Nakamura

et al., 2021). However, Radiolaria without siliciﬁed skele-

tons of the Collodaria order are able to form large colo-

nies (up to 3 m for the largest reported collodarian;

Swanberg and Anderson, 1981) composed of tens to

thousands of single collodarian cells (Fig. 2K). The pecu-

liar morphology of Collodaria is further exempliﬁed by the

existence of taxa bearing siliciﬁed spicules, resembling

those of sponges, rather than the typical spherical skele-

ton found elsewhere in polycystines. The reduction of

siliciﬁed structures within Collodaria is complete for some

taxa lacking siliciﬁed structure in any forms (referred to

as ‘naked’collodarians). Finally, some Collodaria taxa

can form large solitary cells consisting of one large

(i.e. >1 mm length) collodarian cell surrounded by a large

and dense network of pseudopods (Fig. 2L; Box 1).

Ecology

As many other protistan lineages, there are currently no

cultures of any radiolarian species, despite several

attempts made in the past (Anderson, 1978;

Matsuoka, 1992), which failed to maintain radiolarian

through successive generations. Therefore, knowledge of

radiolarian ecology is limited by our ability to sample liv-

ing organisms from various ecosystems or work with

‘omics’data (e.g. transcriptomics, genomics, etc.; Caron

et al., 2012). Despite the inherent complexity of collecting

and studying them (see above), their relative wide occur-

rence in marine ecosystems has allowed a few studies

that have provided precious information regarding radio-

larian ecology. One of the most striking features of radio-

larians biotic interactions, which was highlighted early on

(e.g. Brandt, 1881), is their intricate relationships with

photosymbionts (Anderson, 2012; Decelle et al., 2015). A

large number of radiolarian species are associated with

diverse types of photosymbionts, from prokaryotes to

dinoﬂagellates, where the radiolarian host could acquire

carbon-rich products from the photosynthetic activity of

the symbionts, and the symbiont could beneﬁt from a

protected micro-environment. However, recurring evi-

dence suggest that only the host could beneﬁt from this

association, the symbionts undergoing substantial mor-

phological and physiological alterations (Uwizeye

et al., 2021). While the complex nature of these associa-

tions has not been thoroughly investigated, it is believed

that photosymbiosis could drive the latitudinal and

Environmental Microbiology

2T. Biard

Acantharia

Sphaerozoidae

Collophidiidae

Collosphaeridae

Oroscenidae

Plectopyramidoidea

Eucyrtidioidea

Artostrobioidea

Acanthodesmioidea

Cycladophoroidea

Lithochytrioidea

Pterocorythoidea

Theopilioidea

Plagiacanthoidea

Archipilioidea

Carpocanioidea

Hexacromyoidea

Spongosphaeroidea

Lithocyclioidea

Spongosdiscoidea

Cladococcoidea

Rhizosphaeroidea

Centrocuboidea

Trematodiscoidea

Haliommoidea

Spongopyloidea

Phorticioidea / Larcospiroidea

Rad-B

Rad-C

Rad-A

Arthracanthida / Symphiacanthida

Arthracanthida

Holacanthida

Chaunacanthida

Holacanthida

Taxopodida

Spumellaria

Nassellaria

Orodaria

Collodaria

Polycystines

Phaeodaria

Radiolaria

Fig. 1. Schematic classiﬁcation of Radiolaria based on ribosomal genes (18S and 28S rDNA) modiﬁed from Sandin (2019). Non-supported nodes are reported by a surrounded question mark.

Environmental Microbiology

Diversity and ecology of Radiolaria 3

ABCD

EFGH

IJKL

Fig. 2. Diversity of the different extant Radiolaria orders.

A–B. Acantharia: Lithoptera fenestra and Amphilonche elongata.

C–D. Taxopodida: Sticholonche zanclea (C: live specimen; D: scanning electron microscopy image).

E–F. Polycystine - Spumellaria: Diplosphaera spinosa and Euchitonia elegans.

G–H. Polycystine - Nassellaria: Pteroscenium sp. and Plectaniscus sp.

I–J. Polycystine - Orodaria: Oroscena regalis and Cytocladus tricladus.

K–L. Polycystine - Collodaria: colonial (Collozoum sp.) and solitary (Thalassicolla nucleata) specimens. All scale bars are 50 μm. Images A–B and E–H courtesy of John Dolan (CNRS). Images

C-D courtesy of Karine Leblanc (CNRS). Monographs I–J from Haecker (1908). Many more images of Radiolaria and plankton can be found online at Aquaparadox and MIO Plankton images.

Environmental Microbiology

4T. Biard

vertical distribution of some radiolarian taxa (Suzuki and

Not, 2015; Biard and Ohman, 2020).

Less is known about preys and predators of radiolarians

and their inﬂuence on such biogeographical patterns. Most

radiolarians are phagotrophs, feeding on a broad range

(both in diversity and size) of prey without any obvious

prey preferences: bacteria, small autotrophs (e.g. diatoms,

dinoﬂagellates), protists (e.g. mostly ciliates and tintinnids)

or even multicellular heterotrophs (e.g. copepods, large

mollusc larvae; Anderson, 1977;Sugiyamaand

Anderson, 1998; Bernstein et al., 1999;Matsuoka,2007;

Sugiyama et al., 2008;SuzukiandNot,2015;MarsBrisbin

et al., 2020). A few reports, however, indicate that some

radiolarians could be detritivores (Anderson, 1983).

Capture and digestion mechanisms are still elusive

processes that often involve a few different types of

pseudopodia (i.e. root-like branching projections arising

from the cytoplasm; Fig. 3). Capture of prey by

Nassellaria involves the ‘axial projection’, the thickest

and longest pseudopod emerging outward from the cen-

tre of the skeleton aperture (Fig. 3A). This AxP can be

accompanied by several ‘terminal projections’, thick

pseudopods emerging outwards too, forming altogether

the ‘terminal cone’(Sugiyama et al., 2008). The extent of

the TC can be as long as two to three cell lengths, all-

owing the nassellarian cell to sweep large volumes for

prey. Once captured, the prey is brought back within a

few minutes toward the oral aperture of the skeleton,

after which the prey will be digested within speciﬁc vacu-

oles. Prey capture among Spumellaria is unlike the TC

found in Nassellaria. Rather than projecting their pseudo-

pods toward one direction, Spumellaria possess a large

number of pseudopods (often referred to as axopodia)

radiating in all directions (Fig. 3B). Prey is captured by

adhesion while swimming in the pseudopodial network.

One detailed study revealed that Spumellaria pseudopo-

dia follow a regular rhythmic cycle of extension and con-

traction, repeating successively every 10 min (Suzuki

and Sugiyama, 2001). Prey capture among Collodaria is

likely passive and can involve a large number of pseudo-

pods radiating in all directions, similar to the feeding

behaviour of Spumellaria (Fig. 3C). Prey (or sometime

predators aiming to prey on Collodaria) swimming near

the pseudopodial network get glued on it and will be

slowly be digested in speciﬁc vacuoles. Prey capture by

Acantharia is still an unresolved process, despite a

recent study (Mars Brisbin et al., 2020) reporting that

acantharians in the near-surface layer consistently dis-

played long pseudopodial extensions terminating in drop-

shaped structures, which could serve as a ﬁshing appa-

ratus similar to those observed in polycystine radiolar-

ians. Oroscenids feeding behaviour is primarily unknown.

However, the absence of mouth-like structures, or similar

openings in their skeleton, would either suggest that

Oroscenids are ﬁlter-feeders like mesopelagic

phaeodarians, or that feeding activities (including diges-

tion) occur outside the skeleton (Nakamura et al., 2021).

To date, thanks mostly from gut content analyses,

12 taxonomic groups have been identiﬁed as potential

predators of radiolarians (Table 1). These include a few

crustacean species of amphipods, decapods, euphausids,

isopods, ostracods and mysids, but the most diverse pred-

ators are undoubtedly copepods, with ca. 63 species in

which radiolarian remains have been found in their gut

contents. Other predators include molluscs (pteropods),

annelids (polychaetes), gelatinous plankton like salps or

doliolids, but also ﬁshes (adults and larvae) whom seem

to preferentially feed on larger radiolarians such as colo-

nial Collodaria. Interestingly, some other taxa avoid con-

sumption due to sterol compounds produced by

collodarian’s photosymbionts (Anderson, 1983;O’Rorke

et al., 2012). While gut content analyses offer a good over-

view of potential predators of radiolarians (thanks to their

skeletal remains being mostly preserved from digestion,

which might not be the case for Acantharia), we cannot

Box 1. Schematic classiﬁcation of Radiolaria

based on molecular and morphological criteria

Phylogeny. Radiolaria are separated into three main

phylogenetic entities: Acantharia, Taxopodida and

Polycystines. Phaeodaria is no longer considered to

belong to Radiolaria. Relationships between the three

radiolarian main entities are still unclear

(Sandin, 2019; Nakamura et al., 2021). See Fig. 1for

an illustration.

Morphology. Radiolaria can be separated into dif-

ferent orders based on two successive morphological

criteria: (i) the composition of biomineralized struc-

tures and (2) the type of siliciﬁed structures. The ﬁrst

criterion separates Acantharia and their strontium sul-

fate skeletons, from Taxopodida and Polycystines,

both having biomineralized structures made out of

opaline silica. The second criterion pertains to silici-

ﬁed radiolarians, i.e. Taxopodida and Polycystines,

the ﬁrst having oar-like siliciﬁed spines (Fig. 2C and

D) while the second having classical siliciﬁed skele-

tons (Fig. 2E–J). Finally, among Polycystines,

Collodaria (Fig. 2K and L) are unique due to the colo-

nial nature of most species.

Diversity. While assessing the number of extant

radiolarian species is a difﬁcult task (Suzuki and

Not, 2015), the three main radiolarian groups differ

widely, not only from a morphological perspective but

also from the putative number of known genera: from

one genus for Taxopodida to several hundred for

Polycystines (Adl et al., 2019).

Environmental Microbiology

Diversity and ecology of Radiolaria 5

AB C

AxP

TC =

{

Axo Axo

Fig. 3. Schematic illustration of prey capture among different radiolarian orders.

A. Nassellaria: the ‘terminal cone (TC)’comprises several ‘terminal projections’(TP) and the ‘axial projection’(AxP). Prey capture (here a ciliate) involves the different pseudopodia of the ‘termi-

nal cone’.

B. Spumellaria: prey capture (here a copepod) involves many axopodia (Axo) radiating in all directions and forming a dense pseudopodial network.

C. Collodaria: prey capture (here some tintinnids) is similar to Spumellaria and involves a dense axopodial network surrounding the specimen.

Environmental Microbiology

6T. Biard

Table 1. Summary of known radiolarian predators and occurrence of radiolarians in gut contents.

Radiolaria group Predator Species Frequency in gut contents

Acantharia (include

Acanthometra,Dorataspis,

Amphilonche)

Copepod Canthocalanus pauper

,Pleuromamma robusta,

Rhincalanus gigas

0.04–0.15

Fish Myctophid

b,c

n.a.

Isopod Acanthamunnopsis milleri

,Munneurycope murrayi

n.a.

Pteropod Cuvierina columnella,Hyalaea trispinosa,

Tiedemannia sp.

n.a.

Collodaria Fish Bathylagus sp.

, Myctophid

b,c

n.a.

Fish larvae Anguilla anguilla

n.a.

Spiny lobster larvae Panulirus cygnus

n.a.

Orodaria Salp Unidentiﬁed species n.a.

Spumellaria (include

Actinomma)

Copepod Scopalatum vorax

n.a.

Pteropod Cuvierina columnella n.a.

Taxopodida Copepod Aetideopsis antarctica,Calanus propinquus,C.

sinicus

,Centropages veliﬁcatus

,Euchaeta

antarctica,Microcalanus pygmaeus,Paracalanus

parvus

,P.quasimodo

,Parvocalanus

crassirostris

,Temora stylifera

,T.turbinata

0.01–0.39

Ostracod Conchoecia belgicae 0.07

Euphausiid Euphausia sp.

n.a.

Radiolaria overall Amphipod Cyllopus lucasii,Epimeriella macronyx,Orchomene

plebs

0.03–0.06

Copepod Aetideopsis antarctica,A.rostrata,Calanus

propinquus,C.similimus,C.tenuicornis,

Centropages violaceus,Chiridius polaris

,Chiridius

sp.

,Chirundina streetsii,Diaxis sp.

,Disseta

palumboi,Euchaeta norvegica,E. sp., Euchirella

rostromagna,Gaetanus simplex,G.tenuispinus,G.

sp., Gaidius afﬁnis,G.columbiae,Haloptilus

ocellatus,Heterorhabdus sp.

,Heterostylites major,

Lucicutia wolfendeni,Metridia discreta,M.

gerlachei

,M.ornata,M.paciﬁca

,M.princeps,

Nannocalanus minor,Neocalanus sp.,

Neoscolecithrix magna

,Oncaea mediterranea

Pleuromamma robusta,P.scutullata

,P.xiphias,

Pseudochirella polyspina,Rhincalanus gigas,

Scaphocalanus subbrevicornis

,S.vervoorti,S.

sp.

,Scopalatum sp.

,Scottocalanus securifrons,

Spinocalanus abyssalis,S.antarcticus,S.magnus,

S.spinosus,S. sp.

,Teneriforma sp.

,Undinella

sp.

,Xanthocalanus sp.

,Zenkevitchiella sp.

0.01–1

Decapod Gennadas valens,Hymenodora glacialis,

Nematocarcinus lanceopes,Penaeus

semisulcatus,Sergestes grandis,S.henseni,

Systellaspis debilis

0.02–0.33

Euphausiid Euphausia crystallorophias,E.superba,Thysanoessa

macrura

0.03–0.46

Fish Bathylagus antarctica,Trimmutom sp. 0.31

Fish larvae Anguilla anguilla

,A.japonica

,Clupea harengus

pallasi,Etrumeus teres

,Paraliparis dipterus,

Sardina pilchardus,Sardinops melanostictus

n.a.

Isopod Acanthamunnopsis milleri

,Bathynomus doederleinii,

Eurycope inermis,Ilyarachna hitrticeps,

Munneurycope murrayi

0.02–0.1

Mysid Antarctomysis ohlinii 0.36

Polychaete Poebius meseres n.a.

Pteropod Tiedemannia sp. n.a.

Salp Salpa thompsoni 0.62

Tunicate Dolioletta gegenbauri

n.a.

Additional information about location and depths of observation and a complete list of references are provided in Supplementary Information

Table S1.

Observation from DNA sequences (clone or high-throughput sequencing).

Observation from ﬁsh gut contents, where organisms could have been attached to plastic particles.

Observation from scanning electron microscopy.

Observation from transmission electron microscopy.

Environmental Microbiology

Diversity and ecology of Radiolaria 7

rule out that radiolarian remains in guts could be the result

of a species feeding on detrital material (i.e. marine snow),

leaving unknown the full extent of radiolarian predators.

Despite the detailed nature of existing information, it

should be noted that these observations are still restricted

to a handful of specimens and studies. More than the lack

of knowledge on speciﬁc predators, it is the lack of rate

measurements that prevent the inclusion of radiolarians in

marine food web models, precluding our understanding of

their role there.

Evolutionary relationships

Hypotheses concerning the evolutionary relationships

among the different radiolarian taxa have evolved con-

stantly in the last century, but many clariﬁcations of unre-

solved placements have been solved within the last two

decades with the advent of integrative morpho-molecular

classiﬁcations. This is particularly true for Acantharia

(Decelle et al., 2012a), Collodaria (Biard et al., 2015),

Nassellaria (Sandin et al., 2019), Spumellaria (Sandin

et al., 2021) and Orodaria (Nakamura et al., 2021). To

date, only Taxopodida are lacking a detailed morpho-

molecular classiﬁcation. However, clariﬁcation of the

overall radiolarian classiﬁcation scheme is still required

as the most recent phylogenies including all known radio-

larian orders have highlighted discrepancies

(Sandin, 2019; Nakamura et al., 2021). These later stud-

ies suggested that Orodaria and Collodaria, rather than

being monophyletic clades independent of other radiolar-

ian orders, could be considered as sub-clades merged

within the order Nassellaria. Despite uncertainties related

to long-branch attraction artefact (known to be present

within Nassellaria/Collodaria/Orodaria), this separation of

polycystine Radiolaria into only two different orders

(Spumellaria and Nassellaria, including Orodaria and

Collodaria) would challenge any previous classiﬁcation

schemes, as Collodaria have never been classiﬁed as

Nassellaria before (but were considered in the past to be

part of Spumellaria due to the resemblance between col-

lodarian and spumellarian skeletons; Anderson, 1983).

Furthermore, this could result in deep modiﬁcation of our

understanding of radiolarian evolution and in particular

the evolution of radiolarian skeleton, Collodaria and

Orosphaeridae likely being the youngest radiolarian line-

ages (Suzuki and Oba, 2015) and possessing either giant

siliciﬁed skeletons or lacking any traces of siliciﬁcation.

Acantharia

Diversity and classiﬁcation

Unlike most radiolarians with siliciﬁed skeletons, there is

no fossil record of any Acantharia (Decelle et al., 2012b).

This is because the high under-saturation of strontium in

seawater leads to rapid dissolution of their skeleton upon

death (Beers and Stewart, 1970; Decelle and Not, 2015).

Therefore, the establishment of classiﬁcation frameworks

has relied from early on, solely on morphological criteria

of extant taxa. Such work has long history: it was initiated

by Müller (1859), then his student Haeckel (1887)) and

later stabilized by Schewiakoff (1926). This later work,

emended in the four last decades, distinguishes

145 morphospecies, classiﬁed into 49–50 genera and

four suborders: Hollacanthida, Chaunacanthida, Sym-

phiacanthida and Arthracanthida (Suzuki and Not, 2015;

Adl et al., 2019). Despite extensive taxonomic analyses

that led to this classiﬁcation framework, morphological

identiﬁcation of Acantharia actually still relies on a hand-

ful of criteria, difﬁcult to diagnose because of the fragility

and alterability of skeletal elements from dissolution

(Decelle and Not, 2015). Therefore, the beneﬁts of

molecular approaches to study acantharian classiﬁcation

have been substantial and helped clariﬁed the evolution-

ary relationships between acantharian suborders.

The latest phylogeny of acantharian ribosomal genes

revealed the existence of six molecular clades (Clades A

to F) (Decelle et al., 2012a). At the suborder level, only

Chaunacanthida (Clade C) are monophyletic while the

three remaining suborders are polyphyletic (Fig. 3A). Hol-

lacanthida of Clade A represents the most basal and

likely the most ancient lineage overall. Evolutionary rela-

tionships between this basal clade, and clades B and C

(Chaunacanthida) are yet to be clariﬁed. However,

molecular clock analyses date the appearance of these

two clades substantially ahead of Clades D, E and F, the

latter two being Arthracanthida and Symphiacanthida

respectively (Decelle et al., 2012a).

Compared with morphological classiﬁcation, molecular

phylogenies not only highlighted some major discrepan-

cies between traditional taxonomical and molecular

schemes but also provided substantial insights into the

evolutionary history of acantharian skeleton and ecology

(Decelle et al., 2012a). The three early clades (A–C) pos-

sess less complex skeleton, with 10–20 needle-like spic-

ules with no junction or loosed attached, while complexity

increases (e.g. tight junction of spicules, presence of

shell, etc.) for the recently diverged Clades D, E and F

(Fig. 3B and C).

Photosymbiosis and biogeography

Acantharians are commonly found in surface waters from

polar to equatorial regions, and from surface illuminated

waters to the deep sea (Bernstein et al., 1999). They are,

however, more commonly found in surface waters of trop-

ical oligotrophic regions where they can represent a sub-

stantial fraction of protist abundances and overall primary

Environmental Microbiology

8T. Biard

production (Michaels, 1988; Michaels et al., 1995). The

ecological success of such Acantharia is likely in part

related to the fact that many species live in photo-

symbiosis with photosynthetic partners (Decelle

et al., 2015). However, not all Acantharia have been

found living such an intricate life (Fig. 4B). Indeed, only

taxa of the Arthracanthida and Symphiacanthida

(e.g. Clades E and F) have been repeatedly observed liv-

ing together with tens to hundreds of haptophyte

Phaeocystis cells (Khmeleva, 1967) and such symbioses

likely date from between 100 and 200 Mya when inor-

ganic nutrients were in short supply in the seas (Decelle

et al., 2012b). Detailed investigation of intracellular pro-

cesses related to this symbiosis revealed that

Phaeocystis symbionts are heavily modiﬁed (both their

morphology and metabolism) upon acquisition by their

acantharian host, suggesting that this seemingly mutual-

ism could be a ‘farming strategy’(Decelle et al., 2019).

These acantharians with symbionts often represent the

majority of acantharian biomass in surface waters

(Michaels, 1988; Stoecker et al., 1996). Despite

Acantharia–Phaeocystis being the most widely distrib-

uted form of symbiosis among Acantharia, a few rarer,

yet more complex, symbiosis cases have been observed

among Chaunacanthida (i.e. Clade C). This symbiosis

involves an acantharian cell with multiple partners

(i.e. dinoﬂagellates and haptophyte) in a three-way origi-

nal symbiosis being a likely remnant of primitive symbio-

sis (Decelle et al., 2012c).

Environmental genetic diversity

Little is known about acantharian diversity in marine eco-

systems and overall, only a handful of studies have

looked speciﬁcally into the distribution patterns at

regional or global scales. This is likely due to a combina-

tion of several factors: rapid dissolution of acantharians

in ﬁxed (e.g. formalin) samples (Beers and

Stewart, 1970), the fact that acantharian skeletons are

easily damaged upon collection, and commonly a lack of

the taxonomic expertise required for identiﬁcation

(Decelle and Not, 2015). Consequently, investigations

relying on the analysis of acantharian DNA from diverse

environments have provided valuable, unique informa-

tion. Of the six molecular clades described by Decelle

et al.(

2012a), only the most basal Clade A

(Hollacanthida) was not retrieved in early molecular envi-

ronmental surveys. The remaining clades are found in

various ecosystems, including a wide range of trophic

conditions, from the surface to deep-sea ecosystems.

Surprisingly, two molecular clades (described as Clade I

and III; Decelle et al., 2012a) are composed exclusively

of DNA sequences retrieved from environmental surveys

and could not be assigned to any known acantharian

morphospecies or suborder (Fig. 4A). Furthermore, these

environmental clades were samples in extreme ecosys-

tems such as deep-water masses, hydrothermal vents or

high-latitude ecosystems. However, the study of such

environmental sequences comes with inherent limitation.

Most of these sequences come from the early age of

molecular environmental surveys, using sequencing tech-

nics (e.g. clone library) nowadays obsolete. Such surveys

only provide a relatively limited snapshot of diversity at a

given location compared to the more recent DNA

metabarcoding approaches.

DNA metabarcoding (i.e. high-throughput sequencing

of a constrained DNA region, often for marine protist

communities the small-subunit ribosomal RNA gene) has

allowed much more extensive exploration of diversity pat-

terns (Caron et al., 2004,2012). Surveys of acantharian

metabarcodes retrieved in the surface and deep-sea

(>200 m) zones suggested higher acantharian diversity

(mainly dominated by Clades E and F) in surface waters

(Decelle et al., 2013; Mars Brisbin et al., 2020). Similar to

clone library surveys (see above), DNA metabarcoding

systematically highlighted an overwhelming dominance

of the environmental Clade I in deep waters, regardless

of the sampling location (i.e. Indian, Atlantic, Antarctic or

Paciﬁc Oceans). Therefore, the existence of a deep-sea

and diverse acantharian population should be considered

and deserves further attention. However, given the evolu-

tionary history of acantharian skeleton (Decelle

et al., 2012a; Decelle and Not, 2015), with basal clades

represented by rudimentary skeletons (see above), we

cannot rule out that this environmental clade I, never

morphologically characterized, lack the typical acan-

tharian strontium sulfate skeleton (Mars Brisbin

et al., 2020). At broader geographical scales, despite a

lack of detailed annotation and pattern analyses, DNA

metabarcoding of surface waters suggested that

Acantharia are one of the 11 most diverse eukaryotic lin-

eages (de Vargas et al., 2015), with 1043 operational tax-

onomic units (can be considered a proxy for species

richness to some extent; Caron and Hu, 2018). In the

global bathypelagic realm, DNA metabarcoding

suggested much more reduced acantharian diversity

(i.e. 75 OTUs), despite any further detailed annotations

(Pernice et al., 2016). Overall, environmental diversity of

Acantharia is yet to be fully characterized but will

undoubtedly reveal their substantial high diversity

throughout a broad range of ecosystems.

Nassellaria

Diversity and classiﬁcation

Initial classiﬁcation schemes for nassellarians have long

relied on expert analyses of speciﬁc morphological

Environmental Microbiology

Diversity and ecology of Radiolaria 9

features, mainly the architecture of the initial spicule

(De Wever et al., 2002). Such schemes have been

derived mainly from fossil specimens, and fewer attempts

have been made to compare these classiﬁcations on living

nassellarians. Despite the inherent complexity of discrimi-

nating such criteria, approximatively 140 genera and

430 species are presently recognized (Suzuki and

Not, 2015). While divided at higher levels into seven

super-families until recently (Suzuki and Not, 2015), the

latest classiﬁcation suggests the existence of 16 super-

families, 14 corresponding to extant nassellarian super-

families (Suzuki et al., 2021). This revision of classiﬁcation

scheme at high taxonomic level was partially due to

Sandin et al.(

2019) latest morpho-molecular classiﬁcation.

In this classiﬁcation, phylogenetic relationships among

Nassellaria discriminate four ‘Molecular lineages’that

can be considered as the suborder level (Fig. 5A). Out of

the 14 extant nassellarian families, 11 are covered by

Symbiotic

algae

Reference

sequences

28S rDNA

18S rDNA

General

morphology

Holacanthida Clade A (Acanthoplegma)

Dinoflagellates

Haptophyte

(Chrysochromulina)

Haptophyte (Phaeocystis)

1 1

5 7

10 diametral spicules

with no central junction

and no shell

10 diametral spicules

(loosely attached) and

no shell

20 radial spicules

(loosely attached) and

no shell

20 fused radial spicules

and no shell

20 radial spicules

(robust junction) and no

shell

20 radial spicules

(robust junction) with

latticed or robust shell

Acantharia

Holacanthida Clade B (Acanthocyrtha,...)

Chaunacanthida Clades C1 to C4

(Gigatarcon, Heteracon,...)

Holacanthida Clades D1/D2

(Acanthocolla, Staurolithium,...)

Arthracanthida Clades E1 to E4

(Lychnaspis, Phractopelta,...)

Arthracanthida

Symphiacanthida

Clades F1 to F3

(Phyllostaurus, Acanthostaurus,...)

Not reported

Unknown

None

Environmental Clade I

Environmental Clade III

Clade B Clade E Clade E Clade F

Fig. 4. Schematic morpho-molecular classiﬁcation of Acantharia based on ribosomal genes (18S and 28S rDNA) from Decelle et al.(2012a).

A. Phylogeny showing molecular clades, taxonomic suborders and environmental clades. Non-supported nodes are reported by a surrounded

question mark.

B. Corresponding extent of the morpho-molecular framework and main morphological features (including the presence and type of symbionts

reported).

C. Scanning electron microscopy illustrations of typical strontium sulfate structures found in three acantharian clades. All scale bars are 50 μm.

Images courtesy of Karine Leblanc (CNRS).

Environmental Microbiology

10 T. Biard

molecular data, thus allowing an extensive description of

evolutionary relationships between super-families. The

ﬁrst lineage (I) corresponds to the Eucyrtidioidea and rep-

resents the most basal and likely the most ancient line-

age overall, according to molecular clock analyses

(Sandin et al., 2019). Hereafter, Lineage II encompasses

four super-families (Acanthodesmioidea, Artostrobioidea,

Carpocanioidea and Plectopyramidoidea) for which phy-

logenetic relationships are still unclear, particularly due to

the weak amount (<20) of reference sequences available.

The phylogenetic position is yet unclear for the two

remaining lineages III (Archipilioidea, Theopilioidea and

Plagiacanthoidea) and IV (Cycladophoroidea, Lit-

hochytridoidea and Pterocorythoidea).

When combined with morphological analyses of skele-

ton (Fig. 5B and C), the phylogeny revealed substantial

discrepancies with the traditional classiﬁcation (based in

particular on the initial spicular system). Molecular phy-

logeny suggests that the overall morphology (i.e. number,

sizes and shapes of the different segments, presence/

absence of projections, etc.) of the specimen must be

considered for accurate taxonomic identiﬁcation at high

taxonomical level.

Photosymbiosis and biogeography

There is a striking difference in our knowledge of the eco-

logical relationships of fossil nassellarians, and poly-

cystines overall, which have been used for

paleoenvironmental reconstructions (i.e. the use of fossils

to determine the climate for a given era in the past; Laza-

rus et al., 2021), and our knowledge of nassellarian ecol-

ogy in modern ecosystems. Most of our knowledge of

extant species is derived from sediment samples

(e.g. Boltovskoy and Correa, 2016) from which ecological

niche preferences of living nassellarians can be inferred.

Temperature appears to be by far the most signiﬁcant

covarying factor structuring ecological assemblages of

nassellarians. However, some species are representa-

tives of particular trophic conditions: Acanthodesmia

vinculata for surface oligotrophic waters, Eucyrtidium

acuminatum for surface temperate areas (Suzuki and

Not, 2015). While all Radiolaria are marine species

inhabiting saline waters (salinities above 30 PSU;

Boltovskoy et al., 2017), there is one nassellarian spe-

cies, Lophophaena rioplatensis, who represents the only

known radiolarian species thriving in brackish waters

(Boltovskoy et al., 2003).

In addition to their phagotrophic behaviour (Fig. 3),

numerous nassellarian species have been observed to

be associated with photosynthetic partners (Fig. 5B).

While most of these observations did not investigate the

exact nature of the symbionts, often described as ‘golden

dots scattered within the cell’, a few studies have

narrowed their identities to two dinoﬂagellate species:

Zooxanthella nutricula (often referred to as Bra-

ndtodinium nutricula; Probert et al., 2014) and

Gymnoxanthella radiolariae (Yuasa et al., 2016). The

numbers of symbionts (5–13 μm in size) per host vary

considerably from a few cells up to more than 50 symbi-

onts scattered within the host cytoplasm. While these

symbiotic dinoﬂagellates have been identiﬁed in several

nassellarian species (Fig. 5B), a few super-families have

never been observed associated with symbionts. How-

ever, given the lack of systematic analyses search for

their presence, it is currently impossible to draw a conclu-

sion regarding a possible co-evolution between the sym-

bionts and Nassellaria.

Environmental genetic diversity

Little is known about biogeographical patterns of

nassellarian diversity in the global ocean. This observa-

tion is clearly contrasted with the extent of knowledge

gathered from their microfossils (De Wever et al., 2002).

Only a handful of study has investigated the extent of

their distribution in modern oceans (Boltovskoy

et al., 2010; Boltovskoy and Correa, 2016). A number of

nassellarian species have been reported from the deep

ocean (Boltovskoy and Correa, 2016), some of them

being characteristic of deep-water masses

(e.g. Cycladophora davisiana for the mesopelagic ocean,

0–1000 m; Cornutella profunda for the bathypelagic,

1000–2000 m; Suzuki and Not, 2015). Yet, all these stud-

ies relied on challenging morphological identiﬁcations

(see above) therefore hampering our understanding of

diversity patterns.

By comparing environmental sequences from the pre-

vious expedition to the latest morpho-molecular frame-

work for Nassellaria, Sandin et al.(

2019) provided the

ﬁrst assessment of nassellarian environmental diversity

from molecular data. A large majority of the environmen-

tal sequences were assigned to the lineages II and III

(in particular the Plagiacanthoidea), from various types of

ecosystems (e.g. open ocean and nutrient-poor, deep-

sea anoxic waters, etc.). However, clone data unlikely

provide the most extensive overview of Nassellaria envi-

ronmental diversity. Considering the two most recent and

global oceanographic expeditions, high-throughput

sequencing for metabarcoding suggests a pool of

approximatively 400 OTUs (i.e. proxy for species rich-

ness) for Nassellaria, equally spread across a size range

from 0.8 to 2000 μm, and in the surface ocean (Tara

Ocean Expedition; de Vargas et al., 2015). This number

is similar to the number of identiﬁed species based on

morphological analyses (Suzuki and Not, 2015), while

they are likely not both covering the same size fraction

(sizes <20 μm rarely being studied in microscopy for

Environmental Microbiology

Diversity and ecology of Radiolaria 11

Symbiotic

algae

Reference

sequences

28S rDNA

18S rDNA

Multisegmented, simple

cephalis, segments with

discrete dividers below

the thorax

712 Yes (undeterm.)

Nassellaria

Eucyrtidioidea

Clade A - (Eucyrtidium)

2 segments, high-angled

conical or

very at umbrella-like

shell

2-3 (4-rare) segments,

oval to spindle shell with

a small cephalis

2-3 segments, tubular or

high-angled conical shell

with some having an

undulated thorax

One D-shaped ring with

well-developed rods

from which many small

appendages arise

Not reported

Dinoagellates

(Gymnoxanthella,

undeterm.

Not reported

Plectopyramidoidea*

Artostrobioidea

Acanthodesmioidea

Carpocanioidea*

Yes (undeterm.)

Clade B - (Polypleuris)

Clade C - (Carpocanium)

Clade D - (Spirocyrtis, Extotoxon,...)

Clade E -

(Lophospyris, Ceratospyris,...)

Not reported

1 segment, cephalic wall

includes many arches or

an arch-like meshwork

2 segments, hat shell

2 segments, helmet

conical shell, 2 apical

horns, 3 wing-like rods

on the upper thorax

3 segments, vertical

apical horn, spherical

cephalis & truncated

conical thorax

2-3 segments, vertical

apical horn, spherical

cephalis, globular thorax

and 3 feets

1-2 segments, with

several arches

1915 Yes (undeterm.)

Not reported

Dinoagellates

(Brandtodinium,

Gymnoxanthella)

Dinoagellates

(Gymnoxanthella,

undeterm.

Cycladophoroidea*

Lithochytridoidea*

Pterocorythoidea

Archipilioidea

Theopilioidea*

Plagiacanthoidea

Clade G

Lophophaenidae (Ceratocyrtis,...)

Sethoperidae (Archiscenium)

Cannobotryidae (Acrobotrissa)

Clade X - (Archipilium, Enneaphormis)

Clade F - (Eucecryohalus)

Clade I -

(Dictyopodium*, Lamprotripus)

Clade H - (Cycladophora, Vaclkyria)

Clade J -

(Pterocorys, Theocorythium,...)

Lineage I Lineage II Lineage III Lineage IV

General

morphology

Fig. 5. Schematic morpho-molecular classiﬁcation of Nassellaria based on ribosomal genes (18S and 28S rDNA) from Sandin et al.(2019).

A. Phylogeny showing molecular clades, taxonomic super-families. Group with an asterisk denotes changes in taxonomic nomenclature from Sandin

et al.(

2019) following the latest classiﬁcation scheme for Radiolaria (Suzuki et al., 2021). Non-supported nodes are reported by a surrounded question

mark. Colours correspond to molecular lineages (ML): blue =ML I, red =ML II, orange =ML III and green =ML IV.

B. Corresponding extent of the morpho-molecular framework and main morphol ogical features (including the presence and type of symbionts reported).

C. Scanning electron microscopy illustrations of typical siliciﬁed structures found in the four main nasellarian molecular lineages. All scale bars

are 50 μm. Images courtesy of Miguel Sandin (Uppsala University).

Environmental Microbiology

12 T. Biard

Nassellaria). From the Malaspina Expedition covering the

global deep bathypelagic ocean, no sequences were

assigned to Nassellaria (Pernice et al., 2016), suggesting

a potential absence in deep oceans. However, since sev-

eral deep-sea nassellarians are well-known (Suzuki and

Not, 2015), the absence of deep-sea sequences

assigned to nassellarians could be due to the paucity of

reference sequences available (Fig. 5B), or to the difﬁcul-

ties in DNA ampliﬁcation for Nassellaria (Sandin

et al., 2019).

Spumellaria

Diversity and classiﬁcation

Traditionally (i.e. sensu Haeckel), radiolarians with spheri-

cal skeletons have been classiﬁed as Spumellaria

(or Entactinaria, although recent molecular evidences

questioned its existence as extant group; Nakamura

et al., 2020; Suzuki et al., 2021). In recent years, several

molecular studies have greatly clariﬁed the phylogeny and

taxonomy of Spumellaria (Yuasa et al., 2005; Kunitomo

et al., 2006; Ishitani et al., 2012; Sandin et al., 2021). As for

Nassellaria, most of the early taxonomic work on

Spumellaria was based on fossil specimens, leaving the

complex task of identifying living spumellarians. Nowa-

days, approximatively 110 genera and 380 species have

been distinguished and divided into nine super-families

(Suzuki and Not, 2015). However, recently Spumellaria

have been classiﬁed into 13 extant super-families, thanks

to the latest morpho-molecular classiﬁcation for this order

(Sandin et al., 2021).

Phylogenetic relationships among Spumellaria (Fig. 6A)

discriminate four ‘Molecular lineages’encompassing dis-

tinct monophyletic super-families. While the existence of

these lineages is statistically supported, inter-relations

between spumellarian lineages are still unclear. Unlike

other radiolarians orders, morphological classiﬁcation and

molecular phylogeny are in general agreement with the

classiﬁcation at the clade level. The overall skeleton sym-

metry and morphology (i.e. overall shape, central structure

patterns, etc.; Fig. 6B and C) of the specimen must be con-

sidered for accurate taxonomic identiﬁcation. Internal

structures like the initial spicular system, believed to be crit-

ical taxonomic criteria for spumellarian identiﬁcation

(De Wever et al., 2002), therefore appear to be a second-

ary criterion enabling ﬁner taxonomic assignation, while

the overall shape and symmetry prove to be efﬁcient and

sufﬁcient at higher taxonomic ranks.

Photosymbiosis and biogeography

Our knowledge of spumellarian ecology has the same

inherent limitations that apply to other extant radiolarians.

Similar to Nassellaria, sea-surface temperatures shape

most spumellarian ecological assemblages (Boltovskoy

and Correa, 2016). However, for most subtropical

spumellarians, vertical distribution patterns are driven by

their mixotrophic strategy, as they host one of the most

diverse number of photosymbionts known among Radio-

laria. In all four spumellarian molecular lineages, the

presence of symbiotic partners has been reported, cover-

ing more than half of the super-families (Fig. 6B). Photo-

symbionts include some of the algae groups identiﬁed

among Nassellaria and Collodaria, such as Zooxanthella

nutricula/Brandtodinium nutricula (Probert et al., 2014)

and Gymnoxanthella radiolariae (Yuasa et al., 2016), but

also other symbionts apparently exclusive to Spumellaria.

These include unidentiﬁed species of Chlorophytes,

Prasinophytes and Haptophytes (Gast and Caron, 2001;

Anderson, 2012; Yuasa et al., 2019), as well the marine

prokaryotes Prochlorococcus (Foster et al., 2006) and

Synechococcus (Yuasa et al., 2012). The number of

symbionts per host varies considerably between species

of symbionts and hosts, from a few dinoﬂagellate cells up

to hundreds of bacterial symbionts scattered within the

host’s cytoplasm. Unlike Acantharia, the lack of consis-

tent search for spumellarian photosymbionts and their

broad distribution among super-families prevent drawing

any conclusion regarding the origin of spumellarian

symbiosis.

Environmental genetic diversity

As for Nassellaria, there is a major imbalance between the

knowledge of spumellarian species distribution based on

the presence of their skeletons in marine sediments

(e.g. Boltovskoy et al., 2010) and diversity patterns in mod-

ern oceans. A few dedicated studies suggest that sets of

few dominant species can characterize a latitudinal gradi-

ent in the North Paciﬁc, ranging from subtropical areas to

subarctic regions (Suzuki and Not, 2015). Similar patterns

are observed between photic and deeper water layers with

both patterns being partially explained by the presence of

symbionts and/or the inﬂuence of temperature. A number

of spumellarian species have been reported from deep

waters (Reshetnyak, 1955) with a few being characteristic

of deep-water masses (e.g. Spongopyle osculosa for the

mesopelagic ocean, Saturnalis circularis for the bathype-

lagic; Suzuki and Not, 2015).

A larger set of environmental sequences in shallow waters

can be assigned to Spumellaria, compared to Acantharia or

Nassellaria (Sandin et al., 2021),andasimilartrendis

observed in deep environments and particular ecosystems

such as anoxic basins. Among the breadth of environmental

sequences, several sequences cannot be clustered with ref-

erence sequences (i.e. one of the 13 molecular clades) and

form six environmental clades (referred to as Env 1 to 6), of

Environmental Microbiology

Diversity and ecology of Radiolaria 13

General

morphology

Symbiotic

algae

Reference

sequences

28S rDNA

18S rDNA

Spumellaria

Clade A - Hexacaryidae* (Haliphormis*,...)

Clade B - Hexacromyidae* (Hexacromyum*,...)

Clade C - Hollandosphaeridae (Hollandosphaera)

Hexacromyoidea*

Clade D - Spongosphaeridae

(Spongosphaera)

Spongosphaeroidea

Clade E1 - Panartidae*

(Didymocyrtis)

Lithocyclioidea

Clade E2 - Spongobrachiidae (Spongasteriscus,...)

Clade E3 - Euchitoniidae (Dictyocoryne, Tricranastrum*)

Spongodiscoidea

Spherical skeleton with

mixed number of spines

and cortical shells

Spherical skeleton with

mixed number of spines

and cortical shells

Cylindrical/ellipsoidal

skeleton with 3+ cortical

shells

Flat skeleton without

spines and one cortical

shell

821

15 Dinoagellates

(Brandtodinium)

Dinoagellates

(Brandtodinium,

Amphidinium,

Gymnoxanthella), Bacteria

Yes (undeterm.)

Bacteria (Proc., Syn.),

Haptophyte,

Dinoagellates (Brand.,

Gymnox.)

Clade F1 - Ethmosphaeridae* (Heliosphaera)

Clade F2 - Cladococcidae* (Cladococcus, ...)

Cladococcoidea*

Clade G - Rhizosphaeridae

(Rhizosphaera, Haliommilla)

Rhizosphaeroidea

Centrocuboidea Clade H - Centrocubidae (Centrocubus)

Clade I - Excentroconchidae (??)

Not reported

Spherical skeleton

without spines and one

cortical shell

Spherical skeleton with

6++ spines and one

cortical shell(s)

Spherical skeleton with

6++ spines and one/two

cortical shell(s)

Dinoagellates,

Chlorophytes,

Prasinophytes,

Haptophytes

Not reported

(Clade L1 - Litheliidae) ?

Clade L2 - Spongopylidae/Cristallosphaeridae

(Schizodiscus, Calcaromma,...)

Trematodiscoidea*

Spongopyloidea

Phorticioidea*

Clade K - Actinommidae (Actinomma)

Haliommoidea*

Clades J1/J2 - Trematodiscidae*

(Stylodictya, Flustrella*)

Clade M2 - Larcospiridae* (Larcopyle)

Clade M3 - Pylodiscidae* (Pylodiscus)

Clade M4 - Zonariidae* (Tetrapyle)

Flat skeleton with 6++

spines and one cortical

shell(s)

Spherical skeleton with

6++ spines and three

cortical shells

Flat skeleton without

spines and one cortical

shell

Mostly cubic skeleton

with +/- 6 spines and

one cortical shell

1219

920

Yes (undeterm.)

Dinoagellates

(Brandtodinium,

Gymnoxanthella)

Not reported

Clade M1 - Amphitholidae* (Tholomura)

Larcospiroidea*

Environmental Clade I

Environmental Clade II

Environmental Clade VI

?Environmental Clade V

Environmental Clade III

Environmental Clade IV

Unknown

None

None Unknown

Unknown

UnknownNone Unknown

Unknown

None Unknown

UnknownNone Unknown

Lineage I Lineage II Lineage III Lineage IV

Fig. 6. Legend on next page.

Environmental Microbiology

14 T. Biard

unknown morphologies (Fig. 6A). Among these environmen-

tal clades, Sandin et al.(

2021) hypothesized that Env1 could

contain a group of naked symbiotic spumellarians developing

within skeleton-bearing species. However, this hypothesis is

currently not supported by any physical sample hampering

any further investigation of the possible naked spumellarians.

At larger geographic scales, a metabarcoding survey in

the surface ocean (i.e. Tara Ocean Expedition)

highlighted a slightly more diverse spumellarian order

(with approximatively 600 OTUs, twice more than the lat-

est estimates from plankton samples; Suzuki and

Not, 2015) compared to Nassellaria (de Vargas

et al., 2015). However, considering metabarcode abun-

dances (referred to as ‘read abundance’, a proxy for cell

abundances), Spumellaria are among the nine most

abundant eukaryotic lineages. Interestingly, 80% of these

sequences/reads were extracted from the 0.8–5μm size

fraction, an unlikely size for typical spumellarians (gener-

ally larger than 60 μm). This imbalance in favour of the

smaller size fraction could, however, strengthen the

hypothesis of naked symbiotic spumellarians. Alterna-

tively, the presence of spumellarian in such a small size

fraction could be explained by cell breakage during sam-

pling or the existence of spumellarian swarmers

(i.e. ﬂagellated cells of 2.5–10 μm corresponding to a

sexual reproductive stage of radiolarians) reproducing in

the deep ocean (Li and Endo, 2020), in a similar fashion

to Foraminifera. Deeper in the oceans, Spumellaria

appear less diverse (68 OTUs) in the Malaspina Expedi-

tion metabarcoding survey, but they comprise one spe-

cies (Cladococcus viminalis) with the fourth largest read

abundance overall (Pernice et al., 2016). This exploration

of spumellarian environmental signatures altogether high-

lights that there are substantial gaps in the understanding

of the likely underestimated spumellarian diversity

(i.e. existence of clades without any known morphol-

ogies) and its distribution in modern oceans.

Collodaria

Diversity and classiﬁcation

Compared to other radiolarian orders, only handful studies

have investigated collodarian diversity, both in fossil and

living organisms. Collodaria were described in the late 19th

century (Haeckel, 1882) but were considered for decades

to be part of the order Spumellaria (e.g. Anderson, 1983).

Nowadays, Collodaria are formerly recognized as an inde-

pendent radiolarian order, separate from Spumellaria

(De Wever et al., 2002). However, recent molecular evi-

dence have highlighted that Collodaria could be included

as a new nassellarian family, rather than an independent

molecular clade (Sandin, 2019; Nakamura et al., 2021).

Nevertheless, until further morpho-molecular analyses and

for ease of understanding, Collodaria are here discussed

as an independent entity separate from Nassellaria.

Approximatively 80 collodarian species have been identi-

ﬁed and regrouped in 20 genera (Suzuki and Not, 2015).

Collodaria were originally separated into three main fami-

lies, Thalassicollidae, Sphaerozoidae and Collos-

phaeridae, based on two key morphological criteria: as

either colonial (Sphaerozoidae and Collosphaeridae) or

solitary (Thalassicollidae) organisms and the presence or

absence of cortical shells (Müller, 1859).

Recent morpho-molecular classiﬁcations have

expanded our knowledge of classiﬁcation at the higher

taxonomic levels among collodarians, nowadays classi-

ﬁed into four families (Fig. 7A): the three former families

in addition to the newly created Collophidiidae, another

family of colonial collodarians (Biard et al., 2015). Inter-

estingly, while coloniality was a key criterion, phylogenies

suggested that Thalassicollidae do not form a monophy-

letic clade, but instead is spread in the three ‘colonial’

families (Fig. 7B). This major conﬂict between morpholog-

ical taxonomy and molecular phylogeny has likely its ori-

gin in collodarian biology, where solitary forms are

putatively reproductive stages of colonial organisms

(Biard et al., 2015). Phylogenetic relationships further

separate Sphaerozoidae from Collophidiidae and

Collosphaeridae, the latter two being more closely

related. Collosphaeridae are separated into six

phylogenic clades where no clear patterns can associate

the different clades with known genera. Historically gath-

ering all skeleton-bearing collodarian species, morpho-

molecular analyses highlighted the coexistence of naked

specimens with skeleton-bearing species among

Collosphaeridae (Fig. 7C). Similar to the mix between

colonial and solitary species, the coexistence of naked

and skeleton-bearing Collosphaeridae specimens is likely

due to different ontogenic stages (Biard et al., 2015). In

Fig. 6. Schematic morpho-molecular classiﬁcation of Spumellaria based on ribosomal genes (18S and 28S rDNA) from Sandin et al.(2021).

A. Phylogeny showing molecular clades, taxonomic super-families and environmental clades. Non-supported nodes are reported by a surrounded

question mark. Group with an asterisk denote changes in taxonomic nomenclature from Sandin et al.(

2021) following the latest classiﬁcation

scheme for Radiolaria (Suzuki et al., 2021). Colours correspond to molecular lineages (ML): blue =ML I, red =ML II, orange =ML III and

green =ML IV.

B. Corresponding extent of the morpho-molecular framework and main morphological features (including the presence and type of symbionts

reported).

C. Scanning electron microscopy illustrations of typical siliciﬁed structures found in the four main spumellarian molecular lineages. All scale bars

are 50 μm. Images courtesy of Miguel Sandin (Uppsala University).

Environmental Microbiology

Diversity and ecology of Radiolaria 15

Symbiotic

algae

Reference

sequences

28S rDNA

18S rDNA

General

morphology

Colony of tens-hundreds

cells with one (or two)

spherical cortical shell

Colony of tens-hundreds

cells without silicified

structure and an elongated

cylindrical shape

Colony of tens-hundreds

cells with or without bi- or

tri-radiated spicules

1013

4343

1018

Dinoflagellates

(Brandtodinium) in all

collodarian species

Collosphaeridae

Sphaerozoidae

Clades A1 to A6

(Disolenia, Collosphaera,...)

+ Thallasicollidae

Collophidiidae

Clades B1/B2

(Collophidium)

+ Thallasicollidae

Clades C1 to C11

(Collozoum, Sphaerozoum,...)

+ Thallasicollidae

Collodaria

Clade A Clade C Clade C

Fig. 7. Schematic morpho-molecular classiﬁcation of Collodaria based on ribosomal genes (18S and 28S rDNA) from Biard et al.(2015).

A. Phylogeny showing molecular clades and taxonomic families.

B. Corresponding extent of the morpho-molecular framework and main morphological features (including the presence and type of symbionts reported).

C. Scanning electron microscopy illustrations of typical siliciﬁed structures found in the three main collodarian clades. All scale bars are 50 μm.

Environmental Microbiology

16 T. Biard

the newly formed family Collophidiidae, all three

phylogenic clades are associated to the genus

Collophidium, where all specimens lack siliciﬁed struc-

tures (Fig. 7B). Finally, the Sphaerozoidae are the most

diverse family with 11 phylogenetic clades, yet not clearly

separating the different genera or morphotypes. Indeed,

spicule bearing species (identiﬁed as Sphaerozoum and

Rhaphidozoum) are spread among naked specimens of

the genus Collozoum, leaving unknown the evolutionary

history of siliciﬁed structures among Collodaria, one of

the most recent Radiolaria lineages (Suzuki and

Not, 2015).

Photosymbiosis and biogeography

As for other radiolarians, Collodaria ecology has long

remained elusive, but a number of studies in the last four

decades have substantially expanded knowledge of this

group. The most striking feature of Collodaria is their

exclusive and tight association with one photosymbiont

species, Zooxanthella nutricula/Brandtodinium nutricula

(Probert et al., 2014). While B.nutricula has been

observed sporadically in other radiolarian taxa, all known

collodarian species have been described associated with

a photosymbiont, later revealed to be B.nutricula. This

dependence of Collodaria on a singular photosymbiont

raises many questions about the speciﬁcity and function-

ing of this association, still unanswered to date. The num-

ber of B.nutricula symbionts per collodarian host varies

considerably between colonial and solitary specimens,

but can include a few hundred cells for solitary, up to sev-

eral thousand scattered in the colonial host’s cytoplasm.

This high number of photosymbionts likely enables the

host to obtain a considerable quantity of carbon-rich

products from its partner photosynthetic activity and likely

thus enhance its survival rates (Anderson, 1983).

Despite what we know about prey and predators of

Collodaria (Fig. 3; Table 1), the inﬂuence of biotic interac-

tions on Collodaria distribution in modern oceans is

unknown. A few studies have nevertheless shown the

inﬂuence of abiotic factors on collodarian populations

(Biard and Ohman, 2020). Mostly restricted to photic

layers, likely to sustain the activity of their photo-

symbionts, Collodaria abundances in off the California

coast appear to be inﬂuenced by temperature and the

depth of the deep Chlorophyll amaximum. However,

these factors only explain a minor fraction (25%) of the

population’s variability, suggesting that other factors

(biotic or abiotic) inﬂuence Collodaria in these regions.

Environmental genetic diversity

The diversity of Collodaria in modern ecosystems has

been assessed sporadically from microscopical analyses

of surface sediment or plankton net samples, but without

any detailed description of diversity patterns

(e.g. Boltovskoy et al., 2010). A major breakthrough

occurred in our understanding of collodarian diversity

thanks to high-throughput environmental sequencing. In

early environmental surveys based on clone libraries,

many sequences have been associated with Collodaria

in various types of ecosystems, suggesting a potential

high hidden pool of diversity for this order (Biard

et al., 2015). A majority of these clone sequences, com-

ing from deep environments for the most part, have been

associated with the newly formed family Collophidiidae,

suggesting that this family could be a major representa-

tive of Collodaria in the deep ocean (Biard et al., 2015).

In more recent environmental surveys, Collodaria have

been in the spotlight as one of the most represented taxa

in metabarcoding surveys. From the Tara Ocean Expedi-

tion global survey of the photic oceans, Collodaria were

described as the fourth most hyperdiverse eukaryotic lin-

eage (with nearly 6000 OTUs, a number exceeding by far

the 80 morpho-species described previously) and would

be the most abundant (read abundances) protistan line-

age, only exceeded in numbers by multicellular organ-

isms (de Vargas et al., 2015). Similar trends were

observed from the Malaspina metabarcoding survey:

Collodaria would be the most abundant (read abun-

dances) and ﬁfth-most diverse (197 OTUs) eukaryotic lin-

eage in the bathypelagic ocean (Pernice et al., 2016).

This study provided more precise identiﬁcations of these

deep bathypelagic collodarian communities, systemati-

cally composed of Collophidiidae, a trend similar to clone

libraries. However, if Collophidiidae represent a deep-

water collodarian family, their morphology in such an

environment is yet to be determined, as no colonial or

solitary collodarian specimens have ever been consis-

tently observed in the deep ocean.

While Collodaria are now often recovered in

metabarcoding analyses, one detailed study enabled a

better understanding of their prominence in such surveys

(Biard et al., 2017). Indeed, the number of DNA gene

copies per collodarian cell is some of the highest in

eukaryotic lineages. These high numbers are further

ampliﬁed when considering collodarian colonies, with

dozens of millions rDNA copies, being the highest rDNA

copy number ever recorded in any marine protists. Such

high numbers of copies are as high as the number of

rDNA copies estimated for multi-cellular crustacean cope-

pods. Combined with the likely high DNA intracellular var-

iability (observed in other radiolarian taxa; Decelle

et al., 2014) and potential cell breakage during collection,

these rDNA copy numbers have likely led to an over-

representation of Collodaria in metabarcoding surveys

and suggest that diversity and relative abundance esti-

mates have been likely over-estimated.

Environmental Microbiology

Diversity and ecology of Radiolaria 17

Despite complex inclusion in metabarcoding surveys

scaled to planktonic communities, direct comparisons

between collodarian metabarcodes revealed marked pat-

terns (Biard et al., 2017). Collodaria diversity shows an

increase from the poles to equatorial regions and an

increase from coastal regions to offshore areas. This sec-

ond trend further revealed a clear distinction between col-

lodarian communities along a gradient of distances to

coasts, with Sphaerozoidae (comprising mostly naked

collodarian, or with reduced siliciﬁed structures) dominat-

ing coastal areas, and Collosphaeridae (collodarian bear-

ing siliciﬁed skeletons) dominating offshore.

Taxopodida and Orodaria

Diversity and classiﬁcation

The last two remaining radiolarian orders, Taxopodida

(Fig. 2C and D) and Orodaria (Fig. 2I and J) are two phylo-

genetic distant entities (Fig. 1) and they do not share any

morphological similarities except their siliciﬁed skeletons.

Nonetheless, both orders are currently the last remaining

radiolarians without detailed morpho-molecular frame-

works, despite the recent placement of Orodaria in the radi-

olarian phylogeny (Nakamura et al., 2021). Orodaria are

phylogenetically close to Collodaria and Nassellaria, but

distant from Spumellaria and Phaeodaria, two families

where Orodaria used to be classiﬁed based on morphologi-

cal resemblances (Haeckel, 1887; Haecker, 1908;De

Wever et al., 2002). Morpho-molecular sequences cur-

rently cover only the family Oroscenidae, leaving unknown

the phylogenetic placement of the second Orodaria family,

Thalassothamnidae (Nakamura et al., 2021).

Taxopodida remains one of the most elusive radiolar-

ian orders to date, due to the almost entire lack of knowl-

edge about this group. It is currently composed of a

single described species Sticholonche zanclea, for which

a handful of molecular sequences have been produced.

The phylogenetic placement of Taxopodida and its rela-

tion with other radiolarian orders is therefore complex

and uncertain. Depending on the gene used, Taxopodida

are alternatively a sister clade of Spumellaria or

Acantharia, and less often a basal clade to Foraminifera

and Radiolaria altogether (Sandin, 2019). Whilst no com-

plete morpho-molecular analyses have been carried out

for Taxopodida, this order seemingly appeared highly

diverse from environmental molecular surveys.

Photosymbiosis and biogeography

Orodaria are inhabitants of the deep ocean, dwelling

below the photic zone (Nakamura et al., 2021). Conse-

quently, unlike other radiolarians, Oroscenids have never

been observed associated with photosymbionts or

bacterial symbionts. However, the lack of observation of

live Oroscenids prevents understanding of their ecology.

Information about Taxopodida ecology is even more

scarce and they remain the most elusive radiolarian order

to date. Like Orodaria, no symbionts have been observed

in any taxopodids. Because of such fragmentary informa-

tion, the place of Orodaria and Taxopodida in marine

food webs or biogeographical patterns is currently

unknown, but given their peculiar natures, they should

attract further attention in the future.

Environmental genetic diversity

Early on, during the environmental clone sequencing era,

Taxopodida revealed a diverse signature, with many

sequences from various ecosystems (surface, meso- and

bathypelagic) clustering with the single S.zanclea reference

sequence (Not et al., 2007). Following the rapid acceptance

of environmental molecular surveys, more sequences were

assigned to the Taxopodida cluster, hereafter described as

the “Rad-B"clade (Suzuki and Not, 2015). Two sister cla-

des, Rad-A and Rad-C, were associated with the Rad-B/

Taxopodida cluster, without any evidence suggesting that

these could altogether represent the order Taxopodida.

However, given the breadth of sequences available (further

exempliﬁed by the 250 and 85 OTUs in the Tara Expedi-

tion and Malaspina metarbarcoding datasets; De Vargas

et al., 2015;Perniceet al., 2016) and the variety of ecosys-

tems from which these sequences were produced, the

Taxopodida hold a substantial pool of unexplored diversity

waiting to be characterized.

Considering the extant of radiolarian diversity in envi-

ronmental surveys, there is a striking difference with

Orodaria. To date, no environmental sequences have

been ever formally retrieved from clone sequencing or

metabarcoding surveys. Due to the lack of reference

sequences for Orodaria until recently (Nakamura

et al., 2021), it cannot rule out that environmental

sequences associated with Orodaria have been mis-

identiﬁed or unlabelled, leaving unknown the extent of

orodarian environmental diversity.

Concluding remarks

The last few decades have seen a growing interest in

modern Radiolaria, likely thanks to large-scale studies

that highlighted their apparently substantial contribution

to marine biodiversity (de Vargas et al., 2015), carbon

pool (Biard et al., 2016) or biogeochemical cycles

(Lampitt et al., 2009; Biard et al., 2018). Coupled with the

democratization and decreasing costs of molecular

approaches, major breakthroughs in the understanding of

radiolarian classiﬁcation, at orders level, have set the

baseline (i.e. extended morpho-molecular reference

Environmental Microbiology

18 T. Biard

sequences available on public platforms; Guillou

et al., 2013; Clark et al., 2016) for further large-scale

observation of their diversity patterns.

Despite this new exciting dynamic in radiolarian

research (Box 2), several elements need to be dealt with

caution. For example, the overwhelming numbers of envi-

ronmental sequences assigned to radiolarians might

have a direct origin in the vast number of rDNA gene

copies per radiolarians (Biard et al., 2017). The regularly

high diversity estimates for Radiolaria could be biased by

their high intracellular variability of gene, leading to artiﬁ-

cial diversity even within the same cell (Decelle

et al., 2014; unpublished data). A full characterization of

such processes is therefore essential to fully understand

their contribution to marine biodiversity.

Nowadays, among the six radiolarian orders

described, two are lacking detailed morpho-molecular

characterization: Orodaria (despite a few sequences

recently acquired; Nakamura et al., 2021)and

Taxopodida. Until these two groups are fully character-

ized, it is impossible to understand the full evolutionary

history of Radiolaria, which would be a key to under-

stand how Radiolaria evolved since the Cambrian, from

simple siliciﬁed spicules (i.e. Archeospicularia) to com-

plex large colonial organisms, sometimes without any

siliciﬁed structures and ﬁlled with photosymbiotic algae.

Furthermore, characterizing the full morpho-molecular

framework of each radiolarian order by achieving

broader sampling efforts would likely enable the descrip-

tion of environmental clades, such as those found

among Acantharia and Spumellaria, but also ﬁnally

naming the clades “Rad-A"and “Rad-C", who appear

close to the only Taxopodida sequence available

(Suzuki and Not, 2015).

In parallel with a full morpho-molecular classiﬁcation,

remains the extensive problem of radiolarian ecology.

There are many questions that remain unsolved and too

many to be fully listed here: for example, who are the

predators of radiolarians? Answering this simple question

would have deep consequences in our understanding of

radiolarian ecology and would ﬁnally allow the inclusion

of Radiolaria in food web models. Given their substantial

abundances in some location, it is clear that Radiolaria

can represent a large amount of food for their putative

predators, from copepods to ﬁshes, both in the photic

layers but also below in the deep ocean; What do radio-

larians eat? As a number of radiolarians are associated

with photosymbionts, what is the amount of ingested food

they need to capture? Basic questions such as will likely

remain partially answered because of our incapacity to

culture radiolarians. Therefore, unless a major break-

through occurs, we will rely on freshly collected radiolar-

ian specimens, difﬁcult to maintain through a complete

reproductive cycle. Alternatively, the emergence of micro-

ﬂuidics technics (also known as Lab-on-a-Chip; Girault

et al., 2019) comes with a new existing alternative for

radiolarian culture and a likely promise for major break-

throughs in our understanding of this wonderful group

that is Radiolaria.

Box 2. Summary of diversity, ecology and

distribution for each radiolarian order

Order (size range in μm);Number of extant genera

and species;Fossil record;Reported numbers of

Operational Taxonomic Units for the Tara Ocean

Expedition and Malaspina Expedition;Trophic strat-

egy;Symbiosis;Horizontal and vertical distribution.

Acantharia (50–1000); 49 genera and 145 species;

Fossil record absent likely due to rapid dissolution;

Tara =1043 OTUs, Malaspina =75 OTUs;

Phagotroph; Mostly Haptophytes, Dinoﬂagellates less

common, all restricted to three clades; Cosmopolite,

with high abundances in coastal areas, and species

distributed from the surface to the deep ocean.

Taxopodida also referred as to Rad-B (200–800);

one genus and one species; Fossil record absent;

Tara =250 OTUs, Malaspina =85 OTUs; Trophic

strategy unknown; No symbionts reported; Cosmopo-

lite and species distributed from the surface to the

deep ocean.

Spumellaria (50–2000); 110 genera and 380 spe-

cies; Extensive fossil record; Tara =600 OTUs,

Malaspina =68 OTUs; Phagotrophs; Diverse symbi-

onts (e.g. Dinoﬂagellates, Haptophytes, Bacteria, etc.)

found in all four molecular lineages but not in all fami-

lies; Cosmopolite and species distributed from the sur-

face to the deep ocean.

Nassellaria (50–300); 140 genera and 430 species;

Extensive fossil record; Tara =400 OTUs,

Malaspina =absent; Phagotrophs; Dinoﬂagellates

and unknown symbionts in half of the families; Cos-

mopolite and species distributed from the surface to

the deep ocean.

Orodaria (1000–7000); At least four genera and an

unknown number of species; Fossil record common;

No OTUs reported; Likely phagotroph; No symbionts

reported; Restricted to the deep ocean.

Collodaria (100–3 000 000); 20 genera and 80 spe-

cies; Fossil record limited to skeleton-bearing species

(i.e. Collosphaeridae); Tara =6000 OTUs,

Malaspina =197 OTUs; Phagotroph; Dinoﬂagellate

Zooxanthella nutricula/Brandtodinium nutricula found

in all collodarian species; Cosmopolite, with higher

abundances in oligotrophic regions. Species are dis-

tributed mostly in the surface ocean, but many envi-

ronmental sequences are found in the deep ocean.

Environmental Microbiology

Diversity and ecology of Radiolaria 19

Acknowledgements

This work was supported by the ANR RhiCycle project, grant

ANR-19-CE01-0006 of the French National Research

Agency. I am grateful to John Dolan (CNRS) for his valuable

comments on the manuscript, and Miguel Sandin for his use-

ful discussions on radiolarian classiﬁcation. I would like to

thank Karine Leblanc (CNRS) for sharing images of plankton

from the Plankton MIO website (http://plankton.mio.

osupytheas.fr), and Inigo Montoya (a.k.a. JRD) for sharing

images of plankton from the Aquaparadox website (http://

www.obs-vlfr.fr/LOV/aquaparadox/).

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Diversity and ecology of Radiolaria in modern oceans

Abstract and Figures