Larval cloning in the crown-of-thorns sea star, a keystone coral predator

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DOI: 10.3354/meps12843
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Abstract
The crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is an iconic keystone predator whose population outbreaks have devastating consequences for Indo-Pacific coral reefs. We tested the effects of algal food supply and larval density on the frequency of larval cloning by culturing the early bipinnaria larvae of COTS under variable conditions. Here we show that larval COTS are able to clone themselves in both low and high food conditions, and that the frequency of larval cloning increases with levels of food, but is unaffected by larval density. Across all density treatments (0.3, 1.0 and 3.0 larvae ml−1), the per-capita rate of cloning increased from 4.3% in low, oligotrophic conditions (0.17 μg chl a l−1) to 7.9% in high food conditions (1.7 μg chl a l−1). Larval cloning has the potential to increase both COTS larval supply and the dispersal distance of plank- tonic larval stages, both of which are critical factors in predicting the timing and location of out- breaks of this species. In addition, the relationship between algal food supply and larval cloning frequency lends support to bottom-up hypotheses (e.g. nutrient enrichment) as predictors of COTS outbreaks. However, cloning was observed even under the oligotrophic conditions characteristic of coral reefs.
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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 609: 271–276, 2019
https://doi.org/10.3354/meps12843 Published January 17
1. INTRODUCTION
Predation by crown-of-thorns starfish (COTS),
Acanthaster cf. solaris, is one of the primary drivers
of coral mortality in the Indo-Pacific (Pratchett et al.
2014) and is responsible for >40% of coral loss on the
Great Barrier Reef (GBR) over the past 30 yr (De’ath
et al. 2012). Outbreaks of COTS are likely stimulated
by a number of factors, but one factor frequently in -
voked to explain outbreaks, and one that has in -
creased since European settlement (McCulloch et al.
2003), is increased nutrient input from terrestrial
sources (Birkeland 1982, Brodie et al. 2005, 2012).
The vast numbers of feeding larvae produced by
COTS (as many as 100 million eggs yr−1 from a single
female; Babcock et al. 2016b) use phytoplankton as
a food source and, while able to develop in oligotro-
phic conditions, have higher survival under enhanced
chlorophyll concentrations (0.5−5 µg chl al−1; Wolfe
et al. 2017). It has been hypothesized, therefore,
that increased delivery of nutrients from terrestrial
sources has supported enhanced phytoplankton con-
centrations that have, in turn, yielded more success-
ful recruitment events (Brodie et al. 2005) for this
keystone coral predator (Paine 1969).
One important and emerging aspect of echinoderm
larval biology that has been ignored in prior studies
of COTS is the potential for larval cloning, which is
© The authors 2019. Open Access under Creative Commons by
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Publisher: Inter-Research · www.int-res.com
*Corresponding author: jdallen@wm.edu
NOTE
Larval cloning in the crown-of-thorns sea star,
a keystone coral predator
Jonathan D. Allen1,*, Emily L. Richardson1, Dione Deaker2, Antonio Agüera3,
Maria Byrne2, 4
1Department of Biology, College of William and Mary, Williamsburg, VA 23187, USA
2School of Medical Sciences, University of Sydney, Sydney, NSW 2006, Australia
3Laboratoire de Biologie Marine CP160/15, Université Libre de Bruxelles, 1050 Brussels, Belgium
4School of Life and Environmental and Biological Sciences, University of Sydney, Sydney,
NSW 2006, Australia
ABSTRACT: The crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is an iconic keystone
predator whose population outbreaks have devastating consequences for Indo-Pacific coral reefs.
We tested the effects of algal food supply and larval density on the frequency of larval cloning by
culturing the early bi pinnaria larvae of COTS under variable conditions. Here we show that larval
COTS are able to clone themselves in both low and high food conditions, and that the frequency
of larval cloning increases with levels of food, but is unaffected by larval density. Across all density
treatments (0.3, 1.0 and 3.0 larvae ml−1), the per-capita rate of cloning increased from 4.3 % in low,
oligotrophic conditions (0.17 µg chl al−1) to 7.9% in high food conditions (1.7 µg chl al−1). Larval
cloning has the potential to increase both COTS larval supply and the dispersal distance of plank-
tonic larval stages, both of which are critical factors in predicting the timing and location of out-
breaks of this species. In addition, the relationship between algal food supply and larval cloning
frequency lends support to bottom-up hypotheses (e.g. nutrient enrichment) as predictors of COTS
outbreaks. However, cloning was observed even under the oligotrophic conditions characteristic
of coral reefs.
KEY WORDS: Cloning · Coral · Crown-of-thorns starfish · Keystone species
O
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CCESS
Mar Ecol Prog Ser 609: 271–276, 2019
common in asteroid larvae found in the oligotrophic
tropical waters of the Gulf Stream (Bosch et al. 1989,
Galac et al. 2016). In addition to increasing the sur-
vival of feeding larvae, phytoplankton abundance
has also been implicated as a potential cue for larval
cloning (reviewed by Allen et al. 2018). In another
keystone starfish, Pisaster ochraceus, both quality
and quantity of phytoplankton food supply were
shown to be significant inducers of larval cloning
(Vickery & McClintock 2000). Recent studies in echi-
noids and holothuroids have found similar results
(Eaves & Palmer 2003, McDonald & Vaughn 2010)
and, taken together, suggest that phytoplankton food
availability may be a general inducer of larval cloning
across echinoderms.
Asexual reproduction occurs in both the larval and
adult life stages of echinoderms through a variety of
methods and has been implicated in adjusting popu-
lation densities with respect to available resources
(Mladenov 1996, Lee et al. 2008, Allen et al. 2018).
Descriptions of larval cloning exist across echino-
derm classes, including the asteroids (Bosch et al.
1989, Balser 1998, Vickery & McClintock 2000, Eaves
& Palmer 2003, McDonald & Vaughn 2010), but
despite being one of the most intensely studied star -
fish species on Earth (Pratchett et al. 2017), cloning
has not been assessed for COTS. We reared COTS to
determine if larval cloning occurred, and then tested
2 factors to determine their role as potential inducers
of larval cloning: larval density and algal food supply.
2. MATERIALS AND METHODS
COTS adults were collected on December 1, 2017
at Rib Reef (~18° 29’ S, 146° 52’ E) and immediately
transported to Orpheus Island Research Station (OIRS;
18° 37’ S, 146° 29’ E). The sex of adults was deter-
mined through visual inspection of gonads through
small incisions and then adults were placed into
flow-through ambient seawater tanks with coarse fil-
tration (~5 µm). For each experiment, gonads were
dissected through small openings made in the body
wall. Testes were kept intact in a test tube at ambient
temperature until use, while ovaries were placed in
0.45 µm filtered seawater (FSW) with 1 µM 1-methyl -
adenine (1-MA) to induce maturation (Strathmann
1987). Eggs were observed under a stereomicroscope
until germinal vesicle breakdown was confirmed
(~50 min following immersion in 1-MA), at which
point a dilute sperm solution was added to fertilize
eggs. The sperm solution was created by macerating
the testes and diluting them in FSW in order to acti-
vate swimming sperm (active swimming of sperm
was confirmed visually under a microscope).
For initial observations of cloning (Expt 1), cultures
were started with a single male/female pair on De -
cember 6. Larvae were cultured at 28°C in individual
glass bowls and beakers (volumes ranging from
0.1 to 1.0 l) that were filled with FSW. Larval density
was approximately 3 larvae ml−1. Cultures were
hand-stirred and visually inspected 3 times d−1 for
signs of cloning. These larvae were not fed. Cloning
larvae were first found on December 13 (Day 7 post-
fertilization) and photographed on Days 8−10 post-
fertilization to document regeneration of both larval
parts.
For Expt 2, which tested the effects of larval and
algal food density on cloning frequency, cultures
were generated on December 14 from 3 male/female
pairs to create 3 independent families. Larvae were
initially cultured as in Expt 1. Larvae were allowed to
develop to the early bipinnaria stage and then iso-
lated on December 17 (Day 3). Symmetrical bipin-
naria larvae (>90% of larvae fit this criterion) were
selected by hand for this experiment to ensure that
poorly developed, but uncloned, offspring were not
mistakenly used as evidence of cloning events. Larvae
were examined for cloning on December 19 (Day 5).
Larvae were maintained in small glass beakers
with 40 ml of FSW and randomly assigned to 1 of 3
larval density treatments (3, 1 or 0.3 larvae ml−1) and
2 larval food treatments (N = 3 beakers treatment−1).
Larvae were fed the unicellular flagellated alga Iso -
chrysis galbana at a low (1000 cells ml−1) or high
(10 000 cells ml−1) concentration. Algal food con -
centrations represent chlorophyll concentrations of
approximately 0.17 µg chl al−1 (range: 0.1−0.28 µg
chl al−1) for the low food treatment and 1.7 µg chl a
l−1 (range: 1−2.8 µg chl al−1) for the high food treat-
ments (Alvarez et al. 2017). These low and high
chlorophyll levels are commensurate with those
found in offshore GBR waters and nearshore coastal
waters, respectively (www.bom.gov.au/marinewater
quality/, see Wolfe et al. 2017). To ensure that larvae
and algal food remained in suspension, the cultures
were stirred by hand several times per day. Larvae
from each beaker were observed under a stereo-
microscope and individually pipetted out of the
beaker and into a new beaker. During this process,
each larva was assessed visually for signs of cloning
(see Fig. 1), and each larva was counted and as -
signed as either a clone or an uncloned larva. The
proportion of cloning for each replicate beaker was
thus assessed, and a 2-way ANOVA was conducted
(IBM SPSS version 23) to test the effects of algal food
272
Allen et al.: Larval cloning in sea stars
supply, larval density and their interaction on the fre-
quency of cloning events. The assumption of equality
of variances was tested with Levene’s test and the
variances were not statistically distinguishable from
one another (p = 0.76). Residuals of the analysis were
tested for normality (another assumption of ANOVA)
with a Kolmogorov-Smirnov test and did not deviate
significantly from a normal distribution (p = 0.140), so
no data transformation was performed. To calculate
effect size for the interaction between density and
food supply, we calculated eta-squared (η2) by divid-
ing the sum of squares of the effect by the total sum
of squares (Lakens 2013). For all analyses, we used
an αlevel of 0.05 as our threshold for significance.
273
Fig. 1. (A) Crown-of-thorns starfish, Acanthaster cf. solaris, early bipinnaria larva at the stage at which larvae were sorted into
cloning trials. (B) Mid-stage bipinnaria larva that has begun to develop lobes at the base, but has lost part of the oral hood due
to a cloning event. (C) Some bipinnaria larvae split relatively evenly into 2 smaller individuals through a bisection event. (D−I)
A single larva on Days 8, 9 and 10 of development, following a cloning event on Day 7; the larva bisected into 2 unequal por-
tions, forming (D−F) a smaller ‘head’ clone which is derived from the oral hood and mouth and (G−I) a larger ‘body’ clone de-
rived from the remainder of the larval body. Arrow in (E) shows regenerating coelomic sac. Letters in (F) show development of
a complete gut in regenerating ‘head’ clone. The ‘body’ clone appeared to shrink on Day 9 before regaining size and restoring
symmetry on Day 10, suggesting rapid reworking of the larval body post-cloning. OH = oral hood, M = mouth, E = esophagus,
S = stomach. All scale bars are 100 µm (scale bar in D also applies to E & F; G also applies to H & I)
Mar Ecol Prog Ser 609: 271–276, 2019
3. RESULTS
We found that larval cloning (Fig. 1) was a fre-
quent occurrence in COTS. Larval cloning was
observed starting from the early bipinnaria larval
stage (Fig. 1A). While the timing of the onset of
larval cloning corresponded to the ability of larvae
to feed, cloning was observed in the absence of
algal food. We observed a variety of modes of lar-
val cloning, consistent with prior reports of
multiple modes of asexual reproduction in starfish
larvae (Jaeckle 1994, Allen et al. 2018). The most
common mode of cloning observed was sponta-
neous autotomy of the preoral lobe (Fig. 1B). This
method of cloning resulted in unequal splitting of
the larva into 2 parts: a smaller anterior portion
containing the oral hood and a larger posterior
portion containing the mouth, esophagus and sto -
mach. We were able to track and photograph both
parts of a single larva to demonstrate that over
subsequent days, not only did both parts survive,
but also that the smaller anterior portion was able
to re grow a complete larval gut (Fig. 1D−F) and
coelomic cavity (Fig. 1E). Simultaneously, the larger
portion changed in size over time and repaired an
asym metry that was possibly related to an unob-
served cloning event (Fig. 1G−I). Less frequently,
we ob served other modes of cloning, including a
bisection of the larval body into 2 roughly equiva-
lent halves (Fig. 1C).
Following confirmation of cloning, we then the
tested whether the frequency of larval cloning was
affected either by larval density or by the pres -
ence of phytoplankton food. Across all treatments,
cloning frequencies ranged from 2.5% to 9.6%
(Fig. 2). We found that after just 2 d of pre sentation
with algal food, there was a significant effect of
algal food supply (2-way ANOVA; F1,12 = 8.662; p =
0.012) on larval cloning frequency (high food: 7.91 ±
0.11%; low food: 4.25 ± 0.12%; mean ± SE; Fig. 2).
There was no effect of larval density on cloning fre-
quency (high density: 6.59 ± 0.29%; medium den-
sity: 4.17 ± 0.17%; low density: 7.50 ± 0.08%; 2-way
ANOVA; F2,12 = 2.631; p = 0.137), nor was there an
interaction between larval density and food supply
on cloning frequency (2-way ANOVA; F2,12 = 1.759;
p = 0.214); however, our small sample size (N = 3
per treatment) limited our statistical power to de -
tect density effects and interactions among treat-
ments. One measure of effect size (η2) revealed that
the interaction between density and food supply
accounted for 13% of the variation in cloning
frequency.
4. DISCUSSION
The inherent ability of COTS larvae to clone in oli-
gotrophic conditions, and for this to be enhanced by
increased phytoplankton abundance, has potentially
significant implications for management of COTS on
the GBR. As evident from plankton samples, asexual
reproduction by cloning appears to be a normal form
of propagation for some tropical asteroids (Bosch et
al. 1989, Galac et al. 2016). Asteroids that exhibit
larval cloning, including COTS, have a diverse sym-
biotic bacterial community that includes photosyn-
thetic bacteria that may contribute to the resilience of
asteroid larvae in low food conditions (Galac et al.
2016, Carrier et al. 2018).
Our results suggest that not only are COTS larvae
surviving (Wolfe et al. 2015, Pratchett et al. 2017) in
areas of high chlorophyll concentration (1−2 µg chl a
l−1), but they may actually multiply asexually. Recent
metagenomic surveying of the surface waters of the
GBR suggests that COTS larvae form a ‘continuous
cloud’ over the GBR (Uthicke et al. 2015), including
areas where outbreaks are not occurring. Our data
suggest that, in addition to the tremendous fecundity
of this species, asexual reproduction of larvae may
contribute to their longevity and widespread disper-
sal in the plankton. Our data further suggest that
274
0
2
4
6
8
10
12
14
Mean per capita cloning (%)
Density treatment
3 larvae ml–1 0.3 larvae ml–1
1 larva ml–1
High food
Low food
Fig. 2. Effects of larval density (3, 1 or 0.3 larvae ml−1) and
algal food supply (fed Isochrysis galbana; high food: 10 000
cells ml−1 or low food: 1000 cells ml−1) on cloning frequency
in larval crown-of-thorns starfish, Acanthaster cf. solaris.
Each treatment combination was replicated in 3 beakers.
Bars represent mean ± SE for each treatment; black circles
represent per capita cloning frequency for individual beakers.
In some cases, only 2 circles are visible because 2 beakers
had identical cloning frequencies
Allen et al.: Larval cloning in sea stars
increased phytoplankton enrichment of the normally
oligotrophic waters of the GBR contributes not only
to increased survival of existing larvae, but may itself
be a direct cause of increased numbers of COTS
larvae as well.
While larval cloning was thought for many years to
be an uncommon but potentially adaptive response to
environmental change (Jaeckle 1994), asexual propa-
gation by larvae is now recognized to be widespread
among marine invertebrates, particularly among
echinoderms (Eaves & Palmer 2003, Allen et al. 2018).
Our results match those of prior studies that have
identified algal food supply as one inducer of cloning
in echinoderms (Vickery & McClintock 2000, McDon-
ald & Vaughn 2010), and suggest that the responsive-
ness of echinoderm larvae to variations in food supply
(including in COTS, see Wolfe et al. 2015) includes
enhancing the number of offspring that arise from a
single zygote. The frequency of cloning we identify in
COTS (~3−10%) is comparable to that demonstrated
in other sea stars (e.g. 1−24% in Pisaster ochraceus;
Vickery & McClintock 2000). The levels of chlorophyll
that induced cloning (0.17− 1.7 µg chl al−1) are com-
parable to that which has been shown to be food-lim-
iting in other echinoderm larvae (0.2 µg chl al−1; Fe-
naux et al. 1994) and include the levels in oligotrophic
GBR waters (Wolfe et al. 2017). The levels we used
are below those at which growth and survival of
COTS are negatively affected by excess phytoplank-
ton (10 µg chl al−1; Wolfe et al. 2015).
The presence of larval cloning both at low food
levels and in response to increased food availability
is likely to contribute to the success of COTS and has
potentially major implications for models of popula-
tion dynamics in COTS. In particular, current models
may be underestimating both the fecundity of this
species and the larval dispersal period by ignoring
the potential for cloning in the plankton (Rogers-
Bennett & Rogers 2008). However, an accurate esti-
mate of the ecological significance of cloned larvae
to COTS populations requires demonstration that
cloned larvae can reach metamorphosis and esti-
mates of the abundance of adults derived from
cloned larvae in existing populations. In future stud-
ies, data on not only the frequency of cloning, but
also the changes in larval size and time to metamor-
phosis that are potentially correlated with cloning
events should be investigated. In addition, the conse-
quences of these changes in size and development
time for larval mortality should be investigated, as
planktonic mortality of larvae is known to be a major
limitation on recruitment (Rumrill 1990, Lamare &
Barker 1999, Vaughn & Allen 2010). Understanding
the interactions between environmental inducers of
cloning and the fitness consequences for larval
cloning will be a fruitful avenue for future work.
Nearly 50 yr ago, Paine (1969) coined the phrase
‘keystone species’ to refer to the outsized effect that
sea stars have on their communities. While widely
cited (and often mis-cited, see Lafferty & Suchanek
2016), Paine’s (1969) paper is frequently overlooked
with regard to his argument that COTS could be con-
sidered a keystone species. In fact, Paine (1969)
argued that the role of COTS as a keystone species
could be very similar to that of P. ochraceus, should
the ‘plague’ of COTS be due to factors other than the
removal of predators. Recent work has not rendered
a final decision on the role of predator removal ver-
sus nutrient enrichment in contributing to COTS out-
breaks (Babcock et al. 2016a). While the nutrient-
enrichment hypothesis has garnered wide support,
both it and the predator-removal hypothesis invoked
by Paine (1969) have been deemed ‘largely unre-
solved’ in a recent assessment of COTS biology
(Pratchett et al. 2017).
On one hand, our work shows that COTS larvae
clone in the natural low-nutrient conditions of the
GBR and, on the other, that cloning is enhanced at
elevated nutrient levels. We provide new evidence
that the nutrient-enrichment hypothesis may help to
explain the occurrence of COTS outbreaks through a
novel mechanism, larval cloning, supporting Paine’s
(1969) suggestion that COTS is indeed a keystone
species in the same vein as the more well-known
keystone sea star, P. ochraceus. Regardless of the
descriptors assigned to this species, the regulation of
COTS populations is likely to be a key factor in man-
agement of rapidly declining coral cover on the GBR,
and our work suggests that reduced concentrations
of phytoplankton may reduce the frequency of
cloning in this species and in turn reduce both re -
cruitment and dispersal of a voracious coral predator.
Acknowledgements. The research was supported by an Ian
Potter Foundation Grant from Lizard Island Research Sta-
tion, a facility of the Australian Museum, to M.B. and J.D.A.
E.L.R. was supported by a Fellowship of Graduate Student
Travel from The Society for Integrative and Comparative
Biology and an International Research Grant from the Reves
Center at the College of William and Mary. We thank the
Australian Marine Tourism Operators for collecting the
COTS, and OIRS staff for their support.
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Proofs received from author(s): January 12, 2019
  • Article
    Sea stars of the family Asterinidae are well known for their diverse life history strategies encompassing sexual reproduction through various larval forms, while others, all multi-armed species, through clonal asexual reproduction. For at least 25 years, a geographically isolated population of the minute (radius < 7 mm) tropical Pacific fissiparous asterinid, Ailsastra heteractis, has been locally abundant in a shallow water coral rubble habitat at One Tree Island, southern Great Barrier Reef. The population biology of this sea star was investigated over 14 years to characterise its asexual reproduction through fission (splitting in half) and to determine whether a size threshold is reached for an expected switch to sexual reproduction. Fission followed by regeneration was the dominant propagation mechanism generating individuals that varied in arm number (2–7) and length (0.5–6.9 mm). Recently split individuals were present year round with regenerating individuals being the most prevalent body form. Growth rates and fission activity, as measured by mean arm length, varied among months but with no overall pattern. Analyses of arm length data based on season indicated that fission activity was lowest in autumn. There was a positive correlation between the incidence of fission and air and water temperature with a decrease in the proportion of recently split individuals in winter and an increase in summer. Fission was present in all size classes with no evidence of a switch to a larger sexual morphotype with growth. Juvenile recruitment from sexual reproduction was not evident. Extensive searches did not reveal the presence of other populations of A. heteractis. The absence of recruitment and reproductive organs coupled with a stable population demography indicates that propagation by fission is the only reproductive mechanism for the A. heteractis population.
  • Article
    Arm loss through a separation at a specialized autotomy plane in echinoderms is inextricably linked to regeneration, but the link between these phenomena is poorly explored. We investigated nervous system regeneration post-autotomy in the asteriid seastar Coscinasterias muricata, focusing on the reorganization of the radial nerve cord (RNC) into the ectoneural neuroepithelium and neuropile, and the hyponeural region, using antibodies to the seastar-specific neuropeptide SALMFamide-1 (S1). Parallel changes in the associated haemal and coelomic vessels were also examined. A new arm bud appeared in 3-5 days with regeneration over three weeks. At the nerve stump and in the RNC immediately behind, the haemal sinus/hyponeural coelomic compartments enlarged into a hypertrophied space filled with migratory cells that appear to be involved in wound healing and regeneration. The haemal and coelomic compartments provided a conduit for these cells to gain rapid access to the regeneration site. An increase in the number of glia-like cells indicates the importance of these cells in regeneration. Proximal to the autotomy plane, the original RNC exhibited Wallerian-type degeneration, as seen in disorganized axons and enlarged S1-positive varicosities. The imperative to regrow lost arms quickly is reflected in the efficiency of regeneration from the autotomy plane facilitated by the rapid appearance of progenitor-like migratory cells. In parallel to its specialization for defensive arm detachment, the autotomy plane appears to be adapted to promote regeneration. This highlights the importance of examining autotomy-induced regeneration in seastars as a model system to study nervous system regeneration in deuterostomes and the mechanisms involved with the massive migration of stem-like cells to facilitate rapid recovery.
  • Article
    Full-text available
    Predation by the crown-of-thorns seastar (CoTS; Acanthaster sp.) is a pervasive stressor attributing to the decline of coral reefs. These outbreaks are suggested to be linked to eutrophy-driven recruitment pulses, where increased nutrients enhance larval success. CoTS larvae, however, are tolerant of oligotrophic conditions typical of tropical ecosystems and outbreaks occur in regions isolated from eutrophy, highlighting the resilience of these larvae to oligotrophic conditions. Here we test the hypothesis that CoTS larvae associate with bacterial communities that are dynamic across an oligotrophic-eutrophic continuum and are specific to each feeding regime. Our analysis of the CoTS larval microbiome suggests that CoTS larvae associate with a bacterial community distinct from the environmental microbiota, and that this community experiences a community-level shift in response to differential feeding that is maintained over development. Symbioses with a diverse and dynamic–and a potentially phototrophic–bacterial community may contribute to resilience of CoTS larvae that enable the success of CoTS and other tropical marine larvae in oligotrophic seas.
  • Article
    Full-text available
    Research on the coral-eating crown-of-thorns starfish (CoTS) has waxed and waned over the last few decades, mostly in response to population outbreaks at specific locations. This review considers advances in our understanding of the biology and ecology of CoTS based on the resurgence of research interest, which culminated in this current special issue on the Biology, Ecology and Management of Crown-of-Thorns Starfish. More specifically, this review considers progress in addressing 41 specific research questions posed in a seminal review by P. Moran 30 years ago, as well as exploring new directions for CoTS research. Despite the plethora of research on CoTS (>1200 research articles), there are persistent knowledge gaps that constrain effective management of outbreaks. Although directly addressing some of these questions will be extremely difficult, there have been considerable advances in understanding the biology of CoTS, if not the proximate and ultimate cause(s) of outbreaks. Moving forward, researchers need to embrace new technologies and opportunities to advance our understanding of CoTS biology and behavior, focusing on key questions that will improve effectiveness of management in reducing the frequency and likelihood of outbreaks, if not preventing them altogether.
  • Article
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    In unicellular phytoplankton, the size scaling exponent of chlorophyll content per cell decreases with increasing light limitation. Empirical studies have explored this allometry combining data from several species using average values of pigment content and cell size for each species. The resulting allometry, includes thus phylogenetic and size scaling effects. The possibility of measuring single-cell fluorescence with imaging-in-flow cytometry devices allows the study of the size scaling of chlorophyll content at both the inter and intra-specific levels. In this work, the changing allometry of chlorophyll content was estimated for the first time for single phytoplankton populations using data from a series of incubations with monocultures exposed to different light levels. Inter-specifically, our experiments confirm previous modelling and experimental results of increasing size scaling exponents with increasing irradiance. A similar pattern was observed intra-specifically but with a larger variability in size scaling exponents. Our results show that size-based processes and geometrical approaches explain variations in chlorophyll content. We also show that the single cell fluorescence measurements provided by imaging-in-flow devices can be applied to field samples to understand the changes in the size dependence of chlorophyll content in response to environmental variables affecting primary production. Importance The chlorophyll concentrations in phytoplankton register physiological adjustments in cellular pigmentation arising mainly from changes in light conditions. The extent of these adjustments is constrained by the size of the phytoplankton cells, even within single populations. Hence, variations in community chlorophyll derived from photoacclimation are also dependent on the phytoplankton size distribution.
  • Article
    Crown-of-thorns starfish, Acanthaster planci (COTS), predation is a major cause of coral reef decline, but the factors behind their population outbreaks remain unclear. Increased phytoplankton food resulting from eutrophication is suggested to enhance larval survival. We addressed the hypothesis that larval success is associated with particular chl-a levels in tightly controlled larval:algal conditions. We used chl-a conditions found on coral reefs (0.1–5.0 μg chl-a L− 1), including nominal threshold levels for disproportionate larval success (≥ 1.0 μg chl-a L− 1). High success to the juvenile occurred across an order of magnitude of chl-a concentrations (0.5–5.0 μg chl-a L− 1), suggesting there may not be a narrow value for optimal success. Oligotrophic conditions (0.1 μg chl-a L− 1) appeared to be a critical limit. With a review of the evidence, we suggest that opportunistic COTS larvae may be more resilient to low food levels than previously appreciated. Initiation of outbreak populations need not require eutrophic conditions.
  • Article
    Full-text available
    The crown-of-thorns starfish Acanthaster planci (COTS) has contributed greatly to declines in coral cover on Australia’s Great Barrier Reef, and remains one of the major acute disturbances on Indo-Pacific coral reefs. Despite uncertainty about the underlying causes of outbreaks and the management responses that might address them, few studies have critically and directly compared competing hypotheses. This study uses qualitative modelling to compare hypotheses relating to outbreak initiation, explicitly considering the potential role of positive feedbacks, elevated nutrients, and removal of starfish predators by fishing. When nutrients and fishing are considered in isolation, the models indicate that a range of alternative hypotheses are capable of explaining outbreak initiation with similar levels of certainty. The models also suggest that outbreaks may be caused by multiple factors operating simultaneously, rather than by single proximal causes. As the complexity and realism of the models increased, the certainty of outcomes decreased, but key areas that require further research to improve the structure of the models were identified. Nutrient additions were likely to result in outbreaks only when COTS larvae alone benefitted from nutrients. Similarly, the effects of fishing on the decline of corals depended on the complexity of interactions among several categories of fishes. Our work suggests that management approaches which seek to be robust to model structure uncertainty should allow for multiple potential causes of outbreaks. Monitoring programs can provide tests of alternative potential causes of outbreaks if they specifically monitor all key taxa at reefs that are exposed to appropriate combinations of potential causal factors.
  • Article
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    Studies of pre-spawning crown-of-thorns starfish Acanthaster (COTS) collected from the Great Barrier Reef showed average female gonad mass of 16 % (±8 s.d.) and for males 12 % (±6 s.d.). In females up to 34 % of the body mass could be devoted to gonad. Based on these data, we also derived relationships between diameter gonad weight and reproductive output for both male and female starfish. Due to the large average size of individuals in this study (39.2 ± 0.3 cm diameter), the potential oocyte production of females was between 29 and 38 million eggs per season for average size starfish. The highest oocyte production was estimated to be >100 million oocytes, which is the highest ever recorded for an individual female starfish. These relationships imply that the largest Acanthaster reported may have fecundities greater than 200 million eggs per season. The gonad mass of male starfish is similarly high, and in combination with measured concentrations of sperm exuded from the gonopore (5.2 × 1010 ml−1) the sperm output of an average sized male is estimated to be 1.1 × 1013 sperm. This high level of sperm production may be a key factor allowing this species to sustain itself and even initiate outbreaks at low population densities. We suggest that management targets for maintaining COTS at pre-outbreak thresholds should take account of starfish size as well as starfish density, especially given extreme reproductive potential of large starfish in pre-outbreak populations.
  • Article
    Full-text available
    "Food Web Complexity and Species Diversity" (Paine 1966) is the most-cited empirical article published in the American Naturalist. In short, Paine removed predatory sea stars (Pisaster ochraceus) from the rocky intertidal and watched the key prey species, mussels (Mytilus californianus), crowd out seven subordinate primary space-holding species. However, because these mussels are a foundational species, they provide three-dimensional habitat for over 300 associated species inhabiting the mussel beds; thus, removing sea stars significantly increases community-wide diversity. In any case, most ecologists cite Paine (1966) to support a statement that predators increase diversity by interfering with competition. Although detractors remained skeptical of top-down effects and keystone concepts, the paradigm that predation increases diversity spread. By 1991, "Food Web Complexity and Species Diversity" was considered a classic ecological paper, and after 50 years it continues to influence ecological theory and conservation biology.
  • Article
    Planktotrophic sea star larvae of several species are abundant in oligotrophic waters of the Gulf Stream, western Sargasso Sea, and Caribbean Sea. One abundant larval morphotype at the bipinnaria stage of development is unusual in its ability to constitutively produce clones and in harboring a community of auto-fluorescent bacteria. We hypothesized that the bacterial community would be distinct in these larvae compared to those that do not consistently reproduce clonally. Three sea star larval morphotypes were collected in the Gulf Stream off the coast of Florida. We used DNA-based maximum likelihood phylogenetic analyses to taxonomically classify the larvae and 16S rDNA profiling by deep sequencing to characterize the bacterial communities harbored within. The cloning bipinnaria and non-cloning brachiolaria morphotypes were determined to be a single species of Asteroidea in the family Oreasteridae. The third morphotype, a non-cloning bipinnaria, was identified as Mithrodia clavigera. With bacterial 16S rDNA profiling, we found that the two species of larvae harbor bacterial communities distinct from each other. The Oreasteridae bacterial community at both developmental stages has a photosynthetic Cyanobacteria Synechococcus sp. as the most abundant bacteria. The M. clavigera larvae host a flora consisting primarily of Gammaproteobacteria. The identification of the larval microbiomes is a step toward understanding their host–microbe interactions. Specifically, the association of photosynthetic bacteria with cloning larvae allows for future assessments of whether the bacteria supplement the nutrition of the larvae during extended periods of development and clonal reproduction in open ocean regions where phytoplankton food for the larvae may be in limited supply.
  • Article
    This review provides a brief update of the occurrence and adaptive significance of asexual reproduction in echinoderms. It then focuses on the state of knowledge of biotic and abiotic factors that influence asexual processes in this group, particularly factors that may play a role in regulating the expression and relative proportion of asexual versus sexual phenotypes within populations of species, as well as factors modulating and triggering asexual processes. The information presented in the review is synthesized into the form of a basic model depicting how environmental factors may interact with physiological factors to regulate asexual reproduction in echinoderms, and to show how such regulatory processes may parallel in some ways those known to regulate sexual reproduction. Since the model is based mainly on correlational evidence derived from comparative field observations, a number of experimental approaches for testing the predictions of the model are discussed briefly.