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Pacific crown-of-thorns starfish (Acanthaster cf. solaris) are known to be widely distributed, and previous studies (showing limited genetic structure) suggest that there are high levels of connectivity at regional scales. It is possible, however, that methods used in previous studies (e.g., microsatellites) did not effectively resolve genetic differentiation and structure for crown-of-thorns starfish. This study examined patterns of genetic structure for populations of the A. cf. solaris in the Ryukyu Archipelago and at Miyazaki, Japan, using restriction-site-associated DNA sequencing (RADseq) analysis, to identify genome-wide single nucleotide polymorphism (SNP) loci to examine genetic diversity and gene flow of CoTS populations. We obtained 186 biallelic SNPs completely shared among 33 individuals by utilizing the published genome sequence of CoTS. Population genomic analysis with biallelic SNPs confirmed that there is limited genetic differentiation and seemingly strong connectivity and gene flow among CoTS populations in the Ryukyu Archipelago and at Miyazaki, Japan. Our findings likely reflect the rapid range expansion of CoTS through southern Japan or panmixia facilitated by the northward flowing Kuroshio Current.

In the last decades, managers and local communities have been turning to active restoration as a mechanism to recover damaged reefs affected at an unprecedented rate because of climate change, anthropogenic activities, and natural events, such as outbreaks of the crown-of-thorns starfish (COTS) Acanthaster spp. A coral restoration experiment was conducted in the southern Gulf of California (24°N, 110°W) starting in December 2017. By early 2018, unusually high numbers of Acanthaster cf. solaris appeared at the restoration plots causing significant coral mortality. Fragment survivorship was significantly influenced by site with corals at Roca Swan showing a longer survival probability time [297.43 (SE 16.38) d] compared to El Corralito [133.81 (SE 7.73) d]. From April 2018 to June 2019, average abundance of A. cf. solaris had surpassed 800 ind ha–1 at three sites, and their feeding resulted in high fragment mortality ranging from 39% at Roca Swan to 88% at El Corralito—a strong contrast to other restored reefs along the eastern Pacific where starfish were absent and annual fragment mortality was A. cf. solaris outbreak in 2017–2018, severely hindering success of coral restoration programs.

The crown-of-thorns starfish (COTS), Acanthaster cf. solaris, is one of the main contributors to declines in coral cover on the Great Barrier Reef (GBR) and remains one of the major acute disturbances on coral reefs throughout much of the Indo-Pacific. Extensive control programs on the GBR involve manual culling of COTS in the field, and research is needed to inform these management efforts. Data from the Great Barrier Reef Marine Park Authority’s (GBRMPA) COTS control program provide near-real-time CPUE (Catch-Per-Unit-Effort, COTS culled per minute) data ideal for operational decision-making but these must be converted to density estimates before they can be related to ecological status of reefs or incorporated into ecological models. We developed conversions between common COTS field survey methods (i.e. manta tow, SCUBA transect searches) and COTS control program CPUE data using estimates of sightability and detectability. We used a population model and COTS size-structure data from COTS control program culling efforts to estimate that, on average, only 19% of 1-yr-old COTS (1–15 cm) are available to be culled. Finally, we developed a CPUE-COTS density relationship to estimate the threshold levels of COTS that prevent net growth of hard corals. Culling programs should therefore aim to achieve CPUEs below these ecological thresholds in order to effectively promote coral growth and recovery. These ecologically sustainable thresholds of COTS density varied depending on hard coral cover. For example, for 35% fast-growing coral cover, COTS culling needs to continue until CPUE decreases to below 0.05 COTS/min (1 COTS per 20 min) in order to prevent coral decline, whereas if coral cover is higher (80%), then a higher target threshold CPUE of ca. 0.08 COTS/min (ca. 3 COTS per 40 min) may be ecologically sustainable. These estimates underpin the current pest management rules being implemented by the GBRMPA in its COTS control program.

Crown-of-thorns starfish Acanthaster planci (COTS) outbreaks are a major cause of coral cover loss on the Great Barrier Reef (GBR), with manual culling having only localised success. The endangered giant triton snail Charonia tritonis is a natural predator of COTS, although aquarium and field observations indicate the intensity of direct predation may be inadequate to significantly mediate outbreaks. However, their mere presence can elicit a chemically induced sensory behavioural response which may suppress COTS populations when in non-outbreak status. While there is increasing knowledge of the sensory biology of both species, little is known regarding giant triton snail numbers on the GBR or about how they move and occupy space, making it difficult to determine their true zone of influence and thus their capacity to disrupt COTS behaviour. We used passive acoustic telemetry to establish short-term activity space and movement patterns of giant triton snails on the GBR. Individuals were tracked for up to 41 d, were observed to travel 234.24 m d-1, with a mean total cumulative distance travelled at night (1923.19 m) nearly double that observed during the day (1014.84 m). These distances encompass those reported for COTS (10.3 m d-1) and align with COTS nocturnal behaviour. Space utilisation distributions (UDs) revealed a mean (±SD) home range of 1179.40 ± 659.40 m2 (95% UD) and a core area of 195.68 ± 141.31 m2 (50% UD). Revealing the short-term movement patterns of this natural COTS predator within a reef environment advances knowledge of its spatial ecology and will provide information for its future conservation and for COTS management efforts.

Population irruptions of crown-of-thorns starfish (Acanthaster spp.) contribute greatly to the degradation of coral reefs throughout the Indo-Pacific. Effective management of these population irruptions is limited, in part, by incomplete knowledge of their early life history. Importantly, there are very limited data on the distribution and abundance of newly settled crown-of-thorns starfish (0 + starfish, in their first year since settlement). Extensive sampling was conducted around the circumference of three distinct mid-shelf reefs (at 1–18 m depths) in the central Great Barrier Reef (GBR), during active population irruptions, in May–June 2017, to quantify the occurrence and densities of settlement-stage starfish (2–65 mm diameter) and relate patterns of abundance to distinct habitat features at the scale of individual reefs. Overall, 140 settlement-stage starfish were detected across 1242 quadrats (1 m2). Settlement-stage starfish were recorded from 31 out of 42 sites (73.8%) at mean densities of 0–0.77 starfish m−2. Both estimated densities and the likelihood of occurrence of settlement-stage starfish within quadrats increased overall with the proportion of coral rubble (and dead intact corals), were greatest at intermediate depths (8–14 m), but decreased with the proportion of live hard coral. At the scale of individual reefs, settlement-stage starfish occurred most frequently in south-western and northern fore reef habitats. Our results suggest that settlement and/or early post-settlement survival of crown-of-thorns starfish is greatest in relatively shallow waters of obliquely exposed fore reef habitats where there is high cover of coral rubble. The specific occurrence of these habitat types (within spur and groove systems and rubble slips) provides an opportunity to concentrate searches and increase effective sampling of settlement-stage starfish, though these habitats are relatively widespread and unlikely to constrain the population replenishment or population irruptions of crown-of-thorns starfish on the GBR.

The estimation of larval dispersal on an ecological timescale is significant for conservation of marine species. In 2018, a semi-population outbreak of crown-of-thorns sea star, Acanthaster cf. solaris, was observed on a relatively isolated oceanic island, Ogasawara. The aim of this study was to assess whether this population outbreak was caused by large-scale larval recruitment (termed secondary outbreak) from the Kuroshio region. We estimated larval dispersal of the coral predator A. cf. solaris between the Kuroshio and Ogasawara regions using both population genomic analysis and simulation of oceanographic dispersal. Population genomic analysis revealed overall genetically homogenized patterns among Ogasawara and other Japanese populations, suggesting that the origin of the populations in the two regions is the same. In contrast, a simulation of 26-year oceanographic dispersal indicated that larvae are mostly self-seeded in Ogasawara populations and have difficulty reaching Ogasawara from the Kuroshio region within one generation. However, a connectivity matrix produced by the larval dispersal simulation assuming a Markov chain indicated gradual larval dispersal migration from the Kuroshio region to Ogasawara in a stepping-stone manner over multiple years. These results suggest that the 2018 outbreak was likely the result of self-seeding, including possible inbreeding (as evidenced by clonemate analysis), as large-scale larval dispersal from the Kurishio population to the Ogasawara population within one generation is unlikely. Instead, the population in Ogasawara is basically sustained by self-seedings, and the outbreak in 2018 was also most likely caused by successful self-seedings including possible inbreeding, as evidenced by clonemate analysis. This study also highlighted the importance of using both genomic and oceanographic methods to estimate larval dispersal, which provides significant insight into larval dispersal that occurs on ecological and evolutionary timescales.

Many predators reported to feed on crown-of-thorns starfish (CoTS, Acanthaster spp.) are generalist and opportunistic feeders. However, research into predation on CoTS tends to examine these predator–prey interactions in isolation, and it remains unknown whether many potential predators will prey on CoTS when other, potentially more palatable, food sources are available. Assessing predatory responses to changes in prey availability is critical for gauging the capacity of predators to regulate prey populations. Here, we explored prey preferences and tested for prey switching across nine species of planktivorous damselfish offered varying densities of Pacific CoTS (Acanthaster cf. solaris) larvae versus larvae of a common and co-occurring starfish, Linckia laevigata. Results show that planktivorous damselfishes will consume crown-of-thorns starfish larvae, even in the presence of alternative prey. Feeding responses varied among the nine planktivorous predators with five damselfishes (Acanthochromis polyacanthus, Amblyglyphidodon curacao, Dascyllus reticulatus, Pomacentrus amboinensis and Pomacentrus moluccensis) exhibiting increased consumption of Acanthaster larvae with increasing density, despite the presence of alternative prey. Moreover, Abudefduf sexfasciatus and P. amboinensis exhibited preference for larvae of A. cf. solaris over larvae of L. laevigata. Despite these predation patterns, prey switching between starfish larvae was not observed. These results add to a growing body of evidence which suggests that predators of the early life stages of A. cf. solaris could be important in regulating settlement and recruitment patterns of this starfish, especially at low, non-outbreak, densities.

The extreme reproductive potential of the crown-of-thorns starfish (CoTS) Acanthaster sp. is a key trait used to explain their population outbreaks. Despite their highly defended morphology, sublethal predation is prevalent, as documented here for 2 outbreak populations in the Great Barrier Reef: Davies Reef and Lynch's Reef (arm damage 73.0 and 58.3%, respectively). It is not known how this trauma affects reproductive potential as a quantitative change in gonad production. We investigated the variability in gonad weight in the arms of uninjured CoTS. For CoTS with injured arms, we assessed the impact of arm injury and regeneration on gonad development at the level of the whole individual and among arms. The gonad index (GI) was lower in CoTS with 3 or more injured arms on Davies Reef, but not Lynch's Reef. At the level of individual arms, arm damage (25-100% of individual arm lost at Davies Reef and 10-100% at Lynch's Reef) resulted in a lower gonad weight compared to intact arms. Arms regenerating following total arm loss had a lower gonad weight than intact arms, even for arms that were 50-74% (Davies Reef) and 75-99% regenerated (Lynch's Reef). For uninjured CoTS, we show strong support for GI estimates based on extrapolation from one arm, and that inclusion of more than 4 arms resulted in marginal reduction in error. It is clear that sublethal injury can have a significant effect on gonad production in CoTS, with implications for population control given the reproductive potential of this species.

Outbreaks of crown-of-thorns starfish (Acanthaster spp.) represent a major threat to coral reef ecosystems throughout the Indo-Pacific, and there is significant interest in whether no-take marine reserves could moderate the frequency or severity of outbreaks. Herein, we investigate whether the incidence and severity of sublethal injuries among juvenile Pacific crown-of-thorns starfish (Acanthaster cf. solaris, max diameter = 45 mm) differs between areas that are open versus closed to fishing, between microhabitats (i.e. dead coral substratum versus live coral) and with body size. The majority (180 out of 200) of juvenile starfish had conspicuous injuries, presumably caused by predation. The incidence of injuries in juvenile starfish was negatively related to body size, but links between body size and severity of injuries were only evident in individuals collected from dead coral microhabitats. Small (3 mm radius) starfish from dead coral microhabitats had injuries to 68.06% of arms, compared to 12.00% of arms in larger (12 mm radius) starfish from the same microhabitat. Juvenile starfish associated with dead coral habitats had a higher incidence (95 vs. 87% respectively) and severity (i.e. the percentage of injured arms; 21 vs. 6%) of injuries, compared to those associated with live corals. Interestingly, there was no difference in the incidence or severity of injuries between areas that are open versus closed to fishing. Our results show that small juvenile A. cf. solaris are extremely vulnerable to sublethal, if not lethal, predation, and predation risk declines as they grow and change their microhabitat. Predation during and immediately following settlement is, therefore, likely to have a major influence on population dynamics and ontogenetic changes in microhabitat use for A. cf. solaris.

Crown-of-thorns starfish (Acanthaster spp.) is reported to have exceptional reproductive capacity, but this has been largely inferred based on the overall weight of gonads (and mostly for females), and there are limited estimates of the concentration of gametes within gametogenic tissues. This study quantified gamete concentrations for both male and female Pacific crown-of-thorns starfish (Acanthaster cf. solaris), collected on Australia’s Great Barrier Reef in 2014–2019. Gamete concentrations varied greatly among female starfish (12,338–133,810 oocytes g−1), such that the estimated reproductive capacity ranged from 400-mm diameter approached 53 billion sperm. This study reaffirms that crown-of-thorns starfish have exceptional reproductive capacity, which is strongly size-dependent, but also varies greatly among individuals. Importantly, individual variation in reproductive output may be important in understanding for population irruptions of Acanthaster spp., and their concomitant effects on reef ecosystems.

Population outbreaks of the corallivorous crown-of-thorns seastar (CoTS; Acanthaster spp.) are significant threats to the Indo-Pacific reefs. Although recent research demonstrated that environmental DNA (eDNA) techniques could improve CoTS monitoring, the interpretation of surveillance results has been limited by uncertainties about eDNA dynamics. Here, we conducted aquarium experiments to identify biotic and abiotic factors affecting the shedding and degradation rates of CoTS eDNA. In the first experiment, we investigated the effect of two temperatures (24 and 28 °C) and three feeding treatments (no coral, coral accessible and coral inaccessible) on eDNA shedding rate of laboratory-raised 8-month-old juvenile CoTS. In the second experiment, we quantified CoTS eDNA degradation rate under three temperatures (24, 26 and 28 °C). We found that eDNA shedding rate was affected by feeding treatment (p

Coral cover has declined markedly in the recent past in many regions of the world, including the Great Barrier Reef (GBR), Australia. The major causes of this decline are generally considered to be mortality associated with large-scale severe disturbances (i.e. catastrophic mortality), such as Acanthaster planci outbreaks, cyclones and bleaching. However, background rates of mortality (i.e. not associated with catastrophic disturbance), are rarely quantified, but without these it is difficult to assess the relative importance of these 2 types of mortality (catastrophic and background). We quantified spatial and temporal variation in catastrophic and background whole-colony mortality of the common reef coral Acropora millepora over 24 mo at 2 sites in 3 regions separated by 700 km along the GBR. The study period included 2 cyclones and a flood. Overall mortality rates were exceptionally high. Of 180 colonies tagged in April 2009, only 36 (20%) were alive in April 2011, and 68% of this mortality occurred in intervals following the 3 large disturbances. Background mortality rates were also high in the Palm Islands, where they approached 40% yr-1 compared to

We thank John Lucas, Peter Moran and the late Masashi Yamaguchi for their help with and encouragement of this work. We also thank Russ Babcock and 3 anonymous reviewers for critical review of the manuscript and Carina Cartwright for her assistance in the field and in the pain staking microscope work involved in this study. Funding for the original study was provided by the Crown of Thorns Starfish Research Committee of the Great Barrier Reef Marine Park Authority, Townsville, Australia.

Declines in coral cover across the Indo-Pacific are associated with multiple stressors of global ocean change, and this is exacerbated by population outbreaks of the coral-eating crown of thorns starfish (COTS), Acanthaster spp. We investigated the growth and feeding responses of the corallivorous juvenile stage of COTS and their coral food, Acropora spp., to near-future ocean warming and acidification. Juvenile COTS and Acropora spp. were exposed to three temperatures (26, 28, 30 °C) and three pH levels (NIST scale: 8.0, 7.8, 7.6) in a flow-through cross-factorial experiment. Increased temperature and acidification had independent positive effects on growth and feeding of juvenile COTS. They grew faster and were larger under these conditions, but weighed less at low pH. Accelerated growth of juvenile COTS under ocean acidification conditions was decoupled from skeletogenesis with increased size largely due to development of soft tissue. Juveniles reared at 30 °C also had the greatest number of spines. To assess the recovery of Acropora spp. following sublethal COTS predation in warm and acidified conditions, corals that had been preyed on by the juveniles for 1 week were removed and held in experimental conditions to monitor their growth and survival. The stress from warming and predation had an interactive effect in reduction in coral growth. Mortality of corals was highest following predation and in the 30 °C treatments. There was no direct effect of acidification on growth or survival of Acropora spp. following predation. It appears that COTS juveniles may benefit from near-future warming and acidification. Thus, changing climate may increase the threat of this predatory starfish to coral reefs.

Large-scale population outbreaks of the crown-of-thorns starfish (COTS, Acanthaster spp.) have occurred in every nation with substantial coral reefs in the Indo-Pacific during the past few decades. While a multitude of control efforts have been trialed to combat the outbreaks, an efficient solution has remained elusive. Recent developments have however introduced single-shot injection methods using bovine derivatives (bile salts) and household products (vinegar) that have proven effective at culling COTS. In this study we use a Multiple-Before-After-Control-Impact (MBACI) experimental design to conduct an assessment of the environmental impact of using household vinegar as a chemical for culling COTS. During our six-week assessment we found no changes in coral cover or coral disease. Furthermore, while video recordings documented a number of fish species consuming decaying COTS, we found no evidence of changes in fish abundance or the presence of fish disease, despite densities of injected COTS far exceeding those recorded during natural outbreaks. The efficiency of scavenging fish ensured that few traces of decaying COTS were visible 72 h post-injection. The lack of any observed environmental impact supports our conclusion that vinegar is a low-risk chemical for use in COTS control. Because vinegar is highly accessible, low in cost, and effective at killing COTS without harm to non-target organisms, it has the potential to become a powerful new tool to defend the reef against COTS outbreaks.

We investigated how density and aggregation influence crown-of-Thorns starfish Acanthaster planci reproductive success, using an empirically-Tuned, individual-based simulation model that incorporates spatial and temporal biological stochasticity associated with spawning, and a kinetics model of fertilisation that explicitly incorporates the probability of polyspermy. Greater aggregation of individuals relieved Allee fertilisation dynamics, particularly at low densities, leading to higher rates of successful monospermic fertilisation and allowing populations to produce many more zygotes. This is likely more important to smaller, rather than larger, populations, due to limited and more variable reproductive success. In higher density populations a fertilisation optimum was observed at moderate levels of aggregation, above which monospermic fertilisation plateaued or even declined. This was likely due to 2 factors: The spatial dynamics of gamete plume dispersal and polyspermic fertilisation. Comparison of in situ natural spawning aggregation with model results indicates a cost-benefit equilibrium may exist between aggregation and reproductive success, and that relief from mechanisms limiting aggregation (for example, decreased relative predator abundance) may permit increased aggregation resulting in greater fertilisation and zygote production. We propose an Allee threshold of 3 starfish ha-1 (for starfish of a mean diameter 345 mm), below which reproductive capacity is greatly reduced regardless of aggregation level. These preliminary findings posit aggregation as a key factor in outbreak formation that may feasibly be incorporated into preventative management strategies to detect and define incipient outbreak conditions and to mitigate subsequent risk.

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 (0.1-5.0μgchl-aL-1), including nominal threshold levels for disproportionate larval success (≥1.0μgchl-aL-1). High success to the juvenile occurred across an order of magnitude of chl-a concentrations (0.5-5.0μgchl-aL-1), suggesting there may not be a narrow value for optimal success. Oligotrophic conditions (0.1μgchl-aL-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.

The authors are particularly grateful to Richard Farman, director ofthe Aquarium des Lagons in Noumea, for hosting our experiments. Wealso wish to express our warmest thanks to the aquarium staff for theirlogistical support, in particular Sylvain Govan and Jeff Dubosc.; International audience; Outbreaks of the coral-eating crown-of-thorns starfish Acanthaster spp. (COTS) have become to be amongst the most severe threats to coral reefs worldwide. Although most research has focused on COTS early development, it remains unclear how COTS populations will keep pace with changing ocean conditions. Since reproduction is a key process contributing to outbreaks, we investigated the reproductive success of adult COTS acclimated for 3-4 months to different treatment combinations of ambient conditions, ocean warming (+2 • C) and acidification (− 0.35 pH). Our results suggest that the optimal breeding season in New Caledonia is concentrated around the end of the calendar year, when water temperature reaches >26 • C. We found negative effects of temperature on egg metrics, fertilisation success, and GSI, conflicting with previously documented effects of temperature on echinoderm reproductive outputs. Fertilisation success dropped drastically (more than threefold) with elevated temperature during the late breeding season. In contrast, we detected no effects of near-future acidification conditions on fertilisation success nor GSI. This is the first time that COTS reproduction is compared among individuals acclimated to different conditions of warming and acidification. Our results highlight the importance of accounting for adult exposure to better understand how COTS reproduction may be impacted in the face of global change.

Using monitoring data from 1998 to 2017, we examined the spatiotemporal pattern of disturbances that occurred in Sekisei Lagoon, surrounded by Iriomote and Ishigaki islands in Okinawa, Japan. Outbreaks of crown- of-thorns starfish ( Acanthaster cf. solaris ), coral bleaching, typhoon damage, snail predation, and occurrences of growth anomalies, black band disease, and white syndrome were investigated using generalized linear models. Likelihood ratio test indicated that the type of coral assemblage, topographic features, and geographic range of research sites affected the occurrence of each disturbance. The sites dominated by Acropora spp. and the sites located in Sekisei Lagoon suffered recurrent and multiple disturbances. The impacts of disturbances on coral cover were evaluated using generalized additive models. Model averaging using Akaike information criteria revealed that a loss of coral cover occurred even when only a few A. cf. solaris were present, when the bleaching rate was sufficiently high, or when typhoon damage was severe. Our study describes spatiotemporal patterns of disturbances over a 20-yr period and reveals their relative impacts on coral cover. Our findings may be useful in predicting the responses of coral communities to future climate change scenarios.

Outbreaks of crown-of-thorns seastars (CoTS;Acanthasterspp.) are a major contributor to degradation of Indo-Pacific coral reefs. Understanding the dispersal and fate of planktonic life stages is crucial to understand and manage outbreaks, but visual detection of CoTS larvae is challenging. We apply a quantitative PCR (qPCR) assay to enumerate CoTS larvae in a 3-year time series of plankton samples from two reefs (Agincourt and Moore Reefs) on the Great Barrier Reef. Plankton surveys were complemented with settlement assays, and benthic surveys of juvenile and adult densities over time. Only one out of 109 plankton samples from Agincourt Reef had detectable CoTS mtDNA compared to 41 out of 575 samples from Moore Reef. This may be explained by differences in adult densities, or differences in connectivity and larval retention. Detections of larval CoTS were restricted to summer (November–February), with first detections each year coinciding with water temperatures reaching 28 °C and peak detections late December. A disproportionate number of larval detections occurred in 7 days around full moon. Complementary sampling of settlement and post-settlement life stages confirmed that elevated densities of CoTS larvae at Moore Reef translated to high rates of settlement adding to infestations at this reef. Moreover, there were declines in the detection of larvae, as well densities of juvenile and adult CoTS at Moore Reef, in 2017 and 2018. This study demonstrates that qPCR for genetic identification and quantification of larvae can assist to elucidate life history parameters of nuisance species difficult to obtain with other tools.

High densities of the corallivorous crown-of-thorns starfish (CoTS; Acanthaster sp.) have occurred throughout the Indo-Pacific often resulting in widespread coral loss. Whilst CoTS have previously been recorded at Barrow and the Montebello Islands, in the Pilbara offshore bioregion of northwestern Australia, their densities were relatively low. Outbreak densities of CoTS have been described as the level at which the rate of coral consumption by the starfish is equal to or greater than rate at which the coral grows. In 2014, we recorded densities as high as 320 ± 58 CoTS ha−1 in the region which is well above recognised outbreak densities. Whilst there is little terrestrial runoff and agriculture in the Pilbara region, both temperature and chlorophyll-α levels appear to be sufficient to allow a high degree of CoTS larval success in most years. The region was subjected to anomalously high water temperatures during the summers of 2010–2011 and particularly 2012–2013 which resulted in the mortality of almost 70% of live coral. We hypothesise that the high densities of CoTS observed are a result of CoTS responding to a reduced food supply and aggregating around the remaining live coral resulting in outbreak densities rather than a significant increase in the number of CoTS in the area. The small amount of remaining live coral is concentrated in a few areas and this, combined with high densities of CoTS in these areas, suggest that CoTS represent a significant threat to the recovery of the coral communities of the region.

Successive, extensive bleaching events have reduced coral cover and altered species assemblages significantly in the Montebello and Barrow Islands in north-western Australia. In particular, acroporid and pocilloporid populations were dramatically reduced, while poritid and faviid corals were less impacted and now dominate coral assemblages. Subsequent to this perturbation, there have been changes in the abundance and distribution of the coral eating crown-of-thorns starfish, Acanthaster cf. solaris (to densities of up to 320 ha−1) that have had a further impact on coral communities selectively targeting the coral taxa most susceptible to bleaching, often referred to as climate change “losers”. The feeding selectivity of A. cf. solaris under post-bleaching conditions was studied to determine its likely impact on coral communities and coral recovery. We found that even when Acropora and Pocillopora were extremely rare, A. cf. solaris demonstrated a high level of selectivity for these genera, although they consumed non-preferred taxa such as poritid, faviid, and merulinid corals in greater numbers. The consumption of non-preferred genera and families of corals differed between sites and was influenced by their local abundance. This study demonstrates that where crown-of-thorns starfish populations greatly exceed outbreak threshold densities of 10–15 ha−1, they are likely to impact recovery of coral communities from bleaching events and exacerbate bleaching induced changes in assemblage structure.

Coral-eating crown-of-thorns starfish (Acanthaster sp.) often exhibit dramatic population outbreaks, suggesting that their local abundance may be relatively unchecked by predators. This may be due to high concentrations of anti-predator chemicals (saponins and plancitoxins), but the effectiveness of chemical deterrents in protecting Acanthaster sp., especially spawned eggs, from predation remains controversial. We show that planktivorous damselfishes will readily consume food pellets with low proportions (≤80%) of eggs of crown-of-thorns starfish. However, all fishes exhibited increasing rejection of food pellets with higher proportions of starfish eggs, suggesting that chemicals in eggs of crown-of-thorns starfish do deter potential predators. Interestingly, palatability thresholds varied greatly among the nine species of planktivorous fish tested. Most notably, Amblyglyphidodon curacao consumed food pellets comprising 100% starfish eggs 1.5 times more than any other fish species, and appeared largely insensitive to increases in the concentration of starfish eggs. After standardising for size, smaller fish species consumed a disproportionate amount of pellets comprising high proportions of starfish eggs, indicating that abundant small-bodied fishes could be particularly important in regulating larval abundance and settlement success of crown-of-thorns starfish. Collectively, this study shows that reef fishes vary in their tolerance to anti-predator chemicals in crown-of-thorns starfish and may represent important predators on early life-history stages.

Population outbreaks of the coral-eating predator crown of thorns starfish (COTS), Acanthaster planci are responsible for large-scale disturbance of coral reefs throughout the Indo-Pacific. In response, attempts are often made to control COTS outbreaks in protected areas. For instance, volunteers remove thousands of sea stars every year in Malaysia. This study reports the status of the COTS population in the Pulau Tioman Marine Park and examines the effectiveness of the seasonal sea star removal programme. After the 2009 removal season, we monitored COTS densities and coral assemblages before and after a 6-month no-removal season at sites with and without COTS removal efforts. We recorded high COTS densities up to 330 ind. ha−1 at a few sites independent of removal effort. In fact, removal only temporarily reduced large individuals from local populations. Moreover, after the no-removal season, sites with COTS removal had increased live coral cover, but sites without COTS removal had a drastic decrease in live coral cover, with Acropora spp. being most affected. Therefore, this study suggests that the current seasonal removals could promote coral health, despite the high density of COTS.

Examining the functional response of predators can provide insight into the role of predation in structuring prey populations and ecological communities. This study explored feeding behaviour and functional responses of planktivorous damselfishes when offered captive reared larvae of crown-of-thorns starfish, Acanthaster sp., with the aim of determining whether these predators could ever play a role in moderating outbreaks of Acanthaster sp. We examined predatory behaviour of 11 species of planktivorous damselfish, testing: (1) the relationship between predator size and predation rate, both within and among fish species; (2) consumption rates on larvae of Acanthaster sp. versus larvae of a common, co-occurring coral reef asteroid Linckia laevigata; (3) maximal feeding rates upon both Acanthaster sp. and L. laevigata; and (4) functional responses of planktivorous fishes to increasing densities of Acanthaster sp. Consumption rates of crown-of-thorns larvae by damselfishes were independent of predator size; however, there was a significant negative relationship between predator size and consumption rate of L. laevigata, when pooling across all predatory species. Some damselfishes, including Acanthochromis polyacanthus and Amblyglyphidodon curacao, consumed larval Acanthaster sp. at a greater rate than for L. laevigata. Most predatory species (all except A. curacao and Pomacentrus amboinensis) exhibited a Type II functional response whereby the increasing feeding rate decelerated with increasing prey density. In addition to revealing that a wide range of planktivorous fishes can prey upon larvae of Acanthaster sp., these data suggest that planktivorous damselfishes may have the capacity to buffer against population fluctuations of Acanthaster sp. Importantly, predators with Type II functional responses often contribute to stability of prey populations, though planktivorous fishes may be swamped by an abnormally high influx of larvae, potentially contributing to the characteristic population fluctuations of Acanthaster sp.

Population outbreaks of the corallivorous crown-of-thorns starfish, Acanthaster planci, are a major contributor to the decline in coral reef across the Indo-Pacific. The success of A. planci and other reef species in a changing ocean will be influenced by juvenile performance because the naturally high mortality experienced at this sensitive life history stage maybe exacerbated by ocean warming and acidification. We investigated the effects of increased temperature and acidification on growth of newly metamorphosed juvenile A. planci and their feeding rates on crustose coralline algae (CCA) during the initial herbivorous phase of their life history. The juveniles were exposed to three temperature (26, 28, 30 °C) and three pH (NIST scale: 8.1, 7.8, 7.6) levels in a flow-through cross-factorial experiment. There were positive but independent effects of warming and acidification on juvenile growth and feeding. Early juveniles were highly tolerant to moderate increases in temperature (+2 °C above ambient) with the highest growth at 30 °C. Growth and feeding rates of A. planci on CCA were highest at pH 7.6. Thus, ocean warming and acidification may enhance the success of A. planci juveniles. In contrast to its coral prey, at this vulnerable developmental stage, A. planci appears to be highly resilient to future ocean change. Success of juveniles in a future ocean may have carry-over effects into the coral-eating life stage, increasing the threat to coral reef systems.

Outbreaks of the predator crown-of-thorns starfish Acanthaster planci cause widespread coral mortality throughout the Indo-Pacific. Identifying mediating processes that promote coral survival during Acanthaster outbreaks can help support reef resilience. During surveys in French Polynesia, large colonies of the calcifying hydrozoan Millepora platyphylla appeared to be avoided by the starfish, providing predator-free sanctuaries to scleractinian corals in their proximity during an intense Acanthaster outbreak. Such refuges particularly benefited susceptible taxa such as Acropora and Pocillopora, whose populations were normally targeted by the starfish. These biological sanctuaries provided by Millepora to scleractinians are newly identified forms of associational refuges that can increase coral resistance to predation by Acanthaster. Our observations thus identify a key ecological role in reef resilience for Millepora, which is a ubiquitous component of tropical reefs usually seen as a strong competitor of scleractinians.

Nha Trang Bay (NTB), located off the south-east coast of Vietnam, is a famous tropical tourist site. In the past, the bay was characterised by flourishing coral reefs but, by the end of 2019, more than 90% of the corals were gone. In addition to chronic and complex anthropogenic effects, there was an extensive outbreak of crown-of-thorns starfish (Acanthaster planci), the main natural predator of reef-building corals in the Indo-Pacific, among the remnant coral communities in NTB. By 2019, the mean abundance of A. planci in NTB reached 4.2 starfish per 100m2. Coral surveys conducted on 10 target sites over a 3-year period ending in 2019 revealed that mean coral cover decreased by 64.4% (coral loss varied from 43 to 95%). The greatest declines were in Acropora and Montipora, with 80.6 and 82.3% reductions in the coverage of these taxa respectively. The combination of nutrient enrichment and predator removal is thought to determine the extent of the outbreak of A. planci in the bay. The present trends in environmental conditions in NTB do not indicate a positive outcome for coral reef recovery in in this area in the near future.

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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, perhaps, other tropical marine larvae in oligotrophic seas.

Numerous hypotheses have been put forward to account for population outbreaks of crown-of-thorns starfishes (CoTS, Acanthaster spp.), which place specific importance on either pre- or post-settlement mechanisms. The purpose of this review is to specifically assess the contributions of pre- versus post-settlement processes in the population dynamics of CoTS. Given the immense reproductive potential of CoTS (>100 million eggs per female), persistent high densities would appear inevitable unless there were significant constraints on larval development, settlement success, and/or early post-settlement growth and survival. In terms of population constraints, pre- and post-settlement processes are both important and have additive effects to suppress densities of juvenile and adult CoTS within reef ecosystems. It is difficult, however, to assess the relative contributions of pre- versus post-settlement processes to population outbreaks, especially given limited data on settlement rates, as well as early post-settlement growth and mortality. Prioritising this research is important to resolve potential effects of anthropogenic activities (e.g., fishing) and habitat degradation on changing population dynamics of CoTS, and will also improve management effectiveness.

The reefs surrounding the island of Moorea, French Polynesia, experienced two large pulse perturbations between 2008 and 2010, an outbreak of the crown-of-thorns seastar (Acanthaster planci) followed by a cyclone, that resulted in the reduction in live coral cover on the fore reef from ~40 to

In this paper, we postulate a unique environmental triggering sequence for primary outbreaks of crown-of-thorns starfish (COTS, Acanthaster planci) on the central Great Barrier Reef (GBR, Australia). Notably, we extend the previous terrestrial runoff hypothesis, viz. nutrient-enriched terrestrial runoff → elevated phytoplankton 'bloom' concentrations → enhanced COTS larval survival, to include the additional importance of strong larvae retention around reefs or within reef groups (clusters) that share enhanced phytoplankton concentrations. For the central GBR, this scenario is shown to occur when El Niño-Southern Oscillation (ENSO) linked hydrodynamic conditions cause the 'regional' larval connectivity network to fragment into smaller 'local' reef clusters due to low ocean current velocities. As inter-annual variations in hydrodynamic circulation patterns are not amenable to direct management intervention, the ability to reduce the future frequency of COTS outbreaks on the central GBR is shown to be contingent on reducing terrestrial bioavailable nutrient loads ~20-40%.

We compared spatial variability in re- cruitment patterns between acroporid corals and the predatory crown-of-thorns starfish Acanthaster planci along the Onna village coast of Okinawa Island, Japan. Our objective was to understand population dynamics of both predator and prey. To quantify re- cruitment, we established artificial recruitment plates for acroporid corals and conducted quadrat surveys for juvenile A. planci at 7 sites. We also examined dis- tributional patterns of adult populations of both spe- cies and observed surface currents during larval dis- persal periods using GPS-equipped surface drifters. Acroporid recruits typically arrived in greater num- bers at more northern sites. Despite high variability, A. planci recruits displayed similar spatial patterns. Drifter trajectories suggest that these spatial recruit- ment patterns are probably driven by near-shore currents in the summer. Moreover, current data imply a complicated metapopulation relationship in this region, especially for A. planci, new recruits of which may mainly originate outside of the Onna area. Meta - population dynamics within the study area and among surrounding regions must be explained by the com- plex surface current circulation, because simpler ex- planations involving only the Kuroshio Current do not adequately describe the relationship between these populations.

Many echinoderm larvae exhibit phenotypic plasticity: a change in phenotype in response to environmental food levels. We investigated phenotypic plasticity in the larvae of the crown-of-thorns seastar Acanthaster planci, an opportunistic boom-and-bust species with larvae that have a strong response to food conditions. The increased predation pressure resulting from outbreaks (population explosions) of A. planci is deleterious to coral reefs, but the link between population outbreaks and larval ecology is poorly understood. We hypothesised that the larvae of A. planci would have a different morphological profile in the oligotrophic conditions typical of tropical waters than in the eutrophic conditions associated with increased nutrients. We predicted that larvae reared in low food conditions would increase their ciliated band length to enhance feeding potential. Larvae were fed algal concentrations representing starvation (0 cells ml-1), low food (oligotrophic; 1000 cells ml-1), high food (eutrophic; 10000 cells ml-1) or excessive food (100000 cells ml-1) conditions. A phenotypic response was evident. Larvae in the 2 high food treatments had a shorter ciliated band length relative to body size. Conversely, larvae in the starvation and low food treatments had longer ciliated bands relative to body size, a change that would enhance particle capture capacity and facilitate larval success. This plastic response of the larvae of A. planci could have flow-on effects to adult populations, potentially facilitating population outbreaks.

Outbreaks of the destructive coral-eating crown-of-thorns starfish, Acanthaster planci, present a considerable threat to coral reefs worldwide, and mitigating their impact has proven challenging. The most effective methods to control A. planci require injecting individual starfish with lethal chemicals. While some of these are highly effective, their administration often requires permits, training and access to specialised equipment. We aimed to identify a widely available and highly efficient alternative. We discovered that common household vinegar is lethal to A. planci individuals when injected at the base of one their arms. A single injection of 25 ml vinegar induced functional mortality in

Outbreaks of the crown-of-thorns starfish (COTS), Acanthaster planci, have occurred at many locations throughout the Indo-Pacific and are a major contributor to widespread coral loss and reef degradation. The causes of outbreaks remain controversial, but are commonly attributed to anthropogenically elevated nutrients and/or over-fishing. If so, it seems unlikely that outbreaks would occur in reef systems that are largely isolated from anthropogenic disturbances. However, high densities of COTS were recently observed on reefs in the Chagos Archipelago, a remote group of atolls and banks within the central Indian Ocean, which experience very limited anthropogenic influence. Aggregations of COTS were first noticed at Eagle Island in 2012, which, although unquantified, appeared to be at outbreak levels, and very high densities (1624 km−2) were subsequently recorded at Danger Island in 2013. While these islands are uninhabited by humans, it is possible that nutrient inputs result from upwelling zones around the Archipelago, or high densities of breeding seabirds. Among islands within the Great Chagos Bank, densities of the red-footed booby Sula sula ranged from 8 to 7888 individuals km−2, with associated guano input ranging from 96 to 25,381 kg island−1 year−1. However, Danger and Eagle Islands where high COTS densities were recorded, had both high and low levels of guano production, respectively, which suggests that outbreaks may not be directly linked to guano nutrient enrichment. Other factors which might be responsible for intermittent COTS outbreaks should be considered in isolated reef systems such as the Chagos Archipelago.

Coral reefs are under threat from a variety of sources including the corallivorous seastar, Acanthaster cf. solaris. (Crown of Thorns Seastar; CoTS). Outbreak prediction is a strategic component of managing the impact of this boom and bust species. Details on the fate and dispersal of planktonic life stages are limited, with CoTS larval stages indistinct morphologically from many other asteroid larvae. Given the similarity of many larvae, quantification of marine larvae stages is a major challenge for marine ecologists. We describe a quantitative polymerase chain reaction (qPCR) assay that enables the enumeration of CoTS larvae in field collected plankton samples. Specific primers for the mitochondrial cytochrome oxidase subunit 1 gene (mtCOI) were developed and validated for specificity and sensitivity. Larval culture experiments with CoTS allowed us to determine the mtCOI copy number per larval stage which aided in relating copy numbers in the plankton to actual larval densities. We found the mtCOI copy number varied 3.6-fold across all CoTS planktonic life stages from unfertilised oocyte to competent brachiolaria and this variation was taken into account when determining CoTS larval densities in field samples on the Great Barrier Reef (GBR). CoTS larvae were detected at many locations in the CoTS ‘initiation box’ on the GBR between Cairns and Lizard Island in December 2014 with the highest mean CoTS larval density at 36.9 (23.7–84.3) CoTS larvae m3 between Rudder and Tongue Reef (16.233°S, 145.643°E). Field negative samples taken outside spawning season along with DNA extraction recovery experiments confirmed our sampling, extraction and assay methods were robust. This method will greatly help further studies on understanding CoTS larval ecology and outbreaks, with methods developed also important tools for other ecologically and commercially important marine species.

Given the threat posed by population outbreaks of Acanthaster planci to coral reefs throughout the Indo-Pacific, significant investment is being made to reduce the number of sea stars and their effects on coral assemblages, both through ongoing direct control programs and indirectly, through targeted improvements in water quality and fisheries management. In Australia, bile salts have recently replaced sodium bisulfate as the chemical used to inject, and thereby quickly and efficiently kill, individual sea stars. This study reports on results of experimental studies conducted prior operationalizing bile salts for widespread use on Australia's Great Barrier Reef, both to optimize doses of bile salts and further examine potential side-effects of administering low doses of bile salts into individual sea stars when found at high concentrations. This study showed that injecting A. planci with 10 mL of 8 g l−1 Bile Salts No. 3 or 12 g l−1 of Oxgall solution into the base of the arm with a new gun adapted with a 16 Gauge x1/2” needle is the most rapid and effective way to kill individual A. planci, which were up to 42 cm in diameter. No immediate flow-on effects on reef fish, corals, and other benthic invertebrates were observed in laboratory experiments and field surveys. Efficient control measures using bile derivatives can offer immediate relief from ongoing COTS predation, and when done in conjunction with improved land use practices that reduce nutrient input and establishment of protected areas to protect predator species, can offer benefits for the resilience of reef ecosystems.

Boom and bust dynamics are a prominent characteristic of the crown-of-thorns starfish (Acanthaster planci) populations. Outbreaking starfish populations damage coral communities extensively and then often disappear quickly. In contrast, Onna village, on the central west coast of Okinawa Island, Japan, has been experiencing chronically high population densities of A. planci for decades. In this study, we examined spatial and temporal population dynamics of A. planci along the coast of Onna village, using removal data gathered over 24 years by a local fisheries cooperative. The results demonstrate that A. planci populations along the Onna coast often experienced high densities prior to 2003 and that recently, populations have again been increasing drastically. Size-frequency distributions suggest multiple, successive recruitment, as one of the driving factors maintaining A. planci populations in this location. Moreover, within the Onna area, the timing of outbreak peaks varied among different locations, showing a northward progression.

Brown band (BrB) disease manifests on corals as a ciliate-dominated lesion that typically progresses rapidly causing extensive mortality, but it is unclear whe- ther the dominant ciliate Porpostoma guamense is a pri- mary or an opportunistic pathogen, the latter taking advantage of compromised coral tissue or depressed host resistance. In this study, manipulative aquarium-based experiments were used to investigate the role of P. gua- mense as a pathogen when inoculated onto fragments of the coral Acropora hyacinthus that were either healthy, preyed on by Acanthaster planci (crown-of-thorns starfish; COTS), or experimentally injured. Following ciliate inoc- ulation, BrB lesions developed on all of COTS-predated fragments (n = 9 fragments) and progressed up to 4.6 ± 0.3 cm d -1 , resulting in *70 % of coral tissue loss after 4 d. Similarly, BrB lesions developed rapidly on experimentally injured corals and *38 % of coral tissue area was lost 60 h after inoculation. In contrast, no BrB lesions were observed on healthy corals following experi- mental inoculations. A choice experiment demonstrated that ciliates are strongly attracted to physically injured corals, with over 55 % of inoculated ciliates migrating to injured corals and forming distinct lesions, whereas ciliates did not migrate to healthy corals. Our results indicate that ciliates characteristic of BrB disease are opportunistic pathogens that rapidly migrate to and colonise compro- mised coral tissue, leading to rapid coral mortality, par- ticularly following predation or injury. Predicted increases in tropical storms, cyclones, and COTS outbreaks are likely to increase the incidence of coral injury in the near future, promoting BrB disease and further contributing to declines in coral cover.

Edge-of-range populations are often typified by patterns of low genetic diversity and high genetic differentiation relative to populations within the core of a species range. The "core-periphery hypothesis", also known as the "central-marginal hypothesis", predicts that these genetic patterns at the edge-of-range are a consequence of reduced population size and connectivity toward a species range periphery. It is unclear, however, how these expectations relate to high dispersal marine species that can conceivably maintain high abundance and high connectivity at their range edge. In the present study, we characterize the genetic patterns of two tropical echinoderm populations in the Kermadec Islands, the edge of their southwest Pacific range, and compare these genetic patterns to those from populations throughout their east Indian and Pacific ranges. We find that the populations of both Acanthaster planci (Linnaeus, 1758) and Tripneustes gratilla (Linnaeus, 1758) are represented by a single haplotype at the Kermadec Islands (based on mitochondrial cytochrome oxidase C subunit I). Such low genetic diversity concurs with the expectations of the "core-periphery hypothesis". Furthermore, the haplotypic composition of both populations suggests they have been founded by a small number of colonists with little subsequent immigration. Thus, local reproduction and self-recruitment appear to maintain these populations despite the ecologically marginal conditions of the Kermadec Islands for these tropical species. Understanding rates of self-recruitment vs reliance on connectivity with populations outside of the Kermadec Islands has implications for the persistence of these populations and range stability of these echinoderm species.

Effective control of outbreaks of Acanthaster planci represents the most immediate and practical intervention to reverse sustained declines in coral cover on reefs in the Indo-Pacific. This study explored the minimum doses of oxbile, oxgall, and thiosulfate-citrate-bile-sucrose agar (TCBS) that result in reliable and comprehensive mortality when injected into adult A. planci. The minimum doses required to induce 100% mortality among starfish (n=10) were 4 g l(-1) of oxbile, 8 g l(-1) of oxgall and 22 g l(-1) of TCBS. Moreover, there was no evidence of unintended side effects for other coral reef organisms (e.g., scleractinian corals, echinoderms and fishes) when using oxbile, oxgall, or TCBS at minimum doses. The effectiveness of peptones in killing crown-of-thorns starfish was also tested, but inconsistency in the results revealed that these proteins are unreliable.

The corallivorous crown-of-thorns starfish (COTS), Acanthaster planci, is recognised as a major cause of coral reef degradation throughout much of the Pacific Ocean. However, the effects of COTS on the high diversity reefs in Indonesia have been largely overlooked. In 2007, high densities of COTS were observed in two regions of Indonesia: Aceh and Halmahera. Densities of COTS ranged from 0 to 52 starfish 2,000 m2 across 24 sites in Aceh and from 0 to 18 starfish 2,000 m2 at 10 sites in Halmahera. Mortality rates of Acropora spp. were very high at affected sites: over 50 % of colonies had been killed at seven of the 16 affected sites. A review of historical sources going back to 1969 suggests that COTS have damaged many reefs throughout Indonesia, including much activity within the Indonesian section of the Coral Triangle. Furthermore, the data suggest that COTS activity has increased rapidly since 2000. Very little of this activity has been reported in the primary literature, and there is a general lack of awareness in Indonesia of COTS as a potential cause of reef degradation. This lack of awareness, combined with limited monitoring efforts, means that damage caused by COTS is often attributed to other causes, such as destructive fishing, bleaching or tsunami. COTS are clearly a major source of coral mortality in Indonesia of which scientists and government need to be more cognizant.

Previous studies of the population genetic structure of the corallivorous crown-of-thorns starfish (COTS) Acanthaster planci in the Pacific Ocean showed high levels of gene flow that were assumed to reflect a high dispersal potential. However, the phylogeographic analyses of the Pacific crown-of-thorns starfish species of this study, using the highly variable mitochondrial control region and the most complete geographic coverage to date, contradict this view. Results show high levels of overall genetic structure (ΦST = 0.198), suggesting a complex history of range restrictions and expansions, a pattern that we hypothesize results from changes in topography and oceanography associated with sea-level changes. However, results also show signatures of ongoing gene flow between populations isolated in the past and high levels of genetic connectivity even among distant populations. Combined, these results indicate that while there are significant limits to genetic exchange among populations among Pacific Ocean populations of the crown-of-thorns starfish, the high larval dispersal potential of this species is often achieved as well.

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.

Thiosulfate–citrate–bile–sucrose agar (TCBS) has been shown to induce pathogenesis in the crown-of-thorns starfish (COTS), Acanthaster planci, providing potentially novel options to control COTS outbreaks, but the mechanistic basis for this effect is unknown. This study explores reactions of COTS to individual ingredients of TCBS, testing for allergic reactions versus pathogenesis. Four out of nine TCBS chemical ingredients tested induced allergic reactions and death in injected COTS. Peptone 10 g l− 1 and oxgall 8 g l− 1 induced 100% mortality, while yeast extract and agar induced death in 40% and 20% of COTS, respectively, 48 h after injection. Peptone was evaluated at three different concentrations (10 g, 5 g, and 1 g l− 1). Peptone 10 g l− 1 induced 100% mortality, peptone 5 g l− 1 killed 60% of injected COTS, and peptone 1 g l− 1 induced death in only 20% of challenged COTS, indicating that toxicity of peptone is concentration-dependent. Sodium citrate induced moderate mucus production in all COTS challenged, but disease did not progress and COTS completely recovered after 52 h. The remaining ingredients in TCBS did not produce any kind of clinical signs of disease. Peptone, oxgall, and yeast are potentially useful in controlling outbreaks because these protein extracts can be safer to use compared to previously used noxious chemicals. In addition, lowered concentrations are required to kill COTS, therefore increasing efficiency by saving time, money, and effort in COTS control programs.

Biotic disturbances are important drivers of community structure, but interactions among community members can determine trajectories of response and recovery. On coral reefs in French Polynesia, epibiotic amphipods induce the formation of branch-like “fingers” on flat colonies of encrusting Montipora coral. The fingers form as coral encrusts the amphipods’ tubes and lead to significant changes in colony morphology. I tested whether the induced morphological changes affect Montipora’s susceptibility to predation by pincushion (Culcita novaeguineae) and crown-of-thorns sea stars (Acanthaster planci). Montipora with fingers were less likely to be attacked and more likely to survive attack than colonies without fingers. Furthermore, the presence of fingers altered A. planci prey preference. Sea stars preferred Montipora without fingers over other common coral genera, but preferred other genera when Montipora had fingers. Amphipods indirectly affected Montipora’s resistance and resilience to predation, and the susceptibility of other coral genera to predation, through induced morphological changes. Such trait-mediated indirect interactions likely play an important role in determining how species respond to periodic sea star outbreaks.

The submerged coral reefs of Brunei, little-impacted by human activity and characterized by high live coral cover, have no recorded history in recent decades of the presence of the crown-of-thorns (COT), Acanthaster planci. This sea star, first recorded on Brunei reefs in 2008, attained outbreak densities in 2010. At Littledale Shoal its impact on corals at permanent transect sites has been quantified; mean live coral cover reduced by half from 2006 to 2010 due predominantly to predation. Line intersect transect data confirm a predisposition for tabular Acropora species, a prominent feature at this site, although other scleractinian taxa were also predated. Other regional outbreaks are reviewed, including episodes, and their timing, within the neighbouring Coral Triangle (CT). Mounting evidence implicates nutrient-enhanced increases in primary production as a primary cause of COT outbreaks. However, this stands in contrast with a report of global oceanic phytoplankton decline in the past century, and there is little evidence of such a link in the CT, even though this region is characterized by high precipitation, erosional plumes and seasonal upwelling-associated phytoplankton blooms. Furthermore, until survivorship and competency for a wider spectrum of mass-spawned invertebrate planktotrophs in relation to elevated phytoplankton densities is better understood, such evidence, suggesting release from food limitation as the principal cause of enhanced COT recruitment, should be interpreted with caution.