Are infestations of Cymomelanodactylus killing Acropora cytherea in the Chagos archipelago?

Associations between branching corals and infaunal crabs are well known, mostly due to the beneficial effects of Trapezia and Tetralia crabs in protecting host corals from crown-of-thorns starfish (e.g., Pratchett et al. 2000) and/or sedimentation (Stewart et al. 2006). These crabs are obligate associates of live corals and highly prevalent across suitable coral hosts, with 1–2 individuals per colony (Patton 1994). Cymo melanodactylus (Fig. 1) are also prevalent in branching corals, mostly Acropora, and are known to feed on live coral tissue, but are generally found in low abundance (<3 per colony) and do not significantly affect their host corals (e.g., Patton 1994). In the Chagos archipelago, however, infestations of Cymo melanodactylus were found on recently dead and dying colonies of Acropora cytherea

Age and growth of an outbreaking Acanthaster cf. solaris population within the Great Barrier Reef

Despite having been studied for more than 40 years, much about the basic life history of crown-of-thorns starfish (CoTS; Acanthaster spp.) remains poorly understood. Size at age-a key metric of productivity for any animal population-has yet to be clearly defined, primarily due to difficulties in obtaining validated ages and potentially indeterminate growth due to factors such as starvation; within-population variability is entirely unknown. Here we develop age and growth estimates for an outbreaking CoTS population in Australian waters by integrating prior information with data from CoTS collected from multiple outbreaking reefs. Age estimates were made from un-validated band counts of 2038 individual starfish. Results from our three-parameter von Bertalanffy Bayesian hierarchical model show that, under 2013-2014 outbreak conditions, CoTS on the GBR grew to a 349 ( 326, 380) mm (posterior median (95% uncertainty interval)) total diameter at a 0.54 (0.43, 0.66) intrinsic rate of increase. However, we also found substantial evidence (Delta DIC > 200) for inter-reef variability in both maximum size (SD 38 (19, 76)) and intrinsic rate of increase (SD 0.32 (0.20, 0.49)) within the CoTS outbreak initiation area. These results suggest that CoTS demography can vary widely with reef-scale environmental conditions, supporting location-based mechanisms for CoTS outbreaks generally. These findings should help improve population and metapopulation models of CoTS dynamics and better predict the potential damage they may cause in the future

Bleaching susceptibility and mortality among corals with differing growth forms

Differences in bleaching susceptibility and mortality are apparent among coral species, and have been variously ascribed to differences in physiology and morphology, in particular overall growth form (e.g., branching versus massive corals). However, coral morphology is highly confounded with taxonomy, and no studies have tested for differences in bleaching susceptibility among corals with varying morphology within (rather than between) coral families. For this study, data were compiled for bleaching susceptibility and mortality from 65 published studies that monitored coral health throughout the bleaching event. Overall patterns of bleaching susceptibility were significantly different among coral growth forms, whereby a much higher proportion of branching, tabular, and submassive corals bleached compared to encrusting, massive, and free-living corals. However, differences in bleaching susceptibility and mortality were not consistent among growth forms within families. Branching faviid species (e.g., Echinopora) had much lower incidence of bleaching compared to massive species, but the reverse was true for Acroporidae and Poritidae. Moreover, mortality was markedly different amongst growth forms within families, such that massive Acroporidae (e.g. Montipora) suffered highest mortality when compared to Faviidae and Poritidae, but branching Acroporidae suffered lowest mortality compared to branching Faviidae and Poritidae. Our data suggest that generalities about the susceptibility of branching versus massive corals (and among other major growth forms) arise at least in part because certain growth forms are over-represented by highly susceptible coral taxa (e.g., Acropora) or perhaps because branching corals generally maintain higher dominance than massive corals

Does genetic distance between parental species influence outcomes of hybridization among coral reef butterflyfishes?

Christmas Island is located at the overlap of the Indian and Pacific Ocean marine provinces and is a hot spot for marine hybridization. Here, we evaluate the ecological framework and genetic consequences of hybridization between butterflyfishes Chaetodon guttatissimus and Chaetodon punctatofasciatus. Further, we compare our current findings to those from a previous study of hybridization between Chaetodon trifasciatus and Chaetodon lunulatus. For both species groups, habitat and dietary overlap between parental species facilitate frequent heterospecific encounters. Low abundance of potential mates promotes heterospecific pair formation and the breakdown of assortative mating. Despite similarities in ecological frameworks, the population genetic signatures of hybridization differ between the species groups. Mitochondrial and nuclear data from C. guttatissimus × C. punctatofasciatus (1% divergence at cyt b) show bidirectional maternal contributions and relatively high levels of introgression, both inside and outside the Christmas Island hybrid zone. In contrast, C. trifasciatus × C. lunulatus (5% cyt b divergence) exhibit unidirectional mitochondrial inheritance and almost no introgression. Back-crossing of hybrid C. guttatissimus × C. punctatofasciatus and parental genotypes may eventually confound species-specific signals within the hybrid zone. In contrast, hybrids of C. trifasciatus and C. lunulatus may coexist with and remain genetically distinct from the parents. Our results, and comparisons with hybridization studies in other reef fish families, indicate that genetic distance between hybridizing species may be a factor influencing outcomes of hybridization in reef fish, which is consistent with predictions from terrestrially derived hybridization theory

Damselfishes alleviate the impacts of sediments on host corals

Mutualisms play a critical role in ecological communities; however, the importance and prevalence of mutualistic associations can be modified by external stressors. On coral reefs, elevated sediment deposition can be a major stressor reducing the health of corals and reef resilience. Here, we investigated the influence of severe sedimentation on the mutualistic relationship between small damselfishes (Pomacentrus moluccensis and Dascyllus aruanus) and their coral host (Pocillopora damicornis). In an aquarium experiment, corals were exposed to sedimentation rates of approximately 100 mg cm−2 d−1, with and without fishes present, to test whether: (i) fishes influence the accumulation of sediments on coral hosts, and (ii) fishes moderate partial colony mortality and/or coral tissue condition. Colonies with fishes accumulated much less sediment compared with colonies without fishes, and this effect was strongest for colonies with D. aruanus (fivefold less sediment than controls) as opposed to P. moluccensis (twofold less sediment than controls). Colonies with symbiont fishes also had up to 10-fold less sediment-induced partial mortality, as well as higher chlorophyll and protein concentrations. These results demonstrate that fish mutualisms vary in the strength of their benefits, and indicate that some mutualistic or facilitative interactions might become more important for species health and resilience at high-stress levels

Known predators of crown-of-thorns starfish (Acanthaster spp.) and their role in mitigating, if not preventing, population outbreaks

Predatory release has long been considered a potential contributor to population outbreaks of crown-of-thorns starfish (CoTS; Acanthaster spp.). This has initiated extensive searches for potentially important predators that can consume large numbers of CoTS at high rates, which are also vulnerable to over-fishing or reef degradation. Herein, we review reported predators of CoTS and assess the potential for these organisms to exert significant mortality, and thereby prevent and/or moderate CoTS outbreaks. In all, 80 species of coral reef organisms (including fishes, and motile and sessile invertebrates) are reported to predate on CoTS gametes (three species), larvae (17 species), juveniles (15 species), adults (18 species) and/or opportunistically feed on injured (10 species) or moribund (42 species) individuals within reef habitats. It is clear however, that predation on early life-history stages has been understudied, and there are likely to be many more species of reef fishes and/or sessile invertebrates that readily consume CoTS gametes and/or larvae. Given the number and diversity of coral reef species that consume Acanthaster spp., most of which (e.g., Arothron pufferfishes) are not explicitly targeted by reef-based fisheries, links between overfishing and CoTS outbreaks remain equivocal. There is also no single species that appears to have a disproportionate role in regulating CoTS populations. Rather, the collective consumption of CoTS by multiple different species and at different life-history stages is likely to suppress the local abundance of CoTS, and thereby mediate the severity of outbreaks. It is possible therefore, that general degradation of reef ecosystems and corresponding declines in biodiversity and productivity, may contribute to increasing incidence or severity of outbreaks of Acanthaster spp. However, it seems unlikely that predatory release in and of itself could account for initial onset of CoTS outbreaks. In conclusion, reducing anthropogenic stressors that reduce the abundance and/or diversity of potential predatory species represents a “no regrets” management strategy, but will need to be used in conjunction with other management strategies to prevent, or reduce the occurrence, of CoTS outbreaks

Effects of elevated temperature on the performance and survival of pacific crown-of-thorns starfish (Acanthaster cf. solaris)

Population irruptions of Pacific crown-of-thorns starfish (Acanthaster cf. solaris) have caused substantial damage to coral reefs, but it is largely unknown how this asteroid will fare in a warmer ocean. We exposed these starfish to one of four thermal treatments, with final temperatures of 26 °C (control, annual average), 28 °C (summer average), 30 °C (summer maximum) and 32 °C (predicted summer maximum by 2100). We measured the righting time, movement rate, standard metabolic rate and probability of survival of the crown-of-thorns starfish at various timepoints over ~ 60 days. We found that while tempera- ture did not affect righting time, it did significantly affect movement rate. The movement rate of starfish increased across the 26 to 30 °C range, with those at 28 °C and 30 °C moving 18 and 27% faster than those at the control temperature. Similarly, the standard metabolic rate of starfish increased from 26 to 30 °C, with metabolism 100% and 260% faster at 28 °C and 30 °C compared to those at the 26 °C control. At 32 °C, individual starfish exhibited a 14% slower movement rate, a 33% slower metabolic rate, and also exhibited a fourfold lower probability of survival than those at 30 °C. These results indicate that 32 °C is above the thermal optimum of crown-of-thorns starfish, suggesting that prolonged exposure to temperatures that are expected to be regularly exceeded under near-future climate change may be detrimental to this species

Impacts of ocean warming on the settlement success and post‐settlement survival of Pacific crown‐of‐thorns starfish (Acanthaster cf. solaris)

Ocean warming and population irruptions of crown-of-thorns starfish (CoTS; Acanthaster cf. solaris) are two of the greatest threats to coral reefs. As such, there is significant interest in understanding how CoTS may be directly impacted by rising ocean temperatures. Settlement of planktonic larvae and subsequent metamorphosis is purported to be a major population bottleneck in marine invertebrates, yet it is unknown how ocean warming will impact these processes in CoTS. Herein, the effect of temperature (28 °C ambient, 30 °C, 32 °C, 34 °C) on the settlement success, metamorphic success, and post-settlement survival of this corallivore was explored. While larval settlement was robust to elevated temperature, with at least 94% of larvae settling after 48 h across all temperatures, it was observed that settlement success was lower on substrate that had been pre-treated ≥ 32 °C. Metamorphic success was also significantly constrained at temperatures ≥ 32 °C. At 32 °C and 34 °C metamorphic success was 16% and 63% lower than at ambient temperature, respectively. Significant adverse effects of warming on post-settlement survival were observed at even cooler temperatures, with 10% lower survival at 30 °C compared to at ambient temperature, and at 34 °C, survival was 34% lower. Substantial reductions in metamorphic success and early post-settlement survival at elevated temperatures, as well as negative impacts of warming on the settlement substrate and its capacity to induce settlement, may present a bottleneck for recruitment in a warmer ocean

Recent advances in understanding the effects of climate change on coral reefs

Climate change is one of the greatest threats to the persistence of coral reefs. Sustained and ongoing increases in ocean temperatures and acidification are altering the structure and function of reefs globally. Here, we summarise recent advances in our understanding of the effects of climate change on scleractinian corals and reef fish. Although there is considerable among-species variability in responses to increasing temperature and seawater chemistry, changing temperature regimes are likely to have the greatest influence on the structure of coral and fish assemblages, at least over short–medium timeframes. Recent evidence of increases in coral bleaching thresholds, local genetic adaptation and inheritance of heat tolerance suggest that coral populations may have some capacity to respond to warming, although the extent to which these changes can keep pace with changing environmental conditions is unknown. For coral reef fishes, current evidence indicates increasing seawater temperature will be a major determinant of future assemblages, through both habitat degradation and direct effects on physiology and behaviour. The effects of climate change are, however, being compounded by a range of anthropogenic disturbances, which may undermine the capacity of coral reef organisms to acclimate and/or adapt to specific changes in environmental conditions

Impacts of ocean warming on echinoderms: A meta‐analysis

Rising ocean temperatures are threatening marine species and populations worldwide, and ectothermic taxa are particularly vulnerable. Echinoderms are an ecologically important phylum of marine ectotherms and shifts in their population dynamics can have profound impacts on the marine environment. The effects of warming on echinoderms are highly variable across controlled laboratory-based studies. Accordingly, synthesis of these studies will facilitate the better understanding of broad patterns in responses of echinoderms to ocean warming. Herein, a meta-analysis incorporating the results of 85 studies (710 individual responses) is presented, exploring the effects of warming on various performance predictors. The mean responses of echinoderms to all magnitudes of warming were compared across multiple biological responses, ontogenetic life stages, taxonomic classes, and regions, facilitated by multivariate linear mixed effects models. Further models were conducted, which only incorporated responses to warming greater than the projected end-of-century mean annual temperatures at the collection sites. This meta-analysis provides evidence that ocean warming will generally accelerate metabolic rate (+32%) and reduce survival (−35%) in echinoderms, and echinoderms from subtropical (−9%) and tropical (−8%) regions will be the most vulnerable. The relatively high vulnerability of echinoderm larvae to warming (−20%) indicates that this life stage may be a significant developmental bottleneck in the near-future, likely reducing successful recruitment into populations. Furthermore, asteroids appear to be the class of echinoderms that are most negatively affected by elevated temperature (−30%). When considering only responses to magnitudes of warming representative of end-of-century climate change projections, the negative impacts on asteroids, tropical species and juveniles were exacerbated (−51%, −34% and −40% respectively). The results of these analyses enable better predictions of how keystone and invasive echinoderm species may perform in a warmer ocean, and the possible consequences for populations, communities and ecosystems