Protecting Great Barrier Reef resilience through effective management of crown-of-thorns starfish outbreaks.

Resilience-based management is essential to protect ecosystems in the Anthropocene. Unlike large-scale climate threats to Great Barrier Reef (GBR) corals, outbreaks of coral-eating crown-of-thorns starfish (COTS; Acanthaster cf. solaris) can be directly managed through targeted culling. Here, we evaluate the outcomes of a decade of strategic COTS management in suppressing outbreaks and protecting corals during the 4th COTS outbreak wave at reef and regional scales (sectors). We compare COTS density and coral cover dynamics during the 3rd and 4th outbreak waves. During the 4th outbreak wave, sectors that received limited to no culling had sustained COTS outbreaks causing significant coral losses. In contrast, in sectors that received timely and sufficient cull effort, coral cover increased substantially, and outbreaks were suppressed with COTS densities up to six-fold lower than in the 3rd outbreak wave. In the Townsville sector for example, despite exposure to comparable disturbance regimes during the 4th outbreak wave, effective outbreak suppression coincided with relative increases in sector-wide coral cover (44%), versus significant coral cover declines (37%) during the 3rd outbreak wave. Importantly, these estimated increases span entire sectors, not just reefs with active COTS control. Outbreaking reefs with higher levels of culling had net increases in coral cover, while the rate of coral loss was more than halved on reefs with lower levels of cull effort. Our results also indicate that outbreak wave progression to adjoining sectors has been delayed, probably via suppression of COTS larval supply. Our findings provide compelling evidence that proactive, targeted, and sustained COTS management can effectively suppress COTS outbreaks and deliver coral growth and recovery benefits at reef and sector-wide scales. The clear coral protection outcomes demonstrate the value of targeted manual culling as both a scalable intervention to mitigate COTS outbreaks, and a potent resilience-based management tool to "buy time" for coral reefs, protecting reef ecosystem functions and biodiversity as the climate changes.

Increasing densities of Pacific crown-of-thorns starfish (Acanthaster cf. solaris) at Lizard Island, northern Great Barrier Reef, resolved using a novel survey method.

Recurrent population irruptions of Pacific crown-of-thorns starfish (CoTS, Acanthaster cf. solaris) are among the foremost causes of coral mortality on Australia's Great Barrier Reef (GBR). Early intervention during the initiation of new population irruptions represents the best opportunity to effectively manage this threat. However, current survey methods are not sufficiently sensitive to detect changes in CoTS densities during the early onset of population irruptions. Using scooter-assisted large area diver-based (SALAD) surveys, this study revealed increasing densities of CoTS at Lizard Island from 2019 to 2022. Inferred densities of adult CoTS (which account for distinct sets of observed feeding scars where starfish were not detected) increased from 4.90 ha-1 (± 0.85 SE) in 2019 to 17.71 ha-1 (± 2.3 SE) in 2022. A wide range of size classes were recorded suggesting that recruitment over several years is contributing to increasing densities. Importantly, the sustained density increases reported here denote that renewed CoTS population irruptions may soon become fully established at Lizard Island and more broadly in the northern GBR, especially without early intervention through effective population management.

Induction of larval settlement in crown-of-thorns starfish is not mediated by conspecific cues.

Population irruptions of crown-of-thorns starfish (COTS; Acanthaster spp.) remain a major cause of coral reef degradation throughout the Pacific and Indian Oceans and are inherently modulated by larval settlement and recruitment success. Gregarious larval settlement, as exhibited by many other ecologically important marine invertebrates, can catalyse population growth and replenishment. However, whether conspecific cues induce or influence the settlement of COTS larvae remains a critical information gap. This experimental study examined the induction of COTS settlement in response to a range of conspecific cues associated with early- and late-stage herbivorous juveniles, corallivorous juveniles and adults. Competent COTS larvae were generally not induced to settle by the presence of conspecifics or cues associated with conspecifics, while the settlement success of COTS in the presence of coralline algae was not inhibited or enhanced by adding conspecific conditioned seawater. Rather than being reinforced by gregarious settlement, the recruitment of COTS populations appears dependent on associative settlement cues (i.e., coralline algae and/or associated microbial communities) signalling suitable benthic habitat.

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.

Settlement cue selectivity by larvae of the destructive crown-of-thorns starfish.

Population irruptions of crown-of-thorns starfish (COTS) cause extensive degradation of coral reefs, threatening the structure and function of these important ecosystems. For population irruptions to initiate and spread, large numbers of planktonic larvae have to successfully transition into their benthic life-history stage (i.e. settlement), whereby larval behaviour and the presence of settlement cues may shape spatial patterns of recruitment and adult densities. Our results demonstrate that a wide range of coralline algae species induce COTS larvae to settle; however, the capacity to promote settlement success varied manyfold among algal species, ranging from greater than 90% in Melyvonnea cf. madagascariensis to less than 2% in Lithophyllum cf. kotschyanum and two Porolithon species at 24 h. Because many coralline algae species that promote high settlement success are prevalent in shallow reef habitats, our findings challenge the hypothesis that COTS larvae predominantly settle in deep water. Considering both larval behaviour and algal ecology, this study highlights the ecological significance of coralline algae communities in driving recruitment patterns of COTS. More specifically, the local abundance of highly inductive coralline algae (especially, Melyvonnea cf. madagascariensis) may explain some of the marked spatial heterogeneity of COTS populations and the incidence of population irruptions.

The upper thermal limit of epaulette sharks (Hemiscyllium ocellatum) is conserved across three life history stages, sex and body size.

Owing to climate change, most notably the increasing frequency of marine heatwaves and long-term ocean warming, better elucidating the upper thermal limits of marine fishes is important for predicting the future of species and populations. The critical thermal maximum (CTmax), or the highest temperature a species can tolerate, is a physiological metric that is used to establish upper thermal limits. Among marine organisms, this metric is commonly assessed in bony fishes but less so in other taxonomic groups, such as elasmobranchs (subclass of sharks, rays and skates), where only thermal acclimation effects on CTmax have been assessed. Herein, we tested whether three life history stages, sex and body size affected CTmax in a tropical elasmobranch, the epaulette shark (Hemiscyllium ocellatum), collected from the reef flats surrounding Heron Island, Australia. Overall, we found no difference in CTmax between life history stages, sexes or across a range of body sizes. Findings from this research suggest that the energetically costly processes (i.e. growth, maturation and reproduction) associated with the life history stages occupying these tropical reef flats do not change overall acute thermal tolerance. However, it is important to note that neither embryos developing in ovo, neonates, nor females actively encapsulating egg cases were observed in or collected from the reef flats. Overall, our findings provide the first evidence in an elasmobranch that upper thermal tolerance is not impacted by life history stage or size. This information will help to improve our understanding of how anthropogenic climate change may (or may not) disproportionally affect particular life stages and, as such, where additional conservation and management actions may be required.

Metabolic Responses of Pacific Crown-of-Thorns Sea Stars (Acanthaster sp.) to Acute Warming.

AbstractClimate change and population irruptions of crown-of-thorns sea stars (Acanthaster sp.) are two of the most pervasive threats to coral reefs. Yet there has been little consideration regarding the synergies between ocean warming and the coral-feeding sub-adult and adult stages of this asteroid. Here we explored the thermosensitivity of the aforementioned life stages by assessing physiological responses to acute warming. Thermal sensitivity was assessed based on the maximal activity of enzymes involved in aerobic (citrate synthase) and anaerobic (lactate dehydrogenase) metabolic pathways, as well as the standard metabolic rate of sub-adult and adult sea stars. In both life stages, citrate synthase activity declined with increasing temperature from 15 °C to 40 °C, with negligible activity occurring >35 °C. On the other hand, lactate dehydrogenase activity increased with temperature from 20 °C to 45 °C, indicating a greater reliance on anaerobic metabolism in a warmer environment. The standard metabolic rate of sub-adult sea stars increased with temperature throughout the testing range (24 °C to 36 °C). Adult sea stars exhibited evidence of thermal stress, with metabolic depression occurring from 33 °C. Here, we demonstrate that crown-of-thorns sea stars are sensitive to warming but that adults, and especially sub-adults, may have some resilience to short-term marine heatwaves in the near future.

Knowledge Gaps in the Biology, Ecology, and Management of the Pacific Crown-of-Thorns Sea Star Acanthaster sp. on Australia's Great Barrier Reef.

AbstractCrown-of-thorns sea stars (Acanthaster sp.) are among the most studied coral reef organisms, owing to their propensity to undergo major population irruptions, which contribute to significant coral loss and reef degradation throughout the Indo-Pacific. However, there are still important knowledge gaps pertaining to the biology, ecology, and management of Acanthaster sp. Renewed efforts to advance understanding and management of Pacific crown-of-thorns sea stars (Acanthaster sp.) on Australia's Great Barrier Reef require explicit consideration of relevant and tractable knowledge gaps. Drawing on established horizon scanning methodologies, this study identified contemporary knowledge gaps by asking active and/or established crown-of-thorns sea star researchers to pose critical research questions that they believe should be addressed to improve the understanding and management of crown-of-thorns sea stars on the Great Barrier Reef. A total of 38 participants proposed 246 independent research questions, organized into 7 themes: feeding ecology, demography, distribution and abundance, predation, settlement, management, and environmental change. Questions were further assigned to 48 specific topics nested within the 7 themes. During this process, redundant questions were removed, which reduced the total number of distinct research questions to 172. Research questions posed were mostly related to themes of demography (46 questions) and management (48 questions). The dominant topics, meanwhile, were the incidence of population irruptions (16 questions), feeding ecology of larval sea stars (15 questions), effects of elevated water temperature on crown-of-thorns sea stars (13 questions), and predation on juveniles (12 questions). While the breadth of questions suggests that there is considerable research needed to improve understanding and management of crown-of-thorns sea stars on the Great Barrier Reef, the predominance of certain themes and topics suggests a major focus for new research while also providing a roadmap to guide future research efforts.

Contributions of pre- versus post-settlement processes to fluctuating abundance of crown-of-thorns starfishes (Acanthaster spp.).

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 Effect of Larval Diet on Adult Survival, Swarming Activity and Copulation Success in Male Aedes aegypti (Diptera: Culicidae).

Control of Aedes aegypti (L.) (Diptera: Culicidae) populations is vital for reducing the transmission of several pervasive human diseases. The success of new vector control technologies will be influenced by the fitness of laboratory-reared transgenic males. However, there has been relatively little published data on how rearing practices influence male fitness in Aedes mosquitoes. In the laboratory, the effect of larval food availability on adult male fitness was tested, using a range of different fitness measures. Larval food availability was demonstrated to be positively correlated with adult body size. Larger males survived longer and exhibited greater swarming activity. As a consequence, larger males may have more mating opportunities in the wild. However, we also found that within a swarm larger males did not have an increased likelihood of copulating with a female. The outcome of the mating competition experiments depended on the methodology used to mark the males. These results show that fitness assessment can vary depending on the measure analyzed, and the methodology used to determine it. Continued investigation into these fitness measures and methodologies, and critically, their utility for predicting male performance in the field, will increase the efficiency of vector control programs.