Enrico Montalbetti

Outbreaks of crown-of-thorns sea stars (CoTS) threaten coral reef integrity and biodiversity, yet local dynamics and short-term responses to control remain insufficiently described. This study characterised an outbreaking Acanthaster population in two specific sites of the coast of Khor Fakkan (Gulf of Oman, United Arab Emirates) to resolve species identity, population composition, prey selection and the effects of targeted removals. All sequenced individuals clustered in two related haplotypes belonging to the species Acanthaster planci. Benthic surveys showed moderate live-coral cover, dominated by massive Porites sp. colonies. Moreover, the observations of 139 preyed colonies revealed pronounced genus-level selectivity, with branching and complex morphologies suffering disproportionately and massive forms largely avoided. However, the selection of massive Plesiastrea and Favites genera as preferred coral prey might suggest a shift towards less preferred coral in the CoTS diet, posing a severe threat to coral reefs’ integrity. Intensive removal reduced the local density, up to 86%, and provided substantial short-term relief, but continued monitoring is required to secure long-term reef resilience.

Current climate changes cause increasing stress to corals, which can disrupt host-symbiont relationships, leading to bleaching and mortality. While stress responses are well-documented at the coral colony and reef scales, less is known about how stress responses affect different compartments within the coral holobiont. Here, we investigated the impact of thermal stress on macro– and microscale compartments, including the endoderm-lined gastrovascular cavity (GVC) and the ectoderm-lined external tissue surface, in two tropical corals, Caulastrea curvata and Galaxea fascicularis . We assessed the coral physiological status via respirometry, variable chlorophyll fluorometry, O ₂ and H ₂ O ₂ microsensor analyses, chlorophyll measurements, and oxidative stress biomarkers during a 14-day thermal stress exposure. Under heat stress, both species showed a reduced O ₂ concentration in the GVC by up to 90% in the light, reaching hypoxic (< 50 µmol O ₂ L ⁻¹ ) to anoxic levels, while the O ₂ concentration in the external surface tissue of C. curvata decreased by 20%. Moreover, no changes in the H ₂ O ₂ dynamics of the coral’s external tissue surface were detected. Lipid peroxidation in the gastrodermis of both species increased significantly by 90–198% indicating oxidative damage, although antioxidant enzyme activity (SOD and CAT), chlorophyll content, and bulk metabolic rates (respiration and photosynthesis) remained stable. Our findings indicated that thermal stress could affect the microenvironment of corals, particularly in the GVC, without visible bleaching or major disruptions in bulk physiology. This suggested the importance of microenvironmental metrics for the early detection of coral physiological stress.

The current study investigated the morphological dietary preferences of an outbreaking population of corallivorous crown-of-thorn sea stars (Acanthaster sp.) in Koh Tao, situated in the Gulf of Thailand. The local effects of such populations deemed to be in outbreak are currently poorly understood. Therefore, the aim of this study was to investigate the localised feeding behaviours of Acanthaster sp. within an outbreak population, so as to better understand the reefs vulnerable to future outbreaks. Data was collected over a 2-year period, using underwater visual census methods. Survey parameters included sea star abundance, as well as substrate composition and frequency of observed predation scarring. Acanthaster sp. populations were observed to consistently exceed regional outbreak definitions (34.4 individuals ha⁻¹), and demonstrated unimodal size distribution consistent with a primary outbreak. Analysis of feeding behaviour displayed some evidence to support previous claims of density-dependent predation within Acanthaster sp. populations. Van der Ploeg and Scavia tests of dietary preference appeared to suggest that Acanthaster sp. have a marked preference for coral morphologies belonging to tabular and arborescent coral types, and a strong avoidance for massive, laminar, submassive, and solitary coral morphs. Furthermore, Acanthaster sp. displayed a preference towards small coral colonies between 0 and 20 cm. This finding has potential significance for reef resilience and diversity within areas at risk of CoT outbreaks, and seemingly opposes previous literature in other regions. Overall, the current study displays a host of evidence shedding light on Acanthaster sp. ecology and dietary preferences within an outbreaking population density in the Gulf of Thailand. Moreover, there is clear evidence highlighting large spatial variation in the ecology of crown-of-thorns sea stars feeding ecology, and as such demonstrates the requirement for increased localised studies of Acanthaster sp. that accounts for environmental and ecological heterogeneity.

Capitate hydrozoans are a morphologically and ecologically diverse hydrozoan suborder, currently including about 200 species. Being grouped in two clades, Corynida and Zancleida, these hydrozoans still show a number of taxonomic uncertainties at the species, genus and family levels. Many Capitata species established symbiotic relationships with other benthic organisms, including bryozoans, other cnidarians, molluscs and poriferans, as well as with planktonic dinoflagellates for mixotrophic relationships and with bacteria for thiotrophic ectosymbioses. Our study aimed at providing an updated and comprehensive phylogeny reconstruction of the suborder, at modelling the evolution of selected morphological and ecological characters, and at testing evolutionary relationships between the symbiotic lifestyle and the other characters, by integrating taxonomic, ecological and evolutionary data. The phylogenetic hypotheses here presented shed light on the evolutionary relationships within Capitata, with most families and genera being recovered as monophyletic. The genus Zanclea and family Zancleidae, however, were divided into four divergent clades, requiring the establishment of the new genus Apatizanclea and the new combinations for species in Zanclea and Halocoryne genera. The ancestral state reconstructions revealed that symbiosis arose multiple times in the evolutionary history of the Capitata, and that homoplasy is a common phenomenon in the group. Correlations were found between the evolution of symbiosis and morphological characters, such as the perisarc. Overall, our results highlighted that the use of genetic data and a complete knowledge of the life cycles are strongly needed to disentangle taxonomic and systematic issues in capitate hydrozoans. Finally, the colonization of tropical habitat appears to have influenced the evolution of a symbiotic lifestyle, playing important roles in the evolution of the group.

Microplastics are a persistent and ubiquitous source of pollution in the marine environment, representing a severe threat to tropical coral reefs. The effects of microplastics on reef-building (hard) corals have been documented (interference with normal digestion process, polyp retraction, oxidative stress, impairment of the photosynthetic machinery, bleaching). However, the impact of microplastics on soft corals, the second most abundant benthos of tropical reefs, remains to be thoroughly studied. In this work, we analysed the effects of a short-term microplastic exposure on the cellular physiology of the soft coral Coelogorgia palmosa. We found that samples exposed to >50 mg l⁻¹ of microplastic showed significant increase in the activities of the antioxidant enzymes glutathione reductase, catalase, and superoxide dismutase, suggesting a rise in oxidative stress. Furthermore, exposure to microplastics increased lipid peroxidation, indicating oxidative damage. Overall, our results show that similar to hard corals, microplastic ingestion causes oxidative stress and cellular damage in soft corals. Our study provides a first assessment of physiological effects of microplastic exposure on the soft coral, Coelogorgia palmosa, highlighting the need for further investigations about these contaminants and their influence on marine benthic fauna. Such information is crucial to understand how different reef organisms respond to microplastic pollution and who the ecological winners or losers will be in an increasingly polluted marine environment.

Waminoa spp. are acoel flatworms mainly found as ectosymbionts on scleractinian corals. Although Waminoa could potentially represent a threat to their hosts, not enough information is available yet regarding their ecology and effect on the coral. Here, the Waminoa sp.–coral association was analyzed in Singapore reefs to determine the prevalence, host range, and preference, as well as the flatworm abundance on the coral surface. Moreover, the impact of Waminoa sp. on the expression of putative immune- and stress-response genes (C-type lectin, C3, Hsp70 and Actin) was examined in the coral Lobophyllia radians. The association prevalence was high (10.4%), especially in sites with lower sedimentation and turbidity. Waminoa sp. showed a wide host range, being found on 17 coral genera, many of which are new association records. However, only few coral genera, mostly characterized by massive or laminar morphologies appeared to be preferred hosts. Waminoa sp. individuals displayed variable patterns of coral surface coverage and an unequal distribution among different host taxa, possibly related to the different coral growth forms. A down-regulation of the expression of all the analyzed genes was recorded in L. radians portions colonized by Waminoa individuals compared to those without. This indicated that Waminoa sp. could affect components of the immune system and the cellular homeostasis of the coral, also inhibiting its growth. Therefore, Waminoa sp. could represent a potential further threat for coral communities already subjected to multiple stressors.

Coral restoration initiatives are gaining significant momentum in a global effort to enhance the recovery of degraded coral reefs. However, the implementation and upkeep of coral nurseries are particularly demanding, so that unforeseen breaks in maintenance operations might jeopardize well‐established projects. In the last 2 years, the COVID‐19 pandemic has resulted in a temporary yet prolonged abandonment of several coral gardening infrastructures worldwide, including remote localities. Here we provide a first assessment of the potential impacts of monitoring and maintenance breakdown in a suite of coral restoration projects (based on floating rope nurseries) in Colombia, Seychelles, and Maldives. Our study comprises nine nurseries from six locations, hosting a total of 3,554 fragments belonging to three coral genera, that were left unsupervised for a period spanning from 29 to 61 weeks. Floating nursery structures experienced various levels of damage, and total fragment survival spanned from 40 to 95% among projects, with Pocillopora showing the highest survival rate in all locations present. Overall, our study shows that, under certain conditions, abandoned coral nurseries can remain functional for several months without suffering critical failure from biofouling and hydrodynamism. Still, even where gardening infrastructures were only marginally affected, the unavoidable interruptions in data collection have slowed down ongoing project progress, diminishing previous investments and reducing future funding opportunities. These results highlight the need to increase the resilience and self‐sufficiency of coral restoration projects, so that the next global lockdown will not further shrink the increasing efforts to prevent coral reefs from disappearing.