The Indian Ocean Dipole and El Niño Southern Oscillation as major drivers of coral cover on shallow reefs in the Andaman Sea.

Shallow reefs are a major feature of coral assemblages in the Andaman Sea. At Phuket, Thailand sheltered reefs are dominated by massive corals, together with an increasing abundance of branching species during favourable growth conditions. The growth of coral on these reefs is moderated by long-term increases in sea temperature and relative sea level but fluctuating decadal/intradecadal climate processes of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), which modulate sea level and temperature, are the main drivers of coral cover. In this study, the contribution of these two climate processes was identified and also quantified. Over a 34-year study of fluctuating coral cover, the three major reductions in cover in 1997, 2010 and 2019 were linked to overlapping positive IOD (pIOD) and El Niños in 1997 and 2019, and with an El Niño alone in 2010. Combined pIOD and El Niño depressed sea level was the major factor in reducing cover in 1997 while El Niño extreme sea temperatures were responsible for large reductions in 2010. In 2019, a bi-phasic pIOD and El Niño resulted in lowered cover at a time of both decreased sea level and high sea temperature. Under global warming scenarios, it is projected that extreme pIODs, such as those seen in 1997 and 2019, will occur more frequently while El Niño frequencies will continue to increase even after global mean temperature stabilization. In these circumstances, and with steadily rising background sea temperatures, the future risks to the shallow reefs of the Andaman Sea are substantial, despite any temporary respite gained from climate related or land subsidence sea-level rise. Such findings have wider implications for all reefs affected by climatic-driven sea-level depressions, particularly those around Indonesian shores where similar El-Niño-related reductions in coral cover have been reported.

Selective consumption of sacoglossan sea slugs (Mollusca: Gastropoda) by scleractinian corals (Cnidaria: Anthozoa).

Recent studies revealed that reef corals can eat large-sized pelagic and benthic animals in addition to small planktonic prey. As follow-up, we document natural ingestion of sea slugs by corals and investigate the role of sacoglossan sea slugs as possible prey items of scleractinian corals. Feeding trials were carried out using six sacoglossan species as prey, two each from the genera Costasiella, Elysia and Plakobranchus, and four free-living solitary corals (Danafungia scruposa, Fungia fungites, Pleuractis paumotensis and Heteropsammia cochlea) as predators. Trials were carried out under both in-situ and ex-situ conditions with the aim to observe ingestion and assess signs of prey consumption based on tissue loss of prey individuals over time. Significant differences were observed in both ingestion time and consumption state of prey between prey species, with three of them being ingested more rapidly and preferentially consumed over the others. Additionally, prey size was found to be a significant factor with larger prey (>12 mm) being ingested more slowly and rarely than smaller ones (<6 mm and 6-12 mm). Comparisons of consumption capability among predators showed no significant difference with all coral species showing similar preferences for prey species. While no specific mechanism of prey capture is proposed, we also document instances of kleptoparisitism and resuspension of prey items by wrasses. This study highlights the important distinction between opportunistic prey capture and true predation events.