Impact of Thermal Bleaching Events on the Non-scleractinian Reef Inhabitants of the Lakshadweep Atolls

Abstract

The Lakshadweep Islands, renowned for their highly biodiverse and pristine reef ecosystems, face increasing stress from climate change, notably rising sea surface temperatures (SST), leading to recurrent bleaching events. In 2024, the Lakshadweep Islands experienced mass coral bleaching when the SST peaked to 32.2°C and Degree Heating Weeks (DHW) reached an unprecedented 9.2°C-weeks, the highest ever recorded for the Lakshadweep Islands. Concurrently, several non-scleractinians, such as Radianthus magnifica (Ritteri anemone) and Tridacna maxima (Small giant clam), experienced complete bleaching, while Heliopora coerulea (Blue coral) and Anemonia cf. majano (Carpet anemone) exhibited 60% and 55% bleaching, respectively. In contrast, during another comparatively less intense bleaching event that occurred in 2025, characterised by SST above the bleaching threshold but DHW <4°C-weeks, bleaching was evident only in Heliopora coerulea. This study indicates that DHW >4°C-weeks can induce widespread bleaching in non-scleractinians similar to scleractinians, and the distinct responses and bleaching thresholds of the diverse non-scleractinians to varying bleaching intensities.