First Field Observations of Drupella feeding on Millepora in Indo Pacific reefs

Outbreaks of the corallivorous gastropod Drupella spp. are increasingly recognised as a major driver of coral reef degradation across the Indo-Pacific. While Drupella is typically associated with predation on scleractinian corals, particularly Acropora , interactions with milleporids remain understudied. Here, we report the first feeding association between Drupella and Millepora spp. in the Indo-Pacific, based on field observations from shallow reefs (1–4 m) at Gili Asahan, Lombok, Indonesia (8.739482 N, 115.890852 E). During a 90-minute survey, Drupella snails were observed within six colonies of Millepora dichotoma , M. tenera and M. intricata. Adult snails were found in deep skeletal recesses, with denuded skeletons indicating active feeding. Occasional spillover of Drupella aggregations onto adjacent Acropora colonies was also observed. Statistical analyses revealed no significant differences in snail size or abundance among Millepora species, although effect size estimates suggest potential host-related variation. The predominance of adult Drupella within Millepora colonies suggests an ontogenetic shift in habitat use and prey choice, likely driven by feeding plasticity, refuge availability and energetic demands. We propose that the complex skeletal structure of Millepora provides both shelter and an alternative prey source, facilitating Drupella persistence under variable resource conditions. This interaction may have cascading ecological consequences, as milleporids provide important habitat for reef-associated fauna and are vulnerable to climate change. These findings highlight an underappreciated corallivore-hydrozoan interaction and emphasise the need for targeted monitoring of Millepora during Drupella outbreaks to better understand and manage emerging reef trophic dynamics.