ZMT - Leibniz Zentrum für Marine Tropenforschung

Corallivorous gastropods of the genus Drupella can contribute to considerable coral tissue loss when they occur in dense feeding aggregations. However, the behavioural mechanisms underlying host detection and aggregation formation remain poorly understood, especially for Drupella rugosa in Indonesian reefs. This study investigated field occurrence, coral associations and cue-mediated behaviour of D. rugosa around Tidung Island, Kepulauan Seribu, Indonesia. Field surveys recorded 73 live Drupella individuals across five reef sites, comprising of 62 D. rugosa and 11 D. cornus. Most individuals were associated with acroporid corals, particularly Isopora brueggemanni. Overall, Drupella density was 7.3 ind. m-2, with the highest density recorded at Site 4 (17.00 ind. m-2). To assess cue-mediated foraging behaviour, D. rugosa individuals were tested in a two-arm choice flume using pairwise combinations of coral-only cues (A), coral with feeding conspecific (F), conspecific-only cues (D) and an empty control (E). Of 48 trials, 41 resulted in valid choices, D. rugosa selected the coral-only cue over the empty control in 8 out of 9 valid trials (88.9%; exact binomial test: p = 0.039), although this did not remain significant after Holm correction (p = 0.156). Choices between coral-only (A) and coral-with-feeding-conspecific (F) cues were evenly split. A short-distance conspecific attraction assay showed that contact success decreased from 8/10 trials at 1 cm to 1/10 trials at 10 cm. Contact probability declined significantly with increasing distance (likelihood-ratio χ2 = 12.19, p < 0.001) and survival analysis showed that the rate of contact also decreased with distance (hazard ratio = 0.74, 95% Cl = 0.62-0.88, p = 0.001). Qualitative observations included siphon contact, climbing and stacking interactions. Examination of faecal material further revealed dark-brown faecal strings containing organic material and diatom frustules, highlighting faeces as a potential source of information on ingestion and egestion in Drupella. Together, these findings suggest that coral-derived cues may support host detection, while short-range conspecific interactions may contribute to aggregation behaviour in D. rugosa.