Feeding of Luidia maculata on bioeroding Diadema setosum in Hong Kong: Possible diet shift for the starfish?

Abstract

The long-spined sea urchin Diadema setosum is well documented as causing bioerosion in coral reefs. In Hong Kong, increasing impacts of bioerosion by D. setosum are attributable to the absence of its key predator, the wrasse Choerodon schoenleinii. On 25th June 2012, we observed a starfish, Luidia maculata (maximum length >50 cm) feeding on D. setosum at 5 m depth in Chek Chau, northeastern Hong Kong (22.496°N, 114.359°E). The site has a gently sloping rocky shore with patchy sandy substratum and is next to a coral assemblage. The whole living urchin, including the spines (∼8 cm in diameter), was engulfed (Fig. 1a) and devoured by the starfish using extraoral digestion (Fig. 1b). This mode of digestion is atypical for luidiids, as most of them perform intraoral digestion (Lawrence 1987). Luidia spp. are noted scavengers and predators (Jangoux 1982; Lawrence et al. 2013). The diet of L. maculata includes mollusks and some echinoderms (Chiu et al. 1986). This is, however, the first time to observe its feeding on D. setosum. The prevalence of this prey-predator relationship is currently