Distribution, dietary preference and larval settlement preference of three scleractinain-coral-eating nudibranchs Phestilla spp. (Nudibranchia: Trinchesiidae) from Hong Kong waters

Phestilla is a small genus of scleractinain-coral-eating nudibranchs that comprises 11 valid species distributed throughout the Indo-Pacific region. We conducted a qualitative survey in Hong Kong waters and found Phestilla fuscostriata, P. goniophaga and P. melanobrachia in low densities. Phestilla fuscostriata and P. goniophaga mainly occurred in the northeast to east region while P. melanobrachia mainly occurred in the east to southeast region. They exhibit different levels of host specificity, with Phestilla fuscostriata associated with Pavona decussata, P. goniophaga associated with Goniopora spp., and P. melanobrachia associated with dendrophylliids. Laboratory experiments showed that they fed on their host corals only and were killed when placed on the surface of non-host corals. The feeding rate was size-dependent, with the bigger P. goniophaga and P. melanobrachia on average removed 1.44–8.33 polyps/day/individual and 0.10–2.75​ polyps/day/individual from the coral respectively, and the smaller P. fuscostriata removed only 0.031polyp/day/individual from the coral. Chemical cues released from the hosts had significant inducing effect on the larval metamorphosis of P. goniophaga and P. melanobrachia, but in P. fuscostriata such effect was insignificant. Their larvae were able to disperse to location nearby Hong Kong waters due a shorter period of 1–3 days required for metamorphic competence.