Herbivorous fishes Siganus rivulatus (Siganidae) and Zebrasoma desjardinii (Acanthuridae) feed on Ctenophora and Scyphozoa in the Red Sea

Abstract

Individuals of the rabbitfish Siganus rivulatus (Siganidae) and surgeonfish Zebrasoma desjardinii (Acanthuridae) were observed feeding on ctenophores and scyphozoans in the northern Red Sea during late spring and early summer seasons between 2010 and 2015. Siganus rivulatus and Z. desjardinii, considered nominally herbivorous, preyed on relatively large ctenophores until disintegration, and on the moon jellyfish Aurelia aurita (Scyphozoa) until they sank to the bottom. Siganus rivulatus was the most dominant predatory fish feeding on gelatinous zooplankton (37 % of attacks on ctenophores and 51 % of attacks on A. aurita). Furthermore, prey individuals were usually approached by two or more fish simultaneously. Eleven additional fish species fed on gelatinous zooplankton, with Chaetodon fasciatus (Chaetodontidae) accounting for 27 and 12 % of the attacks on ctenophores and A. aurita, respectively, and Z. desjardinii for 17 and 24 % of the attacks. Other fishes attacked gelatinous zooplankton in ≤6 % of the observations. This study constitutes the first record of S. rivulatus and Z. desjardinii preying on ctenophores and also confirms preliminary observations of S. rivulatus feeding on scyphozoans. Although herbivorous fishes may accidentally ingest small invertebrates while feeding on algae, the deliberate feeding on gelatinous zooplankton observed for S. rivulatus and Z. desjardinii suggests that their trophic role should be reevaluated.