Intense capture of swarming pteropods by large-polyp corals

Abstract

Snorkeling about an hour after sunrise, November 6, 2016, we observed high densities of pteropods (Creseis acicula (Rang, 1828); visually estimated >3 pteropods l) swarming in the waters above the shallow coral reef of Eilat, Israel. Many pteropods were captured by massive corals with large polyps (Merulinidae: Dipsastraea spp., Favites spp., Platygyra sp. and others). Some of the captured pteropods exceeded one cm in length. In some corals, we observed single polyps ingesting up to three pteropods (Fig 1 A, B). Empty shells were found on the bottom beneath the corals. Neither captured pteropods nor empty shells were found in or around corals with small polyps, including Acropora spp., Pocillopora damicornis (Linnaeus, 1758), and Stylophora pistillata Esper, 1797. Corals with large polyps are thought to feed on larger zooplankton, mostly crustaceans (e.g., Porter 1976). Although Janssen (2007) anecdotally mentions an observation of live corals feeding on pteropods following the mass stranding of C. acicula in the shallow waters of Dahab, Red Sea, mentioning of pteropods as a component in coral diet are scarce (Comeau et al. 2013). Our observation provides evidence, however qualitative, for a remarkable difference between corals with large vs. small polyps in their ability to prey on large zooplankton. As repetitive swarming of pteropods were reported from certain reefs (Goswami et al. 1982; Janssen 2007), our findings suggest that their contribution to the corals’ diet may be significant.