Anemonefish musical chairs and the plight of the two-band anemonefish, Amphiprion bicinctus

Abstract

Anemonefishes’ obligatory mutualism with sea anemones dictates their occurrence in marine habitats. We examined whether the spatial distribution, number, and size of the host anemones Heteractis crispa and Entacmaea quadricolor affected the settlement, habitat usage, and survival of the two-band anemonefish, Amphiprion bicinctus. In a 200 × 50 m study site off the coast of Israel in the Gulf of Eilat, we followed the fish and anemone populations initially in 13 censuses from October 1996 to August 1997 and follow-up censuses from 1998 to 2015. Based on size, anemonefish were categorized as adults, juveniles, or settlers. Settlers tended to cluster together but displayed significantly dispersed distributions in relation to adult individual fish and breeding pairs. Adult and juvenile anemonefish associated more with, and exhibited higher survival in, E. quadricolor. Settlers primarily inhabited H. crispa, with similar survival rates in the two anemone species. H. crispa was less occupied compared to E. quadricolor, but 95 % of the 233 anemones hosted fish during at least one census. From 1997 to 2015, anemone and anemonefish numbers plummeted by 86 % and 74 %, respectively. In 2015, all 27 remaining anemones were occupied, with most E. quadricolor inhabited by adults. The anemones left at the study site, on average, hosted more fish per anemone than those in the original population. This saturated habitat could hinder new anemonefish individuals from settling. These results indicate that if the anemone population does not recover, the anemonefish could face local extinction.