Role of food source and predator avoidance in habitat specialization by an octocoral-associated amphipod

Small marine invertebrates often use sessile organisms as microhabitats, which can provide food sources and/or serve as refugees from predators. Because of the availability of external food items such as epibionts and detritus in the marine environment, these invertebrates may not depend on the sessile organisms as sole food sources. In this study, I hypothesized that habitat specialization by a marine invertebrate is determined by factors other than food. Results of field surveys off the coast of the Izu Peninsula, on the eastern coast of Japan, showed that, with few exceptions, the distribution of the amphipod Incisocalliope symbioticus was restricted to the octocoral Melithaea flabellifera. When presented with several habitat options, I. symbioticus selected M. flabellifera most frequently, although some individuals chose the octocoral Acabaria japonica. The selection was proximately determined by water-borne cues from M. flabellifera that appear to be unrelated to the octocoral as a food source, since the amphipod preferred detritus to the octocoral. As a chemical refuge, M. flabellifera had an allelopathic effect that deterred fish predation on the exposed epifauna. With regard to octocoral habitat in the study area, I. symbioticus may be restricted to M. flabellifera because this was the only abundant octocoral consistently occurring in shallow water (< or =10 m), where predation is intensive. The relationship between I. symbioticus and M. flabellifera was commensal and was ultimately driven by the value of M. flabellifera as a chemical refuge from predation, rather than its food value. This study supports the idea that protection from predators, rather than food utilization, can promote ecological specialization.