Host selection by <i>Synalpheus stimpsoni</i> (De Man), an ectosymbiotic shrimp of comatulid crinoids, inferred by a field survey and laboratory experiments

During a survey made on symbionts of comatulids in Hansa bay (Bismark Sea, Papua New Guinea), the shrimp Synalpheus stimpsoni was observed on 4 species out of the 25 which occur in the bay: Comaster multibrachiatus (P.H. Carpenter), C. multifidus (Müller), C. gracilis (Hartlaub) and C. alternans (P.H. Carpenter). Many individuals (ca. 70%) of C. multifidus—the most common comatulid of the bay—were infested. The shrimp are ectosymbionts that occur alone or in a pair (one male, one female) under the calyx of their host. While Synalpheus stimpsoni has no apparent effect on its host, the comatulid provides, at least, protection against predators. When isolated from its natural host and placed close to different comatulid species, S. stimpsoni significantly prefers to reassociate with its natural host. Experiments made for testing the role of vision and olfaction in the recognition and selection of shrimps' hosts show that S. stimpsoni is unable to visually distinguish its natural host either from other comatulids or from branched corals. In contrast, S. stimpsoni differentiates significantly a water flow carrying the odour of its natural host from any other water flow. It is suggested that vision is the first sense involved, enabling the symbiont to move near any comatulid-like object; olfaction then acts, allowing the shrimp to recognize and select an appropriate comatulid host