HOST SELECTION BY SYNALPHEUS-STIMPSONI (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 o n 4 species out of the 25 which occur in the bay: Comaster multibrachi atus (P.H. Carpenter), C. multifidus (Muller), C. gracilis (Hartlaub) and C. alternans (P.H. Carpenter). Many individuals (ca. 70%) of C. mu ltifidus-the most common comatulid of the bay-were infested. The shrim p are ectosymbionts that occur alone or in a pair tone male, one femal e) under the calyx of their host. While Synalpheus stimpsoni has no ap parent effect on its host, the comatulid provides, at least, protectio n against predators. When isolated from its natural host and placed cl ose to different comatulid species, S. stimpsoni significantly prefers to reassociate with its natural host. Experiments made for testing th e role of vision and olfaction in the recognition and selection of shr imps' hosts show that S. stimpsoni is unable to visually distinguish i ts natural host either from other comatulids or from branched corals. In contrast, S. stimpsoni differentiates significantly a water flow ca rrying the odour of its natural host from any other water flow. It is suggested that vision is the first sense involved, enabling the symbio nt to move near any comatulid-like object; olfaction then acts, allowi ng the shrimp to recognize and select an appropriate comatulid host. ( C) 1998 Elsevier Science B.V.