HOST USE PATTERNS AND DEMOGRAPHY IN A GUILD OF TROPICAL SPONGE-DWELLING SHRIMPS

Demographic consequences of the commensal lifestyle for shrimps Synalpheus spp. were assessed by sampling sponges and coral rubble on reefs in Caribbean Panama. Eight of 22 species were found solely or primarily within the internal canals of sponges. Among these sponge-dwellers, host specificity ranged from generalism (occurrence in greater-than-or-equal-to 4 host species) to specialization on a single host species, and sponges used in the field were also preferred in laboratory choice assays. Living in sponges had important consequences for shrimp populations. Parasitism by epicaridean isopods averaged 6 times higher in obligate sponge-dwellers (17 %) than in free-living species (2.5 %). Sponge species differed in the mean size and size range of habitable spaces they provide, number of potentially competing Synalpheus species they support, and vulnerability of associated shrimps to parasitism. Similarly, conspecific shrimp populations occupying different hosts differed demographically. Specifically, populations of Synalpheus brooksi in the sponge Spheciospongia vesparium were significantly less dense, less parasitized, had larger body sizes, and tended toward higher proportions of mature females than conspecifics in the co-occuring sponge Agelas clathrodes. High host specificity, regional variation in host use, and demographic and genetic differentiation among conspecific shrimps in different host species suggest that the commensal lifestyle has pervasive, and potentially evolutionarily important, consequences for the population biology of this diverse group of shrimps.