Sponge host characteristics shape the community structure of their shrimp associates

Both body size and habitat architecture have pervasive effects on the form, function, and interactions of organisms, and can play especially important roles in structuring intimate associations between host organisms and their obligate associates. In this study, we examined how body size is related to host use in a diverse clade of closely related snapping shrimp species (Synalpheus) that live in the interior canals of sponges. Using data from an extensive survey of sponge-dwelling Synalpheus from Jamaica, we tested how sponge morphology (interior canal size and individual sponge volume) was related to the identity and diversity of Synalpheus inhabitants. In cross-species comparisons, we found a strong positive correlation between Synalpheus species body size and sponge host canal size, using both raw species correlations and phylogenetic independent contrasts. Shrimp abundance increased with sponge volume in all sponge hosts tested, and species richness increased with volume in 2 host sponge species. Despite this evidence for a strong constraining influence of habitat architecture on shrimp communities, simulation studies demonstrated that shrimp used only a subset of appropriately sized sponges, indicating that size matching is not the sole determinant of sponge host use. Closely related sponges hosted more similar shrimp communities than unrelated sponges (despite moderate similarity in canal size between unrelated sponges), suggesting that additional genus-specific sponge traits also influence host use. Our study suggests multiple sponge traits likely limit Synalpheus host use, and has important implications for understanding how host use influences speciation of this diverse group.