Disentangling the spatial distributions of a sponge-dwelling fish and its host sponge

Characterizing spatial patterns of occurrence can lend insight into the ecological processes that determine how individuals are distributed within their environment. When microhabitat specialist fishes and their invertebrate hosts are co-distributed, disentangling their respective spatial patterns is a complex problem. Here, we use point pattern analysis (PPA) to examine the spatial distributions of a host sponge Aplysina fistularis and its resident goby Elacatinus lori from a fully censused plot at Curlew Cay, Belize (16°47′23″N 88°04′33″W), sampled in summer 2011. The PPA approach allowed us to disentangle the spatial distribution of sponges and the spatial distribution of goby-occupied sponges. After controlling for depth and the distribution of hard substrate, we found that the sponges were clustered at small scales (< 4.5 m) within the censused area. After controlling for sponge clustering, we found that goby-occupied sponges were neither clustered nor over-dispersed within the censused area. Two fish age classes, recent settlers and established residents, were closely associated at small scales (< 3.5 m). We discuss alternative ecological and behavioral hypotheses for the cause of these spatial patterns. Despite the limited application of PPA in marine ecology, we demonstrate the potential use of this statistical analysis in disentangling the spatial structure of co-distributed populations and providing preliminary insights into the processes that may account for their respective distributions.