Sponge species identity and morphology shape occupancy patterns of a Caribbean sponge-dwelling goby (Elacatinus horsti)

Due to disturbance-driven phase shifts on coral reefs, sponges are an increasingly important microhabitat for reef-dwelling organisms, particularly throughout the Caribbean. The abundance of sponge-dwelling species is expected to be heavily influenced by their degree of habitat specialization, as well as the distribution and morphology of potential host sponges. We conducted surveys along the fringing reefs of Curaçao to test how host sponge community composition and morphology drive occupancy patterns of the yellowline goby (Elacatinus horsti). The assemblage of goby-occupied sponges varied across the island, representing habitat heterogeneity across surveyed sites. We found E. horsti to be a sponge generalist, commonly occupying three species: Aplysina lacunosa, Aplysina archeri, and Neofibularia nolitangere, though all host species had some unoccupied sponges. A variable related to overall sponge size—maximum sponge height—significantly influenced the likelihood of goby occupancy. Goby group size in a sponge was also influenced by maximum sponge height along with the number of oscula and the species of host sponge. Specifically, group sizes were bigger in larger sponges with more openings, and the noxious sponge N. nolitangere hosted groups that were estimated to be 2.6–9.0 times larger than groups in other sponges when accounting for host morphology. Therefore, for these site-attached fish, the opportunity for conspecific interactions varies across different hosts. Collectively, our findings lay the groundwork for future investigations into the ecological trade-offs and fitness consequences of occupying distinct host species and examining the nature of fish-sponge symbioses.