Sponge presence increases the diversity and abundance of fish and invertebrates in a subtropical seagrass bed

Ecosystem engineers (e.g., seagrasses) can have profound effects on biodiversity and community structure. Sponges possess many traits (e.g., create complex structure) that suggest they may act as ecosystem engineers, but we know little about their relationships with the animal communities that inhabit seagrass beds. This study explored the effects of the marine sponge, Ircinia felix, on fish and invertebrate taxa richness, abundance, turnover, and community composition in a seagrass bed through a 1-year field experiment performed in The Bahamas. We recorded the fish and invertebrate communities present in 5 × 5 m plots with the addition of either a live sponge, a polypropylene sponge replica (structure), or no additional structure (control). Both taxonomic groups responded similarly to the addition of a live sponge. Taxa richness and abundance increased at least 4-fold over a year in the presence of I. felix but decreased 2-fold or more in the other plot types. Live sponge presence increased the number of new taxa gained by at least 24% and reduced the number of taxa lost compared to other plot types. Finally, we found that the fish community shifted in a more consistent direction in the presence of a live sponge than in the other plots. Our study shows that sponges can act as ecosystem engineers in subtropical seagrass systems; however, additional research is needed to determine the full extent and implications of their ecological effects.