Cryptofauna Associated With the Epilithic Algal Matrix on Subtropical Brazilian Reefs

The epilithic algal matrix (EAM) plays a significant role in the benthic cover of reefs worldwide. In Brazil, the EAM accounts for a substantial proportion of the benthos, and its associated fauna contributes to the majority of the reef's metazoan biodiversity and is an important link into the trophic chain, connecting the production performed by macroalgae to higher levels of consumers such as fish and crabs. Yet, knowledge about this microhabitat is practically null. In this study, we sampled the EAM‐associated cryptofauna at three sites within a subtropical Brazilian coastal reef, at depths from 3 to 15 m. We recorded a total of 13 taxa, belonging to 6 phyla/subphyla. Crustaceans, particularly Caprellidae amphipods, were the most abundant group, followed by gastropod molluscs. The algal component is known to exert great influence over its associated fauna as it offers niches for colonisation, refuge, and food resources. Algal biomass positively influenced the abundance of the cryptofauna, its composition, and the abundance of crustaceans and molluscs. Coarse particulates influenced both the total community abundance and crustaceans, likely reflecting the group's detritivorous feeding habits. Depth had a positive effect on polychaetes, possibly attributed to competition with dominant taxa or size constraints imposed by the more dense algal mats in shallower depths. Our findings suggest that, similar to macroalgal fronds, the EAM algae shapes the abundance and composition of its associated cryptofauna. Given that the EAM and its cryptofauna likely represent the primary food resource for reef fishes and macroinvertebrates, understanding their structure, composition, and dynamics is essential, as any impacts on these components are expected to cascade through trophic levels, ultimately affecting the entire reef ecosystem and the human populations that depend on it.