Epiphytic hydroids (Cnidaria, Hydrozoa) contribute to a higher abundance of caprellid amphipods (Crustacea, Peracarida) on macroalgae

Habitat complexity is an important factor structuring macroinvertebrate communities on macrophytes, and epiphytic hydroids may offer additional habitats for the epifauna. Caprellids are known to benefit from the association with hydroids, but it is unclear if epiphytic hydroids affect their abundance. In this study, we investigated if caprellid abundance is related to epiphytic hydroid abundance on Sargassum cymosum, and whether this is related to caprellids' clinging behavior. We found that hydroid abundance had a positive effect on caprellid abundance, irrespective of species, sex, developmental and reproductive stages of caprellids, with the exception of Caprella scaura that present maternal care behavior. Experiments also showed that caprellids are more abundant on fronds of Sargassum with hydroids. Moreover, juvenile caprellids prefer to cling directly to hydroids and also prefer portions of the alga with hydroids attached. Our results indicate that epiphytic hydroids add structural complexity to macroalgae, favoring the occurrence of caprellids. However, this effect may be conditioned by the reproductive behavior of caprellid species, since hydroids are particularly important for juveniles by offering suitable structures for their small pereopods to grasp. This, in turn, may indirectly benefit adult caprellids, contributing to their choice of habitats that favor the performance of their juveniles.