Temporal variation in richness and composition of recruits in a diverse cnidarian assemblage of subtropical Brazil

Many studies have shown that the processes of colonization of new marine substrata, from settlement to recruitment, can leave long and lasting signals on the richness, composition and general structure of natural epifaunal assemblages. Systematic descriptions of temporal variability in patterns of richness and structure of recruits are scarce, partly because of logistical difficulties of working with multispecies assemblages of recruits. Here we quantify temporal variation in recruit richness, composition and structure of a rich cnidarian assemblage in southeastern Brazil, and evaluate the effect of microhabitat type and time of submersion on these patterns. We conclude that hydrozoan (the prevailing cnidarians in this assemblage) species richness occurs in a temporally bimodal pattern, with the majority of species divided between year-round recruiting species and temporally infrequently, non-seasonal species. This pattern does not depend on the species local abundance, and suggests that local species richness may be better estimated by increasing sampling efforts over time. Moreover, time since substrate submersion had no effect on species richness, but influences species composition, suggesting constant changes in the assemblage instead of accumulation of species through time. Finally, microhabitat variation, measured as differences between sheltered and exposed surfaces, despite influencing species' abundances, was not important for species richness.