Period of the day drives distinctions in the taxonomic and functional structure of reef fish assemblages

Marcos Lucena¹, Thiago Mendes¹, Cesar Cordeiro², Moyses Barbosa¹, Carlos Ferreira¹

¹Universidade Federal Fluminense, Brazil - ²Universidade Estadual do Norte Fluminense, Brazil

Circadian processes are major forces influencing animal behavior and activity to resource partitioning and trying to avoid competition. Studies evaluating circadian changes on the structure of marine assemblages are lacking, especially for reef fish. Evaluating the changes imposed by the day-night cycle on the structure and functioning of these assemblages is of critical importance to understand what are the differences of the diurnal/nocturnal components and what resources are comparatively explored, in order to better modelling and manage them. Here, we assessed the fish fauna through underwater visual census in the same sites comparatively during day and night in a subtropical rocky reef of the Southwestern Atlantic to evaluate differences on the community and functional structure of these assemblages. Fish richness, density, biomass and functional richness were higher in the diurnal assemblage, whereas nocturnal assemblages exhibited higher taxonomic distinctiveness and functional divergence. Both richness and density of nocturnal assemblage, plus richness and diurnal taxonomic distinctness index of diurnal assemblage were influenced by depth. The species of each assemblage showed different combinations of traits, and as expected, the diurnal assemblages being richer, occupying a larger proportion of the multidimensional trait space. Body size was comparatively larger for diurnal assemblage, evidencing niche partition according to ontogeny. These results indicate profound differences in the structure of diurnal and nocturnal reef fish assemblages likely driven by resources and space use as well as predator avoidance. Fish are engaged in many key processes in reef systems, apart from its importance for human nutrition and cultural services. Thus, understand the components and diversity of nocturnal assemblages is important to fill the huge gap existent.