Intraspecific Trait Variation in Body Sizes Is Associated with Diet and Habitat Use: Evidence from <i>Atherinella brasiliensis</i> in a Tropical Estuary

Intraspecific variations in the morphological traits of juveniles and adults of the Brazilian silverside, <i>Atherinella brasiliensis,</i> from three estuarine habitats were studied to understanding whether their morphology interacts with their dietary composition and habitat structure. For each individual, fourteen morphological measurements and eight functional traits were recorded related to food acquisition and locomotion. The highest abundance of <i>A. brasiliensis</i> was recorded in mudflats, which were often associated with a greater number of juveniles. Overall, 392 <i>A. brasiliensis</i> stomachs were examined, and their diet comprised mainly zooplankton organisms, followed by insects and benthic crustaceans. Among the morphological measures, our data revealed that in vegetated habitats (seagrass and riparian vegetation), individuals showed a higher oral gape surface and caudal peduncle and fed predominately on epibiotic or benthic fauna, while for individuals that had bigger eyes in unvegetated habitats (mudflat), this facilitated the ingestion of zooplankton and diatoms. Furthermore, a greater relative body height recorded in unvegetated habitats enhanced swimming performance and was linked to the effects of the lowest habitat structure. The results highlight the significant effects of morphological variation on juvenile and adult food acquisition and swimming ability.