Latitudinal gradients shape fish trait-environment relationships across estuarine and oceanic beaches along the Brazilian coast

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Understanding how environmental filters shape species traits along latitudinal gradients is crucial for evaluating biodiversity. We assessed the relationships between fish functional traits and environmental variables in estuarine and oceanic beaches along the Brazilian coast. We hypothesized stronger trait-environment relationships in estuarine beaches due to greater environmental gradients and habitat heterogeneity, driving more trait variation than highly dynamic but more environmentally homogeneous oceanic beaches. Additionally, we expected latitudinal variation in these relationships, driven by broad-scale abiotic and biotic gradients that influence ecological filtering, such as temperature, productivity, and tides. Our results show stronger trait-environment relationships in estuarine beaches, where well-defined gradients (e.g. salinity, nutrients, turbidity) and high variability (e.g. tides, rainfall) shape fish traits. Conversely, oceanic beaches exhibit more dynamic conditions but weaker environmental gradients, with latitude playing a strong role in shaping traits. Salinity was a driver on estuarine beaches, with higher levels favoring reef-associated fish, non-parental care, and herbivores, while lower levels favored freshwater species and generalists. At higher latitudes on oceanic beaches, elongated-circular species increased while flat-bodied species decreased. Euryhaline fish increased at latitudes >20° S. On estuarine beaches, benthopelagic fish increased and reef-associated species decreased toward higher latitudes. Herbivores were rare and mainly found at latitudes <20° S, while small-sized fish (<15 cm total length) were more common at higher latitudes in both habitats. Together, these findings highlight the importance of environmental filters and latitude in shaping fish functional traits across coastal habitats, offering key insights for biodiversity conservation and ecosystem management in dynamic tropical and subtropical regions.

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