Urbanization impacts on the ecomorphological structure of Neotropical stream fish assemblages

Abstract Urban expansion and agricultural activities are primary drivers of degradation in Neotropical streams, affecting their physical integrity and, consequently, their fish assemblages. This study compared fish assemblages between 12 urban and rural streams in the Paranapanema River basin, using a set of 21 ecomorphological indices. Sampling was conducted in April 2019 in 50 m reaches, and ecomorphological data were analyzed using PERMANOVA, NMDS, SIMPER, RDA, functional diversity indices, and null models. PERMANOVA analysis revealed significant differences in the ecomorphological structure of the ichthyofauna ( p = 0.003), corroborated by a clear separation between groups in the NMDS ordination. SIMPER analysis indicated that the pectoral fin aspect ratio, caudal peduncle compression, lateral compression, mouth aspect, and relative caudal peduncle length were the main contributors to the dissimilarity between streams, explaining > 50% of the variation. RDA showed that PPM (suspended solids) was the most significant environmental variable ( p = 0.04) structuring the assemblages. Fish assemblages in urban streams exhibited an ecomorphological pattern with higher values of caudal peduncle compression and relative caudal peduncle, traits adapted for short-distance swimming in turbid waters. In contrast, rural streams, with greater integrity, featured fish with higher lateral compression and pectoral fin aspect ratios, characteristics of structurally complex environments and adapted for higher speed and maneuverability. The results confirm that environmental changes caused by urbanization select for specific morphotypes and highlight the importance of conserving the physical integrity of streams for maintaining the ecomorphological biodiversity of fish communities.