Biogeography and Diversity Patterns of Hydroids (Cnidaria, Hydrozoa) from the Southwest Atlantic Coast

This study investigates hydroid species distribution across the western Atlantic coastline, focusing on biogeographic patterns, comparing them with Caribbean assemblages, and assess the influence of environmental variables—including salinity, temperature, primary productivity, ocean currents, and chlorophyll—on biogeographic structure. We analyzed 375 species from to 9259 records (1946–2022), spanning the western Atlantic from the Caribbean to southern Argentina (28° N–53° S). Cluster analyses using UPGMA and ordination via nMDS, based on Sorensen-transformed occurrence data. Taxonomic distinctness was assessed with Average Taxonomic Distinctness (Delta+) and Lambda+ variation. UPGMA clustering revealed two main groups: one in the Caribbean and Brazil, and another in southern Brazil, Uruguay, and Argentina. The Amazon River mouth acted as a semi-permeable barrier, with 21.4% species overlap between Caribbean and Brazil. Southeastern Brazil had the highest species richness, likely due to environmental synergy and biodiversity hotspot. Assemblages followed known biogeographic gradients, with lower diversity offshore and on islands. The Río de la Plata showed a distinct, salinity-driven composition. Salinity, chlorophyll, and currents were key distribution drivers.