Fine‐Scale Bioregionalization in a Complex Island System: A Freshwater Perspective From the Greater Antilles

ABSTRACT Aim The isolated nature of islands makes them particularly vulnerable to climate change. The freshwater fishes of the Greater Antilles remain understudied and are further threatened by habitat fragmentation and the introduction of exotic invasive species. A practical solution to protect this diversity is to develop a spatially explicit framework to inform conservation actions. We test whether biogeographically significant subregions can be identified within the Greater Antilles. Location The Greater Antilles, Caribbean Basin. Taxa Freshwater Fishes (Atheriniformes, Cichliformes, Cyprinodontiformes, Lepisosteiformes, and Ophidiiformes). Methods We compiled a dataset of 1704 occurrence records across the Greater Antilles based on museum vouchers and the authors' fieldwork. River basins were used as analytical units for hierarchical agglomerative clustering both among and within islands. We also combined genetic data to examine phylogeographic patterns and uncover potential substructure within islands. Results Ninety‐two per cent of the freshwater fish fauna of the Greater Antilles is endemic. Although diversity is unevenly distributed among and within islands, patterns of species richness and weighted endemism partially align across islands. The region is not a single biogeographic unit but consists of multiple subregions of biogeographic significance, where river basins from the same island are more similar to each other than to basins from other islands, with Hispaniola as the only exception. Main Conclusions The Greater Antilles comprise a mosaic of biogeographically meaningful subregions, each harbouring unique, highly vulnerable, and often range‐restricted species assemblages. The patterns recovered reflect a complex interplay among the region's geological history, multiple colonization events, and diversification processes. Our proposed areas of endemism represent testable hypotheses for investigating the evolutionary history of freshwater taxa in the region. These areas also provide natural management units that can serve as geographical targets for conservation prioritisation and policymaking for freshwater ecosystems across the Greater Antilles.