Dispersal Across Headwaters Determines Fish Population Structure Between Interdigitating River Systems in the Guiana Shield Highlands.

Riverine aquatic species primarily occupy the dendritic networks that define river basins, and population-structure models for riverine taxa, including freshwater fishes, generally assume strictly longitudinal dispersal within these networks. River configurations, however, are dynamic: hydrologic connections change seasonally and over geologic time, potentially enabling out-of-network dispersal, a process that remains poorly studied. Because such dispersal violates key assumptions of existing models, its prevalence may require the development of new frameworks for understanding riverine population structure. To test for out-of-network connectivity between interdigitating river systems, we analyzed genetic population structure in Krobia potaroensis, a cichlid fish endemic to the rivers draining the Pakaraima Mountains in western Guyana, South America. We detected genetic associations among populations that do not correspond to contemporary river configurations, indicating historical or ongoing dispersal between adjacent river systems. These results demonstrate that within-network longitudinal dispersal cannot always be assumed and that alternative dispersal routes can shape population structure, genetic diversity, and genetic differentiation in riverine taxa.