Exploring Riverine Conditions and Their Impact on Fish Diversity: An Ecological Speciation Perspective

The ecological speciation of fish has traditionally been studied in terms of adaptive radiation in lacustrine ecosystems, while such processes in riverine environments have remained comparatively understudied. This review synthesizes current evidence that rivers, with their dynamic flow regimes, ecological gradients, and geomorphological complexity, can promote diversification processes similar to adaptive radiation. Extreme habitats such as caves, sulfide springs, acidic blackwaters and thermal outflows impose strong selective pressures that drive the parallel specialization of physiological and morphological traits across multiple fish lineages. Likewise, hydrological barriers including rapids and waterfalls promote partial isolation, relaxation of natural selection and ecological divergence. Depth gradients within large tropical rivers, such as the Amazon and the Congo, replicate lacustrine conditions, generating convergent adaptations to low light and high pressure. Together, these cases illustrate that flowing waters can provide both ecological opportunity and reproductive isolation, producing evolutionary patterns comparable to those observed in lakes. However, distinguishing adaptive radiations from allopatric or single-axis speciation requires integrated genomic, ecological, and morphological approaches. Combining phylogenomics, niche modelling and experimental assays in future work will clarify the extent to which riverine environments contribute to global fish biodiversity through ecological speciation.