Reproductive Strategies Across the Freshwater-Estuarine-Marine Continuum in Coastal Elsmobranchs: Insights from Comparative Life-History Patterns

Abstract

The freshwater–estuarine–marine continuum represents one of the most dynamic ecological gradients occupied by aquatic vertebrates, yet the reproductive significance of habitat connectivity across this environmental mosaic has rarely been evaluated from a broad comparative perspective. Although numerous elasmobranchs exploit freshwater, estuarine, and marine environments during different stages of their life cycle, most studies have focused on individual species, limiting our understanding of large-scale ecological patterns. Here, we present a comparative synthesis of the reproductive ecology of 42 elasmobranch species, including sharks, rays, and sawfishes, that regularly utilize freshwater, estuarine, and marine habitats. We reviewed available information on distribution, reproductive biology, nursery habitat use, ontogenetic habitat shifts, population structure, and conservation to identify recurring life-history patterns across phylogenetically independent lineages. Despite substantial taxonomic diversity, the analyzed species consistently exhibited reproductive strategies associated with ecological connectivity rather than exclusive dependence on a single habitat type. Estuaries emerged as recurrent centers of reproductive activity, functioning as nursery areas, pupping grounds, juvenile developmental habitats, and ecological interfaces linking freshwater and marine ecosystems. The degree of habitat dependence varied among taxa, ranging from highly specialized river sharks and sawfishes to predominantly marine carcharhinids and batoids that nevertheless relied upon estuarine environments during critical reproductive stages. Collectively, these findings indicate that transitional aquatic habitats constitute functional ecological systems that repeatedly enhance reproductive success across multiple elasmobranch lineages. The observed convergence is consistent with the Dynamic Environmental Reconfiguration Hypothesis (DERH), supporting the interpretation that ecological connectivity represents an adaptive evolutionary strategy shaping life-history traits across the freshwater–estuarine–marine continuum. Beyond advancing understanding of reproductive evolution, this synthesis emphasizes that effective conservation of coastal elasmobranchs requires protecting ecological connectivity among rivers, estuaries, and adjacent marine habitats rather than managing these environments as isolated ecosystems.