Ecological Plasticity and Reproductive Strategies of the Smalleye Hammerhead (Sphyrna tudes) Across the Freshwater-Estuarine-Marine Continuum

This review synthesizes current knowledge on the ecological plasticity and reproductive strategies of the smalleye hammerhead, Sphyrna tudes (Valenciennes, 1822), within the freshwater–estuarine–marine continuum. Despite being one of the least studied hammerhead sharks, S. tudes exhibits one of the strongest known ecological associations with estuarine and river-influenced coastal environments in the western Atlantic. Available evidence consistently demonstrates that the species is closely linked to estuaries, mangrove systems, river mouths, and the Amazon–Orinoco river plume, where dynamic gradients in salinity, turbidity, productivity, and habitat connectivity shape multiple stages of its life history. Unlike highly euryhaline sharks that routinely penetrate freshwater systems, S. tudes appears to exploit ecological conditions generated by freshwater discharge rather than freshwater itself, representing an intermediate ecological strategy among coastal elasmobranchs. This synthesis identifies major knowledge gaps regarding movement ecology, physiological tolerance, reproductive behavior, and population connectivity while highlighting priorities for future research. The ecological significance of these findings is further evaluated within the framework of the Dynamic Environmental Reconfiguration Hypothesis (DERH), which proposes that environmental transitions, rather than discrete habitat categories, constitute major drivers of ecological specialization and life-history evolution. Collectively, the available evidence indicates that S. tudes represents an informative model for understanding how transitional aquatic ecosystems influence reproductive strategies, ecological plasticity, and conservation across coastal elasmobranchs.