Ecological plasticity and evolutionary connectivity in Glyphis: implications for euryhaline shark evolution

The genus Glyphis represents one of the least understood groups among modern elasmobranchs, exhibiting fragmented distribution and a high conservation risk. Traditionally interpreted as predominantly freshwater-associated, recent fossil, molecular, phylogenetic and ecological evidence suggests a more complex evolutionary history, marked by salinity plasticity and functional connectivity among fluvial, estuarine, and marine environments. This narrative review synthesizes studies published between 2008 and 2024, integrating data on taxonomy, genetic connectivity, osmotic physiology and habitat use, in addition to comparative analyses with related euryhaline elasmobranchs. The evidence analyzed indicates that the occupation of fluvial environments in Glyphis may represent a derived condition, consistent with an ancestral euryhaline condition and intermittent marine dispersal. These findings have direct implications for the conservation of tropical ecological corridors and for advancing our understanding of the ecological evolution of threatened elasmobranchs. This review proposes that Glyphis represents a secondarily river-associated lineage, retaining ancestral euryhaline plasticity and recurrent marine connectivity across its life cycle.