Transcriptomic insights into renal osmoregulation and environmental adaptation in two hammerhead shark species (Sphyrna zygaena and Sphyrna lewini).

Elasmobranchs maintain osmotic balance through specialized renal mechanisms regulating ion transport, nitrogen conservation, and epithelial integrity. Although euryhaline species exhibit adaptive physiological responses to salinity variation, the molecular bases of these strategies remain poorly understood at the species level. Here, we performed comparative transcriptomic analysis of kidney tissues from two sympatric hammerhead sharks, Sphyrna lewini and Sphyrna zygaena, collected under comparable marine salinity conditions (~ 26-28‰). RNA-seq identified 16,543 differentially expressed genes (DEGs), of which 11,614 were successfully annotated across multiple databases. Enrichment analysis highlighted pathways related to peroxisome function, amino sugar and nucleotide sugar metabolism, lysosomal activity, and fatty acid metabolism. S. lewini, a more dilution-tolerant species reported from coastal, estuarine, and occasionally riverine habitats, exhibited higher expression of genes related to mitochondrial respiration, ion transport (e.g., Na⁺/K⁺-ATPase, NKCC2, V-type H⁺-ATPase), redox regulation, and membrane remodeling, indicating an energetically demanding osmoregulatory strategy involving active ion transport and renal urea retention. In elasmobranchs, maintaining elevated urea concentrations is metabolically costly due to the need for continuous synthesis, reabsorption, and protection of cellular proteins from urea-induced destabilization. In contrast, S. zygaena, which is more commonly associated with shelf and offshore habitats, showed elevated expression of genes associated with extracellular matrix organization, tight junction structure (e.g., claudins), glycosaminoglycan modification, and endoplasmic reticulum ion buffering (e.g., TRIC-B), supporting a structurally reinforced kidney epithelial phenotype adapted to relatively stable marine salinities. These findings provide comparative molecular insights into species-specific osmoregulatory strategies and offer a framework for understanding physiological differentiation among closely related elasmobranchs.