Homeostatic adaptations of the amphibious fish Polypterus in terrestrial environments

The water-to-land transition in early tetrapods during evolution would have required a compensatory mechanism for the body-fluid homeostatic function originally undertaken by the gills in the water. We compared physiological and renal transcriptomic responses of the amphibious bichir Polypterus senegalus reared in aquatic and experimental terrestrial conditions. Plasma Na⁺ concentration, osmolality, and ammonia did not differ significantly between treatments (water: 106.3 mM, 207.3 mOsm/kg and 270.1 μg/dL; terrestrial: 103.0 mM, 201.5 mOsm/kg and 308.9 μg/dL). By contrast, transcripts for epithelial sodium channel (ENaC) subunits and for ammonia transporters (Rh proteins) were significantly upregulated in kidneys under terrestrial conditions. These results indicate that Polypterus maintains plasma homeostasis on land and that renal transcriptional plasticity — particularly increased expression of sodium and ammonia transporters — may partially compensate for diminished branchial function. These findings suggest that renal plasticity in basal actinopterygians could have facilitated early vertebrate water-to-land transitions.