Ecophysiology of Salinity Tolerance and Ionic Regulation in Estuarine Fishes of the Niger Delta: An Original Mechanistic and Evolutionary Analysis

This dissertation provides an original mechanistic and evolutionary analysis of salinity tolerance and ionic regulation in estuarine fishes of the Niger Delta, covering Rivers, Bayelsa, and Delta States, South South Nigeria. The Niger Delta estuary supports exceptional fish diversity at the freshwater-marine interface, and the ionic regulation capacities enabling fish to exploit this dynamic osmoregulatory environment have not been characterised at the molecular and physiological level for Nigerian estuarine species. The dissertation selects six co-occurring species representing a salinity tolerance gradient from stenohaline freshwater to euryhaline estuarine specialists. Ion transport enzyme activities (Na+/K+-ATPase, V-type H+-ATPase), gill ionocyte ultrastructure by transmission electron microscopy, plasma osmolality and ion concentrations, and whole-body water content are characterised across salinity challenges (0, 5, 15, and 25 ppt). Transcriptomic analysis by RNA-seq on gill tissue compares osmoregulatory gene expression between euryhaline and stenohaline species during salinity transfer. A phylogenetic comparative analysis tests whether osmotic breadth is associated with accelerated rates of ionoregulatory gene evolution across a molecular phylogeny of 40 Niger Delta fish species. The dissertation proposes the Delta Euryhaliny Model (DEM) as an original physiological-evolutionary framework, demonstrating that euryhaliny in Niger Delta fishes evolved through both up-regulation of conserved ionoregulatory mechanisms and recruitment of novel efflux transporter gene families. Original contributions include the first osmophysiology dataset for Niger Delta estuarine fishes and the DEM theoretical framework applicable to estuarine conservation planning. Keywords: osmoregulation, salinity tolerance, estuarine fish, Niger Delta, ecophysiology.