The great ecological tolerance of aphaniid tooth-carps (Aphaniidae) suggests that the genetic-based adaptation to divergent osmoregulatory environments may be an important driver of the aphaniid fish diversity. This study was conducted to assess gill mRNA expression levels of two osmoregulatory genes, i.e., the CLC-3 and the Na+/K+-ATPase in two tooth-carps inhabiting the freshwater (FW) spring and salty water (SW) river. The expression pattern of genes was examined by quantitative real-time PCR. The number of ionocytes in the gill tissues was counted and compared using histological sections. The expression of CLC-3 in the FW fishes was 3.4 fold higher than that in the SW specimens, which highlighted the role of CLC-3 as a hyposmotically activated chloride channel. The expression of Na+/K+-ATPase was increased by 5.6 fold in the SW fishes. This upregulation in the SW fishes leads to the excretion of excess ions and maintains the homeostasis. The histological sections demonstrated that the ionocytes were distributed in the interlamellar region of the gill filament and that the average number of ionocytes was more in the SW fishes. This is probably an ecophysiological response to the saline condition of the habitat. The results of this study revealed that the examined osmogenes likely play an essential role in the adaptive responses of the studied killifishes to the variable osmotic conditions.
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