Why living marine Catfish Plotosus lineatus in Persian Gulf is unique amongst teleosts?

Plotosidae marine catfish Plotosus lineatus reportedly have an extra-branchial salt secreting dendritic organ (DO) unlike other marine teleosts. 10 days acclimation in three salinity ranging from [seawater (SW control) 34‰, brackishwater (BW) 3‰ and hypersaline water (HSW) 60‰] was used to investigate the osmoregulatory abilities of P. lineatus through measurements of physiological parameters including blood chemistry, Na+/K+-ATPase (NKA) activity and ion transporter expression in DO and gills using immunohistochemistry. A significant osmoregulatory challenge results from elevated mortality (36%), plasma osmolality and ions, and hematocrit in HSW. At all salinities DO NKA activity and protein were significantly higher than gills. DO mass thus total DO NKA activity was higher, indicating higher overall capacity at HSW. NKA and NKCC1 co-localization was rare in gills ionocytes while it was observed strong in DO parenchymal cells. Using IB detected NKCC1 expression only in DO which was highest at HSW and DO NKA-IR cells showed apically CFTR localization. Taken together, the observed high NKA activity in DO, strong basolateral localized with NKCC1 and apical CFTR indicates the presence of the conserved secondary active Cl- secretion mechanism found in other ion transporting epithelia.