Adaptation of teleosts to very high salinity

A number of species of euryhaline teleosts have the remarkable ability to adapt and survive in environments of extreme salinity, up to two or even three times the osmolality of seawater. This review looks at some of the literature describing the adaptive changes that occur, primarily with intestinal water absorption and with the properties of the gill epithelium. While there is much that is still not completely understood, recent work has begun to look at these adaptations at the cellular and molecular level. As with seawater osmoregulation, fish adapting to hypersaline conditions generally increase drinking rates and water absorption across the intestine by solute-linked transport. This process requires increased activity, expression levels and possibly changes in subunit isoforms of Na(+)/K(+)-ATPase pumps, along with increases in other components of the NaCl absorptive pathway. Additionally, recent studies have demonstrated the importance of luminal anion exchange activity and of luminal alkalinization, which can support absorption against increasing osmotic gradients by promoting divalent ion precipitation (CaCO(3)) and by conversion of HCO(3)(-) ions to CO(2). The steepness of the lumen to blood osmotic gradient vis-a-vis the NaCl absorptive capacity, along with the accumulation of non-absorbed Mg(++) and SO(4)(--), likely become the limiting factors for survival in hypersaline conditions. Of interest is the observation of predicted hyperosmotic fluid absorption by the intestine in several species. Adaptive changes in the gill epithelium are also critical in this process, allowing for secretion of absorbed NaCl from the extracellular fluids. Most notably there are important changes in the numbers and size of mitochondrion-rich (MR) cells, the sites of active secretion of Cl(-), which ultimately drives the overall process of NaCl secretion. Balance studies of intake and output clearly indicate that a decrease in the osmotic permeability of the gill epithelium must also occur. The molecular correlates of this effect are not known, although decreased expression of one or more aquaporins seems to be a likely possibility. Finally, the regulatory changes seen with hypersaline adaptation may provide important new insights into epithelial function, including the role of organized transport assemblies ("metabolons") and changes in the expression of tight junction proteins such as claudins or occludins, which may modulate electrolyte permeabilities.