The physiology of hyper-salinity tolerance in teleost fish: a review

Abstract

Hyper-saline habitats (waters with salinity >35 ppt) are among the harshest aquatic environments. Relatively few species of teleost fish can tolerate salinities much above 50 ppt, because of the challenges to osmoregulation, but those that do, usually estuarine, euryhaline species, show a strong ability to osmoregulate in salinities well over 100 ppt. Typically, plasma Na(+) and Cl(-) concentrations rise slowly or not at all up to about 65 ppt. At higher salinities ion levels do rise, but the increase is small relative to the magnitude of increase in concentrations of the surrounding water. A number of adjustments are responsible for such strong osmoregulation. Reduced branchial water permeability is indicated by the observation that with the exposure to hyper-salinities drinking rates rise more slowly than the branchial osmotic gradient. Lower water permeability limits osmotic water loss and greatly reduces the salt load incurred in replacing it. Still, increased gut Na(+)/K(+)-ATPase (NAK) activity is necessary to absorb the larger gut salt load and increased HCO(3) (-) secretion is required to precipitate Ca(2+) and some Mg(2+) in the imbibed water to facilitate water absorption. All Na(+) and Cl(-) taken up must be excreted and increased branchial salt excreting capacity is indicated by elevated mitochondrion-rich cell density and size, gill NAK activity and expression of chloride channels. Excretion of Na(+) and Cl(-) occurs against a larger gradient than in seawater and calculation of the equilibrium potential for Na(+) across the gill epithelium indicates that the trans-epithelial potential required for excretion of Na(+) climbs with salinity up to about 65 ppt before leveling off due to the increasing plasma Na(+) levels. During acute transition to SW or mildly hyper-saline waters, some species have shown the ability to upregulate branchial NAK activity rapidly and this may play an important role in limiting disturbances at higher salinities. It does not appear that the opercular epithelium, which in SW acts in a way that is functionally similar to the gills, continues to do so in hyper-saline waters. Little is know about the hormones involved in acclimation to hyper-salinity, but the few studies available suggest a role for cortisol, but not growth hormone and insulin-like growth factor. Despite the increased transport capacity evident in both the gill and gut in hyper-saline waters there is no clear trend toward increased metabolic rate. These studies provide a general outline of the mechanisms of osmoregulation in these species, but significant questions still remain.

Osmoregulatory strategies in natural populations of the black-chinned tilapia Sarotherodon melanotheron exposed to extreme salinities in West African estuaries

“…Laboratory studies conducted in several euryhaline teleosts, including the black-chinned tilapia, showed a signiWcantly increased branchial NKA subunit expression (and in some species also the subunit) in hypersaline conditions (Jensen et al 1998;Deane and Woo 2004;Tine et al 2008;Gonzalez (2011)). In the European sea-bass D. labrax for example, a "U-shaped" salinity dependency has been shown for the branchial NKA ( subunit) with lowest expression levels at the iso-osmotic point (15 ‰) and highest expression levels at extreme salinities (0 and 50 ‰) (Jensen et al 1998).…”

“…To oVset the loss of water, teleosts need to decrease their branchial water permeability and increase their drinking rate. In a recent review on Wsh physiology of hypersalinity tolerance, Gonzalez (2011) stated that in several species, drinking increases with salinity, but only at a slow rate regarding hypersaline media. Active monovalent ion absorption in the intestine followed by osmotic uptake of water (Ando et al 2003), as well as, higher rates of HCO 3 ¡ secretion is necessary to prevent dehydration and ion load.…”

Changes in plasma osmolality, cortisol and amino acid levels of tongue sole (Cynoglossus semilaevis) at different salinities

“…Salinity is one of the most important abiotic factors affecting the growth of fishes and has complex and wideranging biological effects (Boeuf and Payan, 2001;Jamil et al, 2004;Gonzalez, 2012;Webb et al, 2012). Cortisol is an important glucocorticoid that functions in the osmotic acclimation and stress response of teleosts (Mommsen et al, 1999;McCormick, 2001;O'Connor et al, 2011;Pankhurst, 2011).…”

Fortification of an Aquafeed with Potassium Chloride Does Not Improve Survival of Juvenile Australian Snapper Pagrus auratus Reared in Potassium Deficient Saline Groundwater

“…The gastrointestinal tract (GI tract) of marine fish plays a critical role in osmoregulation via the process of NaCl coupled fluid absorption [18][19][20][21]. However, the dual role of the intestine, serving hydromineral functions as well as the digestion and absorption of nutrients, is often in conflict and many of the interactions are not well understood [17][18][19][22][23][24][25]. It is known that consumption of large meals of dry aquafeed immediately increase the load of minerals in the GI tract, drastically changing the ionic balance of the intestine [21,24].…”