Also.. FW Rays are very different in their osmosis abilities and how they handle ammonia secretion.. They have repressed rectal glands and this is why they can't travel to and from seawater. It also makes Ammonia much more lethal to them because they can not process it the same.
Studies of osmoregulation in marine and particularly fresh water elasmobranchs have been intermittently reported in the literature over the last 80 years. Although there has been significant research on elasmobranch osmoregulation, no study exists uniting the previous work into a single comprehensive report. This study examines previous research in elasmobranch osmoregulation and presents the results in a single, comprehensive review, covering topics including body fluid (solute and solvent) volume and concentration variations, body fluid synthesis, retention and secretion, in different elasmobranch species, in different habitats, having varying nutritional states, and in different life history stages.
In elasmobranchs, blood and other body fluids are separated from the surrounding aqueous environment by permeable surfaces. Osmoregulation depends on the relationship between the solute to solvent concentrations of both the internal body fluids and the outside medium that surrounds the animal (Pang et al, 1977). Unless internal and external fluids have the same solute to solvent concentration, water will enter the body when its fluids contain a higher concentration of solute to solvent than does the water comprising the environment. In contrast, water will leave the body when the surrounding medium contains a higher concentration (Pang et al, 1977). Thus, marine animals face problems of dehydration and the elimination of excess salts while freshwater animals must conserve their salts and eliminate excess water (Pang et al, 1977).
Marine elasmobranchs have evolved the technique of reabsorbing and retaining urea and other body fluid solutes in their tissues so that serum osmolarity (solute/solvent concentration) remains just greater than that of the external seawater (Smith, 1931; Thorson, 1962; Poulsen, 1981). This greatly reduces their osmotic challenges so that they do not need to continuously drink seawater, as do teleosts. However, they still face the problem of a natural and continuous diffusion of salts into their bodies from the external seawater, where the concentration is higher. This is compensated for by salt excretion in the urine, by secretions of the rectal gland, and salt transfer at the gill epithelium (Haywood, 1973).
Fresh water elasmobranchs retain and synthesize less urea than that of their marine counterparts. Their body fluid solute concentrations are relatively low and urine is dilute and copious (Thorson et al, 1967; Thorson, 1970; Goldstein and Forster, 1971; Poulsen, 1981). This greatly reduces their osmotic problem of water retention. The freshwater stingrays of South America have abandoned retention of urea, they lack a functional rectal gland and they osmoregulate as much as do the freshwater teleosts (Thorson et al, 1967; Thorson, 1970; Goldstein and Forster, 1971; Thorson, 1976; Poulsen, 1981).
Marine elasmobranchs face the problem of a natural and continuous diffusion of salts into the body from the external sea water, where the concentrations are higher (Haywood, 1973). The rectal gland of marine elasmobranchs functions as a salt secreting mechanism (Conte, 1969; Oguri, 1964; Haywood, 1975); however, disturbances produced by the cessation of its activity can eventually be compensated for internally by other means that are as yet unknown (Burger and Hess, 1960; Conte, 1969; Haywood, 1975).
Freshwater elasmobranchs do not face the problem of continuous diffusion of salts into the body and in fact, the rectal gland of Bull sharks moving from salt to freshwater becomes regressive (Oguri, 1964). No functional rectal gland in the freshwater rays, Potamotrygon, has been found (Goldstein and Forster, 1971).
Potamotrygon vs. Himantura - How they are SO different..
The white-edge whip tail ray Himantura signifer inhabits a freshwater environment but has retained the capability to synthesize urea de novo through the arginine-ornithine-urea cycle (OUC). The present study aimed to elucidate whether the capacity of urea synthesis in H. signifer could be upregulated in response to environmental ammonia exposure. When H. signifer was exposed to environmental ammonia, fairly high concentrations of ammonia were accumulated in the plasma and other tissues. This would subsequently reduce the net influx of exogenous ammonia by reducing the NH(3) partial pressure gradient across the branchial and body surfaces. There was also an increase in the OUC capacity in the liver. Since the ammonia produced endogenously could not be excreted effectively in the presence of environmental ammonia, it was detoxified into urea through the OUC.
In comparison, the South American freshwater stingray Potamotrygon motoro, which has lost the capability to synthesize urea de novo, was unable to detoxify ammonia to urea during ammonia loading. No increase in glutamine was observed in the various tissues of H. signifer exposed to environmental ammonia despite a significant increase in the hepatic glutamine synthetase activity. These results indicate that the excess glutamine formed was channelled completely into urea formation through carbamoyl phosphate synthetase III. It has been reported elsewhere that both urea synthesis and urea retention were upregulated in H. signifer exposed to 20 per thousand water for osmoregulatory purposes. By contrast, for H. signifer exposed to environmental ammonia in freshwater, the excess urea formed was excreted to the external medium instead. This suggests that the effectiveness of urea synthesis de novo as a strategy to detoxify ammonia is determined not simply by an increase in the capacity of urea synthesis but, more importantly, by the ability of the animal to control the direction (i.e. absorption or excretion) and rate of urea transport.
Our results suggest that such a strategy began to develop in those elasmobranchs, e.g. H. signifer, that migrate into a freshwater environment from the sea but not in those permanently adapted to a freshwater environment.
