Freshwater Sharks and Rays
Although most people think of sharks and rays as exclusively saltwater inhabitants, quite a few species actually make use of freshwater living spaces. In a 1995 paper, elasmobranch systematist Leonard Compagno and shark fisheries biologist Sid Cook surveyed the diversity and distribution of freshwater sharks and rays. Compagno and Cook reported that some 43 species of elasmobranch in 10 genera and four families penetrate freshwater environments in Australia, Southeast Asia, western Africa, eastern South America, Central America, and southeastern parts of North America. Coastal elasmobranchs, including some skates (family Rajidae), smooth dogfishes (Triakidae), pajama catsharks (Poroderma spp.), and Sandbar Sharks (Carcharhinus plumbeus) regularly enter estuaries to feed or give birth to young. More impressive yet are stingrays (Dasyatidae, Potamotrygonidae, and others), sawfishes (Pristidae), and the notorious Bull Shark (Carcharhinus leucas), all of which penetrate far up freshwater rivers — the Bull Shark has been recorded some 2,600 miles (4,200 kilometres) from the mouth of the Amazon River — and some even complete their life cycles in freshwater. Yet, in theory, it should be next to impossible for a 'saltier than seawater' elasmobranch to survive in fresh water. How do these freshwater sharks and rays manage this osmoregulatory feat?
Pioneer studies by ichthyologist Thomas Thorson on Bull Sharks of Lake Nicaragua have revealed much about this species' ability to osmoregulate in freshwater habitats. Thorson discovered that Bull Sharks — traditionally thought to be a distinct, land-locked species — readily traverse the rapid-laden Rio San Juan that connects Lake Nicaragua to the Caribbean Sea. Thorson and his co-workers tested the blood and other body fluids of Bull Sharks caught in the river mouth on the Caribbean side and found that their internal concentration of solutes (sodium, chloride, urea and TMAO) are identical to those of Bull Sharks from full-strength seawater elsewhere. Bull Sharks caught in the Lake, however, had a total osmotic pressure about two-thirds that of marine values. Thorson and his co-workers found that the loss of osmotic pressure is due to a 20 percent reduction of bodily sodium and chloride (excreted via the rectal gland) and a more than 50 percent reduction of urea. Bull Sharks therefore adapt to freshwater by reducing the concentration gradients of solutes that favor the uptake of water and the loss of salts and urea to the environment.
Despite a reduced solute concentration in freshwater, Lake Nicaragua Bull Sharks still have body fluids more than twice as 'salty' as typical freshwater fishes. They must, therefore, experience a massive influx of water. This water influx is almost certainly dealt with by the kidneys, resulting in copious excretion of dilute urine. An early measurement of urine production by a freshwater sawfish indicated a daily flow rate of about a cup (250 millilitres) per kilogram of body mass — more than 20 times that of a typical marine elasmobranch. Suck kidney activity must be energetically expensive. Yet tagging studies by Thorson and his co-workers has demonstrated that Bull Sharks can survive in Lake Nicaragua for prolonged periods, with records of four to six years for some individuals. Bull Sharks do not, however, appear to reproduce in Lake Nicaragua or other freshwater habitats. Instead, this species seems to return to brackish water for courtship, mating, and bearing young. Bull Shark pups may not be able to meet the high metabolic cost of osmoregulating in fresh water.
When living tissues of elasmobranchs are isolated for study, urea must be present in the solutions bathing them. Otherwise the urea-deprived cells do not seem to function properly. Thus, shark and ray cells are so adapted to urea that they cannot survive without it. This is probably why elasmobranchs like Bull Sharks and sawfishes that are able to tolerate a wide range of salinities do not completely lose their urea after entering fresh water. But one group of elasmobranchs has lost its dependence on urea. These are the South American river stingrays of the family Potamotrygonidae. This group of about 26 closely related species inhabits at least seven freshwater drainage systems on the South American continent. Their ancestors were marine and most likely trapped on the shallow western side of South America when the tectonic uplift of the Andes Mountains occurred in the early-to-middle Cretaceous Period (about 100 million years ago). By the close of the Cretaceous (about 65 million years ago), their isolation and freshening of their waters by runoff were complete.
Today the potamotrygonid stingrays have an osmotic physiology distinctly different from all other elasmobranchs. Their blood solute concentration measures about half that of freshwater-tolerant elasmobranchs and roughly equals that of freshwater teleosts. Moreover, the body fluids of these river stingrays have very low levels of urea. While they do have rectal glands, this organ is greatly reduced in size and apparently does not excrete salts. Experiments attempting to acclimate potamotrygonids to oceanic salinities have failed, the upper limit of their salt tolerance being about 50 percent that of seawater. At this salinity, the body fluids of these rays are almost in osmotic equilibrium due to the uptake of sodium and chloride by diffusion and a slight increase in urea retention. While the river stingrays have retained the ability to manufacture urea, they have lost the ability to concentrate it and other solutes at levels enabling them to function in the sea. Thus the potamotrygonid rays are prisoners in fresh water, trapped by their highly specialized osmoregulatory physiology.
The infamous White Shark has never been recorded from fresh or brackish waters. Despite its reputation for toughness, the mighty Great White may be just as osmotically helpless in freshwater as the potamotrygonid stingrays are in salt. Yet it may be premature to conclude that White Sharks cannot tolerate reduced salinities: in January 1997, a juvenile Porbeagle (Lamna nasus) — a close relative of the Great White — was captured in brackish water at the mouth of the Mar Chiquita Lagoon, Argentina.