chrisdef15;1981494; said:
...Bit off topic but question for the Aquarium working guys and girls ...have any of you seen Bull sharks kept in fresh there whole life? ....
Pioneer studies by ichthyologist Thomas Thorson on Bull Sharks of Lake Nicaragua revealed much of the preliminary studies which have been conducted on Bulls, and a few others about their 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.
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.
These and other studies have proven Bull Sharks can survive in fresh water for prolonged periods, with records of four to six years for some individuals, and in extreme isolated cases, documented individuals have appeared to live out their lives within a fresh environment, however, reports of reproduction on these isolated individuals - to my knowledge has yet to be submitted, which raises the questions as to how they were introduced to these areas, whether groups have lived in these areas before the time which it became an enclosed body, or if they were introduced by humans. There is a lot of speculation on these topics, a key factor in why research is ongoing for this and similar Elasmo species.
Wide documented studies support that Bull Sharks do not appear to reproduce in freshwater habitats. Instead, this species seems to return to brackish waters for courtship, mating, and bearing young. Bull Shark pups may not be able to meet the high metabolic cost of osmoregulating in fresh water, lacking a significant salt reserve at birth.
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, and this is one theory scientists have focused on for determining the sustainability and reproduction of Bulls in fresh water.
