Sharks are prehistoric creatures that have been around for 450 million years. They lived before land vertebrates. This fact alone makes sharks record breaking evolutionary creatures. Sharks are a member of the "elasmobranch" collection of fish because they have a skeleton made of cartilage instead of bone. Elasmobranch is not the only cartilaginous fish. The species are divided according to shapes, features and body shape. Elasmobranchs are members of a larger group called Chondrichthyes, which include skates and rays which have flattened bodies in lieu of the torpedo-like body of the shark. All have the rigid dorsal fin as well as other similar characteristics. We usually think of sharks as ocean dwellers; however their anatomy has evolved over the last million years making it possible for them to survive in fresh water and inshore marine waters. There are several species of elasmobranch that can survive in both fresh water and salt water. Documented studies have proven that some migratory elasmobranch species travel in both marine and freshwater environments throughout their life cycle. (1). One of these species is the bull shark, Carcharhinus leucas, which is able to exist for long periods of time in fresh water and can travel long distances up river. Although most fresh water elasmobranchs are found in tropical and warm water temperatures, bull sharks are not confined to brackish waters and have been spotted along the Mississippi River as far north as Madison County, Missouri and in the rivers of Nicaragua. Often adults can also be found near estuaries and freshwater inflows to the sea. Certain species of bull sharks reproduce exclusively in freshwater, which explain why their presence is so widespread in the coastal and freshwater bays, estuaries, rivers, and lakes. Although they easily migrate worldwide from warm shallow marine waters into fresh water environments, bull sharks are not true fresh water sharks.
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What may have contributed to the prolific growth of this particular freshwater elasmobranch is its exclusive predator status in freshwater environments. The bull shark is extremely large in length and weight ranging from about 12 feet in length to about 1,700 pounds in weight. Using a bump and bite technique to disorient its prey, the bull shark will eat whatever is readily available including other sharks, fish, turtles, sting rays, mollusk and birds. Depending on its habitat and location, its only predator is alligators, crocodiles and other sharks such as the tiger shark and great white shark. In fresh water, the bull shark is relatively unthreatened. (10) The bull shark is considered to be the most aggressive and most dangerous species of shark because of its near-shore attacks on humans. Yet it is not the stocky body or the aggressive and unpredictable behavior that has allowed the bull shark to survive for millions of years. It is its molecular physiology to evolve into a euryhaline species which can tolerate a wider range of salinity and its ability to flourish in both fresh water and salt water. These physiological factors are the clues to the bull shark's elasmobranch evolution into a freshwater variant of the species.
Osmoregulation is an animal's ability to maintain a constant concentration of water inside its body, even when the external environment should cause it to lose or gain water. The mechanism for osmoregulation in sharks in a marine environment is generally the high concentrations of urea in their blood, and the removal of the extra salt is primarily through the kidneys. The rectal glands play a key role in elasmobranch osmoregulation and their contribution to the drinking and eating processes that maintain osmotic consistency. Sharks evolved in salt water and their blood and body fluids are as salty as the seawater surrounding them. Marine elasmobranchs drink saltwater to balance their osmotic pressure. Their bodily fluids contain small organic molecules; the most important of these are urea and trimethylamine oxide (TMAO). Retention of high concentrations of urea and TMAO add significantly to a shark's osmotic pressure. They reabsorb and retain urea in their blood and other body fluids and excrete excess accumulated salt in the urine to maintain their optimal, constant osmotic pressure. Urea is important in marine elasmobranchs and must be reduced in dilute envirornments. In fresh water, a marine elasmobranch has firm limitations on the amount of time it can survive. It must take in more water to regulate the relationship between the solute and the solvent concentrations which presents the problem of dehydration and the excess elimination of salts. Reduced urea conserving areas of fresh water elasmobranch kidneys have been observed. (3)
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Studies indicate that fresh water sharks have twice the amount of salt in their bodies as other fresh water fish. The bull shark must maintain an optimal constant osmotic pressure in its body in relation to the surrounding fluid to survive its life cycle in a fresh water habitat. Bull sharks have developed an ability to adapt their process of osmoregulation to compensate for the difference in the salinity of the water and consequently can tolerate a wider range of salinity because they evolved the technique of retaining less urea in their blood and other body fluids so that their internally body solute concentrations are lower and the urine secretions are more diluted. The rectal glands show degenerating changes in bull sharks in a fresh water environment, compared with specimens of this species and other species from a marine habitat. (4) Freshwater stingrays exhibit the same molecular markers. These rays have reduced rectal gland size, and are able to excrete salts in higher salinity waters equivalent to their marine equivalent. (9 and 12) The elasmobranch rectal glands indicate how they are able to adapt to euryhaline conditions and freshwater environments. The ability to fully adjust in waters with a wide range of salinity and survive shows how weak the physiological constraints are on elasmobranchs.
Active evolutionary selection can already be observed with freshwater elasmobranchs. As freshwater populations that have limited geographical range are introduced more into human settlements around their habitat, these creatures come closer to extinction. (6) Having been exclusive predators, with limited threats, this new influence may limit their adaptive nature to the environmental threat of humans. Bull sharks are found primarily in shallow waters, and they tend to remain in the same location for long periods of time. (2) They have been known to move away from established home ranges and undertake large scale movement to open ocean areas. Therefore, the coastal zone, predominantly the areas of high freshwater inflow, is a potential essential habitat for this species. The features affecting the penetration of elasmobranchs into fresh water environments are currently unknown.
Rectal glands are composed of epithelial tissue and functions in the secretion of excess sodium and chloride in elasmobranchs. The liver in sharks produces abundant amounts of urea which in turn create a hyperosmotic state to the surrounding environment. When this arises, sharks must perform as a fresh water fish and constantly take in water to stable out the imbalance in the equilibrium. The excess salts that accumulate are then concentrated by the rectal gland and secreted. The rectal gland size is reduced and have degenerating changes in the bull shark that inhabit a fresh-water environment, compared other species that occupy a marine habitat. Studying and researching the life history of elasmobranchs may help discover the evolutionary traits that arose allowing there to be freshwater sharks. Marine sharks and rays in salt water regulate urea so that they can remain hyper-osmotic to their environment.
Elasmobranchs can adapt their mode of osmoregulation to survive in fresh water environments. Urea is produced by the liver. The kidneys transfer urea from the blood to the kidneys. The kidneys of a bull shark can evolve and adjust to the salinity of water surrounding it. When migrating from the ocean to an estuary or upriver into freshwater, a bull shark kidney will progressively begin to remove less salt and excrete more urea from the bloodstream in the urine, which reverses the normal method of osmoregulation in a marine lasmobranch. (5 and 7) Zoology studies indicate that Atlantic stingrays have been known to alter Na+/K+ ATPase expression in gills and rectal glands for osmoregulation in marine and freshwater environments. A negative correlation between salinity and Na+/K+ ATPase in the gills and a positive correlation with Na+/K+ ATPase in the rectal glands were observed; however, there is also indication that the gills lose their function to excrete ions in freshwater environments, exclusively taking ions in. (14 and 16) Absorption K+ in these rays are attributed to the acid-base balance requirements placed on the organism. (13)
Sharks use sound to locate prey and are attracted to low frequency pulses. Marine elasmobranchs have special sensory organs around their snout and mouth known as the ampullae of Lorenzini that detect changes in water pressure and detect electric fields generated by ocean currents or other fish. Marine sharks and rays have the most sensitive electrosensory system of all elasmobranchs. These special sensory organs form a network of jelly filled canals on sharks and stingrays. The canal length varies from animal to animal and is vital to the fitness and survival of the elasmobranchs. Comparing marine and freshwater stingrays shows a shorter and narrower form of these canals in the freshwater forms. These organs take on a larger role in osmoregulation in freshwater elasmobranchs, due the reduction of their electrosensitivity functionality in freshwater environments. The ampullae require stronger signals from shorter distances to detect changes in the water. This adaptive evolution of new functionality, in a vital organ, while still retaining its functionality for electric field sensing demonstrates the broad range of conditions wherein many of the elasmobranch organ systems can be utilized. (16)
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The topic of many new ideas presented on a regular basis is the unique metabolism of elasmobranchs. The metabolic use of lipids for energy expenditure is a highly developed difference among elasmobranchs. Urea is believed to be very important in elasmobranch metabolism, but urea is not the basis for metabolic regulation in elasmobranchs. Recent studies have revealed lipid oxidation rates do not vary significantly with urea levels, despite previously beliefs. (11) The evolution of hypo osmoregulation attributes to the evolution of the teleost fish metabolic pathway. A significant function for oxidative fuel, Ketone bodies, cannot clearly provide an explanation for its evolutionary development in all elasmobranchs. (18)
The evolution of the rectal glands and the adaptive osmoregulation of the bull shark, Carcharhinus leucas, are just two of the many reasons as to why this special family of elasmobranchs can infiltrate freshwater and thrive. The rectal glands are an important characteristic in osmoregulation and their contribution to the drinking and eating processes that maintain osmotic consistency. These glands, however, show degenerating changes in bull sharks in a fresh-water environment to those specimens found in a marine habitat. There has not been sufficient research to determine if in fact just the rectal glands and osmoregulation changes can be accredited to this ability. In the bull shark, the kidney can adapt to the salinity of water as they migrate from the ocean into fresh water rivers and lakes by removing less salt and excreting more urea from the bloodstream in the urine. This act of adaptive osmoregulation fundamentally reverses the normal method of osmoregulation in a marine system. Their exclusive predator status in freshwater environments may also contribute to the prolific growth of freshwater elasmobranchs. The molecular physiology of these sharks allow them to tolerate a wider range of salinity and their evolution into a euryhaline diversity of elasmobranch that can survive in both fresh water and salt water environments. The noteworthy appearance of freshwater bull sharks is an evolutionary mechanism that underlines the theory of Natural Selection proposed by Darwin. However, more data is needed to further the knowledge of this significant change in the elasmobranch species.