Sharks are prehistoric animals that have been around for 450 million old ages. They lived before land craniates. This fact entirely makes sharks record interrupting evolutionary animals. Sharks are a member of the “ selachian ” aggregation of fish because they have a skeleton made of gristle alternatively of bone. Elasmobranch is non the merely cartilaginous fish. The species are divided harmonizing to forms, characteristics and organic structure form. Elasmobranchs are members of a larger group called Chondrichthyes, which include skates and beams which have flattened organic structures in stead of the torpedo-like organic structure of the shark. All have the stiff dorsal five every bit good as other similar features. We normally think of sharks as ocean inhabitants ; nevertheless their anatomy has evolved over the last million old ages doing it possible for them to last in fresh H2O and inshore Marine Waterss. There are several species of selachian that can last in both fresh H2O and salt H2O. Documented surveies have proven that some migratory elasmobranch species travel in both Marine and fresh water environments throughout their life rhythm. ( 1 ) . One of these species is the bull shark, Carcharhinus leucas, which is able to be for long periods of clip in fresh H2O and can go long distances up river. Although most fresh H2O selachian are found in tropical and warm H2O temperatures, bull sharks are non confined to brackish Waterss and have been spotted along the Mississippi River as far north as Madison County, Missouri and in the rivers of Nicaragua. Often grownups can besides be found near estuaries and fresh water influxs to the sea. Certain species of bull sharks reproduce entirely in fresh water, which explain why their presence is so widespread in the coastal and fresh water bays, estuaries, rivers, and lakes. Although they easy migrate worldwide from warm shoal Marine Waterss into fresh H2O environments, bull sharks are non true fresh H2O sharks.
What may hold contributed to the fecund growing of this peculiar fresh water selachian is its sole marauder position in fresh water environments. The bull shark is highly big in length and weight runing from about 12 pess in length to about 1,700 lbs in weight. Using a bump and bite technique to disorient its quarry, the bull shark will eat whatever is readily available including other sharks, fish, polo-necks, biting beams, mollusc and birds. Depending on its home ground and location, its lone marauder is alligators, crocodiles and other sharks such as the tiger shark and great white shark. In fresh H2O, the bull shark is comparatively unthreatened. ( 10 ) The bull shark is considered to be the most aggressive and most unsafe species of shark because of its near-shore onslaughts on worlds. Yet it is non the compact organic structure or the aggressive and unpredictable behaviour that has allowed the bull shark to last for 1000000s of old ages. It is its molecular physiology to germinate into a euryhaline species which can digest a wider scope of salt and its ability to boom in both fresh H2O and salt H2O. These physiological factors are the hints to the bull shark ‘s elasmobranch development into a fresh water discrepancy of the species.
Osmoregulation is an animate being ‘s ability to keep a changeless concentration of H2O inside its organic structure, even when the external environment should do it to lose or derive H2O. The mechanism for osmoregulation in sharks in a marine environment is by and large the high concentrations of carbamide in their blood, and the remotion of the excess salt is chiefly through the kidneys. The rectal secretory organs play a cardinal function in elasmobranch osmoregulation and their part to the imbibing and feeding procedures that maintain osmotic consistence. Sharks evolved in salt H2O and their blood and organic structure fluids are every bit salty as the saltwater environing them. Marine elasmobranchs drink saltwater to equilibrate their osmotic force per unit area. Their bodily fluids contain little organic molecules ; the most of import of these are urea and trimethylamine oxide ( TMAO ) . Retention of high concentrations of urea and TMAO add significantly to a shark ‘s osmotic force per unit area. They reabsorb and retain urea in their blood and other organic structure fluids and excrete surplus accumulated salt in the piss to keep their optimum, changeless osmotic force per unit area. Urea is of import in marine selachian and must be reduced in dilute envirornments. In fresh H2O, a Marine selachian has firm restrictions on the sum of clip it can last. It must take in more H2O to modulate the relationship between the solute and the dissolver concentrations which presents the job of desiccation and the extra riddance of salts. Reduced urea conserving countries of fresh H2O selachian kidneys have been observed. ( 3 )
Surveies indicate that fresh H2O sharks have twice the sum of salt in their organic structures as other fresh H2O fish. The bull shark must keep an optimum changeless osmotic force per unit area in its organic structure in relation to the environing fluid to last its life rhythm in a fresh H2O home ground. Bull sharks have developed an ability to accommodate their procedure of osmoregulation to counterbalance for the difference in the salt of the H2O and accordingly can digest a wider scope of salt because they evolved the technique of retaining less urea in their blood and other organic structure fluids so that their internally body solute concentrations are lower and the urine secernments are more diluted. The rectal secretory organs show devolving alterations in bull sharks in a fresh H2O environment, compared with specimens of this species and other species from a marine home ground. ( 4 ) Freshwater stingrays exhibit the same molecular markers. These beams have reduced rectal secretory organ size, and are able to egest salts in higher salt Waterss equivalent to their marine equivalent. ( 9 and 12 ) The elasmobranch rectal secretory organs indicate how they are able to accommodate to euryhaline conditions and fresh water environments. The ability to to the full set in Waterss with a broad scope of salt and survive shows how weak the physiological restraints are on selachians.
Active evolutionary choice can already be observed with fresh water selachian. As freshwater populations that have limited geographical scope are introduced more into human colonies around their home ground, these animals come closer to extinction. ( 6 ) Having been sole marauders, with limited menaces, this new influence may restrict their adaptative nature to the environmental menace of worlds. Bull sharks are found chiefly in shallow Waterss, and they tend to stay in the same location for long periods of clip. ( 2 ) They have been known to travel off from established place scopes and undertake big graduated table motion to open ocean countries. Therefore, the coastal zone, preponderantly the countries of high fresh water influx, is a possible indispensable home ground for this species. The characteristics impacting the incursion of selachian into fresh H2O environments are presently unknown.
Rectal secretory organs are composed of epithelial tissue and maps in the secernment of extra Na and chloride in selachian. The liver in sharks green goodss abundant sums of carbamide which in bend create a hyperosmotic province to the environing environment. When this arises, sharks must execute as a fresh H2O fish and invariably take in H2O to stable out the instability in the equilibrium. The extra salts that accumulate are so concentrated by the rectal secretory organ and secreted. The rectal secretory organ size is reduced and have devolving alterations in the bull shark that inhabit a fresh-water environment, compared other species that occupy a marine home ground. Analyzing and researching the life history of selachian may assist detect the evolutionary traits that arose leting at that place to be freshwater sharks. Marine sharks and beams in salt H2O modulate carbamides so that they can stay hyper-osmotic to their environment.
Elasmobranchs can accommodate their manner of osmoregulation to last in fresh H2O environments. Urea is produced by the liver. The kidneys transfer urea from the blood to the kidneys. The kidneys of a bull shark can germinate and set to the salt of H2O environing it. When migrating from the ocean to an estuary or upriver into fresh water, a bull shark kidney will increasingly get down to take less salt and egest more urea from the blood stream in the piss, which reverses the normal method of osmoregulation in a marine lasmobranch. ( 5 and 7 ) Zoology surveies indicate that Atlantic stingrays have been known to change Na+/K+ ATPase look in gills and rectal secretory organs for osmoregulation in Marine and fresh water environments. A negative correlativity between salt and Na+/K+ ATPase in the gills and a positive correlativity with Na+/K+ ATPase in the rectal secretory organs were observed ; nevertheless, there is besides indicant that the gills lose their map to egest ions in fresh water environments, entirely taking ions in. ( 14 and 16 ) Absorption K+ in these beams are attributed to the acid-base balance demands placed on the being. ( 13 )
Sharks use sound to turn up quarry and are attracted to low frequence pulsations. Marine selachian have particular centripetal variety meats around their neb and oral cavity known as the ampullae of Lorenzini that detect alterations in H2O force per unit area and observe electric Fieldss generated by ocean currents or other fish. Marine sharks and beams have the most sensitive electrosensory system of all selachians. These particular sensory variety meats form a web of jelly filled canals on sharks and stingrays. The canal length varies from animate being to animate being and is critical to the fittingness and endurance of the selachian. Comparing Marine and fresh water stingrays shows a shorter and narrower signifier of these canals in the fresh water signifiers. These variety meats take on a larger function in osmoregulation in fresh water selachian, due the decrease of their electrosensitivity functionality in fresh water environments. The ampullae require stronger signals from shorter distances to observe alterations in the H2O. This adaptative development of new functionality, in a critical organ, while still retaining its functionality for electric field feeling demonstrates the wide scope of conditions wherein many of the elasmobranch organ systems can be utilized. ( 16 )
The subject of many new thoughts presented on a regular footing is the alone metamorphosis of selachian. The metabolic usage of lipoids for energy outgo is a extremely developed difference among elasmobranchs. Urea is believed to be really of import in elasmobranch metamorphosis, but urea is non the footing for metabolic ordinance in selachian. Recent surveies have revealed lipid oxidization rates do non change significantly with urea degrees, despite antecedently beliefs. ( 11 ) The development of hypo osmoregulation attributes to the development of the teleost fish metabolic tract. A important map for oxidative fuel, Ketone organic structures, can non clearly provide an account for its evolutionary development in all selachian. ( 18 )
The development of the rectal secretory organs and the adaptative osmoregulation of the bull shark, Carcharhinus leucas, are merely two of the many grounds as to why this particular household of selachian can infiltrate fresh water and thrive. The rectal secretory organs are an of import feature in osmoregulation and their part to the imbibing and feeding procedures that maintain osmotic consistence. These secretory organs, nevertheless, show devolving alterations in bull sharks in a fresh-water environment to those specimens found in a marine home ground. There has non been sufficient research to find if in fact merely the rectal secretory organs and osmoregulation alterations can be accredited to this ability. In the bull shark, the kidney can accommodate to the salt of H2O as they migrate from the ocean into fresh H2O rivers and lakes by taking less salt and egesting more urea from the blood stream in the piss. This act of adaptative osmoregulation basically reverses the normal method of osmoregulation in a Marine system. Their sole marauder position in fresh water environments may besides lend to the fecund growing of fresh water selachian. The molecular physiology of these sharks allow them to digest a wider scope of salt and their development into a euryhaline diverseness of selachian that can last in both fresh H2O and salt H2O environments. The notable visual aspect of fresh water bull sharks is an evolutionary mechanism that underlines the theory of Natural Selection proposed by Darwin. However, more informations is needed to foster the cognition of this important alteration in the elasmobranch species.