Osmoregulation in Pisces Osmoregulation is a type of homeostasis which controls both the volume of water and the concentration of electrolytes. It is the active regulation of the osmotic pressure of an organism’s body fluids, detected by osmoreceptors. Organisms in aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids. The nature of osmoregulatory problem is quite different in various groups of fishes in different environments. There is always a difference between the salinity of a fish’s environment and the inside of its body, whether the fish is fresh water or marine. Regardless of the salinity of their external environment, fish use osmoregulation to fight the process of diffusion and osmosis and maintain the internal balance of salt and water essential to their efficiency and survival. Kidneys do play a role in osmoregulation but overall extra-renal mechanisms are equally more important sites for maintaining osmotic homeostasis. Extra-renal sites include the gill tissue, skin, the alimentary tract, the rectal gland and the urinary bladder. 1. Stenohaline and Euryhaline Fishes: Stenohaline (steno=narrow, haline=salt): Most of the species live either in fresh water or marine water and can survive only small changes in salinity. These fishes have a limited salinity tolerance and are called stenohaline. e.g., Goldfish Euryhaline (eury=wide, haline=salt): Some species can tolerate wide salinity changes and inhabit both fresh water and sea water. They are called euryhaline. e.g., Salmon . 2. Osmotic challenges Osmoconformers, are isosmotic with their surrounding and do not maintain their osmolarity. e.g., marine invertebrates. Osmoregulators, are organisms that actively regulate their osmotic pressure, independent of the surrounding environment. e.g., Vertebrates 3. Osmoregulation In Freshwater Fishes Water Balance: The osmotic pressure of body fluids depends on mineral and organic compound content. The osmotic pressure of body fluids in fresh water fishes is always higher than that of the surrounding water (hyperosmotic) and the later diffuses into the body through oral membranes, gills and even intestinal surfaces. In certain species of fish water may enter through the skin also. To counter the continuous inflow of water through gills, a large amount of hypotonic urine is produced by the freshwater fishes in general. The freshwater fishes possess more glomeruli (even more than 10000 in number in the kidney). The kidney is also larger in size and is well vascularized. It has been calculated that the freshwater teleosts produce urine which is normally 5-10 per cent of the body weight per day. Water excretion is the main function of the kidneys in these fishes but small quantity of nitrogenous compounds, containing creatine, creatinine, aminoacids, ammonia and urea are also present in the urine. A freshwater teleost does not drink water as large amount of water enters the body by osmosis and is more than necessary for the fish.

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Salt Balance As the osmotic concentration of the body fluids in freshwater fishes is always higher than the surrounding water, some salts are lost into the water by the process of diffusion which takes place mainly through surface tissues like buccal epithelium, gills and skin. Some salts may be lost in the faeces and urine also. The quantity of salts lost per day varies in different species of fishes. In salmon loss of salts may be upto 17% of the body chlorides while in gold fish, Carassius auratus, it is only 5%. In the proximal and distal part of the renal tubule active reabsorption takes place and as such the loss of salt is minimum in freshwater fishes . Salts are replaced by two ways: firstly through food and secondly by the absorption of salt ions from the surrounding water. Absorption of salt ions from the water takes place mainly through gills and oral membrane. The ions of Sodium (Na+), Calcium(Ca++), Lithium (Li+), Cobalt (Co++), ++ ------Strontium (Sr ), Bromine (Br ), Chlorine (Cl ), Acid phosphate (HPo4 ) and Sulphate(SO4 ) are absorbed by chloride cells located at the base of the gill lamellae. The entire mechanism of salt balance is under endocrine control. 5. Osmoregulation In Marine Fishes: In contrast to freshwater environment marine fishes live in the sea where salt concentration is higher than that of the body fluids of the fish. Naturally, water is lost through the semipermeable membrane by the process of diffusion. Fishes have to drink sea water in order to make up the loss and thus salt content of the body increases. Increase of salt in the blood is then eliminated. There is marked reduction in the urine output, for conservation of water and the uriniferous tubules are modified for this purpose. In marine teleosts, there is considerable reduction in the size of the kidney tubules. The distal segment of the convoluted tubule is absent in most of the cases. The neck segment also is either absent or constricted in certain species. Nitrogenous waste products are excreted mainly through the gills in the form of ammonia. Traces of urea, ammonia and trimethylamine are present in the urine. Water is lost through the gills and other tissues because of lower concentration of blood in comparison to the sea water. The marine teleosts, therefore drink water. There is reduction in the size and number of glomeruli also. Water is reabsorbed in the kidney tubules. There is physiological control over excess of urine formation and as such large amount of water does not pass out through the glomeruli. In many teleosts the glomeruli become degenerate. Even aglomerular kidney may be present in some marine teleosts. Salt secreting cells are present in the gills and oral membrane to help the process of excretion because large amount of salt accumulates in the body tissues because the fishes frequently drink sea water to avoid dehydration. The Hagfishes are isosmotic to their surroundings, so that there is practically no movement of water across the semipermeable membranes (Fig.5). The Hagfishes do not drink sea water and their requirement of water for urine formation is met from the blood of host. The slime produced from their skin is high in Mg++, Ca++and K+content. Their surplus salt is removed through the gills. Monovalent ions reach the blood stream from the intestine and leave through the gills. But, the divalent ions remain in the intestine where they combine with oxides and hydroxides. Thus the insoluble compounds formed in the alkaline medium pass out along with the feces.

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In Elasmobranchs there is no danger of dehydration because they maintain their blood at a higher concentration in comparison to surrounding water. Urine is formed as a result of filtration process in the glomerulus. The urea present in the urine is reabsorbed by special segments of the kidney tubule. Moreover, the gills in elasmobranchs are impermeable to urea and as such, the urea is retained in the blood in large quantities. Thus osmotic concentration of the blood is raised and it becomes higher than that of the sea water. The osmotic balance in these fishes is maintained by urea cycle. Salts are excreted in the feces and in the urine. Some salts are reabsorbed in the kidney tubules. Salts are never excreted through gills as there are no special salt excreting cells. They achieve salt balance by secreting a fluid containing higher concentration of sodium and chloride ions from the rectal gland. There is high concentration of urea in the blood of elasmobranchs living in fresh water. The high osmotic pressure of the blood is maintained also by the presence of trimethylamine oxide (TMAO). It makes up 7 to 12 % or more of that part of osmotic pressure in sharks, rays and skates by organic compounds. It is reabsorbed from the glomerular filtration like urea. It is important in elasmobranch osmoregulation.

Osmoregulation In Diadromous Fishes Diadromous fishes are adjusted both to the freshwater and the salt water phase of the life cycle. Diadromous fishes develop osmoregularity mechanisms for different environmental salinities. Anadromous Fishes (Salmon and Lampreys) Anadromous fishes like Salmon and Lamprey, migrate from the sea to spawn in fresh water. The Salmon spends its adult life in the sea, and migrates up the rivers to spawn. The eggs develop into fry or large fish, depending on the species, before they return to the sea (Fig.6). The diadromous (anadromous and catadromous) fishes are not necessarily euryhaline, but are able to migrate because of hormonal activity that results in the physiological changes required for survival in the new environment. Salmon eggs cannot develop normally in saline solution. Young salmon (Onchorynchus kisutch) cannot enter the sea until salt- secreting cells have developed in the gills, and other physiological changes have taken place. When returning to freshwater adult salmon exhibit a slight decrease in the osmotic concentration of the blood. Some Lampreys (cyclostomes) are also anadromous. Petromyzonmarinus spends some time in seawater but returns to the rivers and dies after breeding. Some species are found only in freshwater and migrate from lakes to streams (potamodromous). In Lampetra fluviatilisin freshwater at 180C.The daily urine output is large, about 45% of the body weight, but the urine chloride content is low (0.5 to 1.5mM). The body surface is moderately permeable to water and considerable quantities of water are absorbed osmotically because of the large area of the branchial surface. At temperatures near freezing (0 to 3oC.) permeability to water and probably to salts is lowered. Since lampreys do not feed at these temperatures, the lowered permeability and continued, though decreased, absorption of chloride by the gills help to maintain the salt balance. The marine lamprey can regulate body fluid until the time of anadromous migration.

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Catadromous Fishes (Eels) Catadromous fishes, Anguilla, spend a major part of their lives in fresh water and migrate to sea for breeding purpose. They lay their eggs in the sea. The young larva drift about for 1 to 3 years as stenohaline marine animals. They undergo metamorphosis and change from Leptocephalus stage to elvers which become euryhaline and eventually enter rivers and fresh waters (Fig.7). The renal unit of the kidney of the eel contains glomerulus, neck segment, proximal and distal convoluted tubules, and initial collecting tubule. When transferred from fresh to salt water and the reverse, eels show significant changes in weight for several hours, presumably because of the osmotic flow of water into (in fresh water) or out of (in salt water) the fish. The quantity of water passing through the surface of the eel is remarkably small, most of the exchange is by mouth or by urine excretion. Osmotic regulation then restores the normal water and salt balance. Eels are able to retain chloride to a remarkable degree and in fresh water maintain their salt supply by absorbing salts from food and reabsorbing practically all chloride contained in the glomerular filtrate in the kidneys. Chloride cells can functionas secreting and also absorbing units. Hormonal activity at the time of migration is responsible for the adjustment in osmotic regulation. Endocrine Control Of Osmoregulation The endocrine glands are responsible for water-salt balance including urine flow in fishes. The filtering rate of renal corpuscle is affected by hormones. Diffusion and absorption of the gills have been found to be greatly influenced by hormones. Cortisol is a sea water adapting hormone in fish and prolactin as the fresh water adapting hormone. Recent evidence indicates that the growth hormone\insulin like growth factor I axis is also important in sea water adaptation in several teleosts of widely differing evolutionary lineages. In salmonids growth hormone acts in synergy with cortisol to increase sea water tolerance. Cortisol under some conditions may promote ion uptake and interact with prolactin during acclimation to fresh water. The osmoregulatory actions of growth hormone and prolactin are antagonistic. In some species thyroid hormone support the action of growth hormone and cortisol in promoting seawater acclimation. In Anguilla, adrenaline has been shown to have strong vasodilatory effect on the gill vessels. It reduces or completely stops chloride secretion. In salmon hypothalamus and gonads have been implicated in the process of mineral balance.

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