Osmoregulation in Freshwater Metazoans

Freshwater and brackish water animals that are live in hypoosmotic(of lower osmotic pressure) environment and keep a hyperosmotic (of higher osmotic pressure) condition in their body fluids. They may be capable to tolerate only a narrow range of salinity of the medium in which they live. These are termed as stenohaline animals. If they tolerate a wide range of salinity they are termed as euryhaline animals. Essentially their living environments are osmotically less concentrated as compared to their body fluids. They face the problem of the water continuously entering into the body and leaching of salts from the body. Such vacuoles are present in fresh water sponges also. Protonephridia of freshwater flatworms, metanephridia of annelids and coxal glands of crustaceans are another such water pumps that are capable of removing large amounts of fluids from the body. In fact such types of organs have the primary function of water balance rather than excretion of nitrogenous wastes. In some animals there are no unique organs for the removal of water. Hydra is one such example. The regulation of both water and salt in Hydra is performed by active transport of sodium. In the nonexistence of calcium or sodium in the environment the osmoregulatory process breaks down in Hydra. The pumping in of sodium into the gut is followed through the passive flow of water along the osmotic gradient. The mesogloea functions such as an extracellular fluid space. It is believed that two pumps may be operational in Hydra, one transporting Na into mesogloea and the second which transports it into gut. Water taken osmotically is expelled by the mouth. Active transport of sodium takes care of both osmotic and volume regulation. So there is an influx of water into the body through the external surface, and the excess water is removed by the gastrovascular cavity, by the mouth. Fluid in the gastrovascular cavity is hypoosmotic to tissue fluid. The gastrovascular cavity is so supposed to act like big contractile vacuole Ability to produce dilute urine has been demonstrated in animals relating to more advanced phyla (arthropods, earthworms and fresh water molluscs). By using the techniques of micropuncture and clearance of tubular fluid in the metanephric tubules, both filtration and active transport have been illustrated. For example, in the antennal gland of fresh water crayfish, the end sac functions as the site of filtration. Chloride is reabsorbed like the filtered urine passes through the long tubule resultant in conservation of salts and reabsorption of water.

Filtration in arthropods and molluscs is essentially performed by the hydrostatic pressure of the blood. In arthropods, the wall of the coelomic sac is extremely vascularised. In molluscs the heart passes through the filtration cavity or pericardial sac. There is filtration by the wall of the heart, into the pericardial cavity. From the pericardial cavity, filtrate passes through the nephrostome into the kidney. Usually the coelomic sac is located near the heart or near the region of high blood pressure. The observed dilution of urine in the distal tubule and ureter could be due to the addition of water or to the reabsorption of salts. But the make use of metabolic poisons that arrests the active uptake has clearly illustrated that absorption of solutes is accountable for the excretion of hypoosmotic urine. It could be said that the capacity of excretory organs to form hypoosmotic urine and to trap ions from ambient fluid played a important role in the colonisation of the fresh water environment.