Osmoregulation in Marine Non-Chordate Metazoans
Studies on the osmotic pressure of body fluids of marine organisms have displayed that their internal osmotic pressure is more or less similar to the sea water where they live. Marine invertebrates are isotonic with the seawater in which they live, but the ionic composition of their body fluids might be markedly different from that of the normal sea water. For instance the mesogloea of the coelentrates has high potassium and a low sulphate concentration than to the seawater in which they live. This is true of polychaetes and echinoderms as well. Marine and brackish water animals have isosmotic or slightly hyperosmotic body fluid. Production of an isosmotic or a little hyperosmotic urine may cause the loss of valuable electrolytes. Hence, there is a continuous regulation of electrolytes of body fluids. Such type of regulation is achieved by several ways. Surface areas permeable to water and ions are usually reduced to a minimum Water pumps in the form of contractile vacuoles and nephridial tubules are present. But the most significant machinery that remains a part of every cell is the active transport. In certain organs like gills of crustaceans, highly specialised tissues exploit the capacity of active transport of large amounts of salts. In decapod crustaceans and cephalopods, ionic regulation might extend to every ion. For example, in these organisms calcium and potassium are more concentrated in body fluids than in the external medium, while magnesium, sulphate and chloride are less concentrated. Reduction in anion concentration like sulphate is compensated by an increase in sodium concentration. So in marine invertebrates including coelentrates, the internal medium has a specialised ionic composition quite different from that of external medium. Excretory organs play a role in ionic regulation.