Marine Biology

Physilogical Adaptation

The adaptations required for estuarine life are those that maintain the ionic balance of body fluids influctuating external salinities. Osmosis is the physical process in which water passes through a semipermeable membrane that separates two fluids of different salt concentration, moving from the area of lower to higher salt concentration. The ability to control the concentration of salts or water in internal fluids is called Osmoregulation . Most marine organisms cannot control their internal salt content and are Osmocon formers. Their ability to penetrate estuaries is limited to their tolerance for changes in their internal fluids. Osmoregulators are organisms that have physiological mechanisms to control the salt content of their internal fluids. Most estuarine animals either are Osmoregulators or they are able to function with fluctuating internal salt concentrations.

Penetrating into an estuary means encountering water with lowered salinity. Since the internal salt concentration of marine species is higher than that of the estuarine water, water tends to move across membrane into their bodies in an attempt to equalize the concentrations. Regulation means excreting the excess water without losing salts or excreting water and salts and replacing salts with active uptake of ions from the environment. For freshwater animals, which move from a more dilute to a less dilute medium while entering estuaries, the reverse is true.

Osmoregulatory ability is formed is several taxa of estuarine animals, primarily polychaste worms, molluscs, and crustaceans. Osmoregulation can operate in three ways: 1) animals may move water; 2) they may move ions; or 3) they may adjust the internal water ion balance. In advanced invertebrates and vertebrates, the osmoregulatory organ is generally the kidney, where excess water is excreted and needed ions resorbed. Among many invertebrates, gills are the most common osmoregulatory structures. However, special cells may also exist in other parts of the body; particularly for taking up or removing certain ions.