Influence of the environment: The Marine, Freshwater and Terrestrial Environments.
The physical properties of salt water, freshwater and air have important implications for anatomy, physiology and behaviour of the animals inhabiting them. Let us take a look at the main features of marine, freshwater and terrestrial environments and their effects on animal life.
Post Two: The freshwater environment
Freshwater environments provide good buoyance for the animals that live in them, an excess of water so nitrogenous wastes can be excreted as ammonia and no danger of water loss by evaporation over respiratory surfaces. Apart from these points, however, they are much more variable than marine environments.
Freshwater bodies may vary greatly in turbidity, velocity and volume, and these factors may change rapidly in response to heavy rain. Drought may also be a factor, sometimes leading to the complete drying of a water body. Salt concentrations may also be variable in fresh water. In general, freshwater environments have lower salt concentrations than those found in animal tissues, so animals tend to gain weight by osmosis. The excess water must be removed while retaining important salts in the tissues, using a process called osmoregulation. However, in times of drought, evaporation may concentrate salt so that an inland water body may become saline, not fresh.
In such cases, freshwater animals must prevent excess water by osmosis into the surrounding saline water. In addition to this natural variability of the freshwater environment, freshwater animals may face other, externally imposed problems. Rivers and streams are often repositories for a range of human wastes and this pollution may have significant effects on freshwater life.
Freshwater animals are much less likely than marine animals to produce free-floating eggs, because they can be swept away by rapid currents. Instead, eggs are usually retained by the parent until they hatch, or attached firmly to the substrate. Larvae are rarer than in the marine environment and the eggs are often provided with a large yolk for nutrition.
So although freshwater animals benefit from buoyancy and the opportunity to excrete nitrogenous wastes as ammonia, they must cope with changes in the extent, turbidity and velocity of the water body that can impose problems.
Look out for Post Three: The terrestrial environment, coming next.
More Info: Gordon, M.S and Olsen, E.C. (1995). Invasions of the land: the transition of organisms from aquatic to terrestrial life. Columbia University Press, New York. Pechenik, J.A. (2005). Biology of the invertebrates. McGraw-Hill, Boston. 5th edn, Chapter 1. The Living Environment: http://bit.ly/1yK6pnv
