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 One: The Marine Environment
Almost half of all phyla are restricted to the marine environment. One reason is that it is the most stable of the three main types of environment. It’s “easier” to live there. Wave action, tides, ocean currents and upwellings ensure that there is little fluctuation in the concentrations of dissolved gases, so although oxygen concentrations are only about 2.5% of those in air they do not fall as low as those in some stagnant freshwater environments. There is no danger of respiratory surfaces drying by evaporation because they are immersed in water, so the external surface of the body or specialised gills exposed in the surrounding water can be used for oxygen uptake.
Dissolved salts are also evenly distributed. Sea water has a similar salt concentration to the tissue fluids of many marine animals, so they can maintain water balance easily because there is no tendency for water to enter or leave the body via osmosis. Small organisms and food particles drift suspended in water and many marine animals specialise in collecting such particles, a process known as suspension-feeding. On land, only web building spiders ‘filter’ food from the surrounding medium. Sea water also provides a resource for excreting nitrogenous (nitrogen-containing) wastes arising from protein metabolism. Ammonia is the basic nitrogenous waste in all organisms and, with the abundance of water available, marine animals such as bony fish commonly excrete it directly. Although ammonia is highly toxic, there is no shortage of water to dilute it.
Water temperature is much more stable than air temperature, because large amounts of energy are needed to raise water temperature. The narrow temperature ranges experienced by marine animals are less demanding on their physiology than the larger ones experienced on land. Similarly, the pH (acidity) of salt water varies much less than that of fresh water. It is maintained close to 8.1 by the presence of bicarbonate ions. Some physical conditions, however, do vary, such as the decline in light and the rise in water pressure with increasing depth. These changes require specialised adaptations from animals living in the ocean depths.
The buoyancy of sea water supports animal bodies, as well as allowing energy-efficient drifting on ocean currents. This is why the largest animals ever known, both living and extinct, have been marine – cool huh? However, water is more viscous than air, meaning that the molecules stick together more readily. This creates resistance to organisms moving through it, so marine animals, especially small ones, may need special physiological or behavioural adaptations for movement or for suspension-feeding.
Lastly, the marine environment is ideal for reproduction involving external fertilisation and development, so the great majority of marine animals use this form of reproduction. Sperm and ova can be shed into the water where they can mix for fertilisation, with no threat of desiccation or salt imbalance. The embryos of marine animals often develop into larvae, which may look quite unlike the adult. Larvae are specialised for feeding, growth and dispersal, rarely requiring large egg yolks to nourish their development. As they grow, they increasingly resemble adults, a process known as metamorphosis.
Marine animals benefit from this buoyant, stable and isotonic environment that provides an ideal medium for fertilisation and development. No wonder life is thought to have started out in the oceans. Look out for Post Two: The freshwater environment, coming next.
~ JM
Image Credit: Photograph by Brian Skerry, National Geographic. Sourced from http://on.natgeo.com/1O8m5Ie 21/04/2015
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
The Marine Environment:http://bit.ly/1JZwdNE
