One of the major challanges faced by animals is the maintenance of a constant internal environment, called Homeostasis (Hickman et al. Ch. 30).  The term Homeostasis is a catch all term that refers to the control of many variables, such as temperature, salt, water, glucose, etc.  The benefit of constant conditions comes in that the chemical reactions on which life depends are affected by physico-chemical conditions within the cytoplasm.  If the internal conditions change, isozymes (alternate forms of an enzyme that function optimally under differing conditions) are often necessary to ensure that the reactions will preceed.  Therefore, there is an energetic cost associated with allowing the internal environment to vary.  If you maintain a constant temperature you only need to make one form of the enzyme but if your temperature is going to vary, you will need to make many isozymes.

     The excretory system is one of the systems involved in maintaining homeostasis in the body.  The excretory system is responsible for the removal of nitrogenous wastes produced during the metabolism of proteins.  Along with nitrogenous waste, the excretory system is responsible for the removal of water from the body.  There are three main forms of nitrogenous waste used by animals.  Many aquatic organisms release ammonia, a highly toxic compound produced in protein metabolism.  Ammonia is released by small animals and those that live in water.  Urea is a less toxic form of nitrogenous waste but the animal must expend energy to produce it from ammonia.  Urea is often used by large animals and terrestrial animals.  The fact that it can build up in the body to higher concentrations than possible for ammonia means that these animals can produce a concentrated urine, therefore, conserving water.  Uric acid is the most complex, and energetically costly, form of nitrogenous waste.  It's advantage is that it is only weakly soluble in water and therefore can be removed from the body with minimal water loss.  This form of waste is found in many terrestrial insects and in reptiles and bird.  It is adaptive for reptiles and birds in that it can be stored inside the shell of developing embryos without incurring a large water demand.  So as we can see, differences in nitrogenous waste are important not only due to differences in toxicity, but also due to differences in the amount of water required for their removal.

     The control of water content is one of the greatest challenges facing aquatic animals.  Freshwater species usually have cytoplasm that is hyperosmotic (see Hickman et al. pages 47-49 for a discussion of osmotic pressure) compared to the environment and saltwater species are often hypoosmotic compared to seawater.  This results in the net gain or loss of water to the body due to Osmosis, the diffusion of a solvent (in this case water) across a semipermeable membrane ( in this case the cell membrane).   Freshwater organisms are constantly removing excess water that is entering the body whereas saltwater organisms are trying to reduce water loss to the environment.  Many saltwater species have an osmolarity (solute concentration -moles of solute per liter of water) that is equal to that of sea water (isosomotic solutions).  These organisms do not expend any energy to control internal osmolarity and therefore will conform to the osmolarity of the environment.  These organisms are called osmoconformers (See spider crab in graph below).  Organisms that maintain a constant osmolarity regardless of the external environment are called osmoregulators (See Shore crab in brackish water - salinity below normal seawater).



Why would osmoconformers be more common in marine environments than freshwater or brackish environments?

Think about the variability in the environment.

We collected a species of crayfish (

Back to Syllabus