MadSci Network: General Biology |
Mammals are unusual in having separate orifices dedicated to urination, defeacation and reproduction. Fish, as with birds and most other vertebrates, have to make do with one, which is usually called a cloaca. The chief purposes of urine production are maintenance of water balance and elimination of waste products of metabolism, especially nitrogenous by- products from the breakdown of protein. Mammals achieve these aims by excreting greater or lesser quantities of water, and by excreting nitrogenous waste in the form of urea, dissolved in the excreted water. Birds excrete their nitrogenous waste as solid uric acid, mixed with their faeces, providing the white element of bird droppings. Fish, being immersed in water, have a more complex problem than simply obtaining and holding on to a suitable quantity of water. Osmosis (the dynamic movement of a solvent such as water across biological membranes) is an important physiological problem, and is dealt with via their large kidney. This can be seen as a dark-coloured tissue distributed along the spine within the body cavity in bony fish, but is a smaller organ in elasmobranchs (sharks and their relatives). Fish living in salt water constantly lose water to the hypertonic (more concentrated) ocean, and have to drink sea water to replace it, obliging them to excrete large amounts of salt. Freshwater fish have the opposite problem, osmosis results in constant absorbtion of water from their surroundings, which would bloat and ultimately disrupt their tissues. They need to be able to excrete (urinate) water continuously. The waste products of all these processes leave the body from the same orifice and I am not aware of any excretion via the gills. This idea may have come from a misunderstanding of one aspect of elasmobranch physiology. Sharks and rays reduce the necessity to constantly drink seawater and excrete salt by increasing the concentration of dissolved substances in their blood to a level almost equal to seawater, thereby preventing osmosis. One of the substances they allow to build up in their blood is urea, at levels which would be toxic in other animals, and which may explain why less-than-fresh shark or ray flesh smells so bad so quickly. This urea would be continuously lost to the seawater by diffusion, especially through the gills, so the membranes of shark gills are unusual in that they are impermeable to urea. You are right to assume that fish urinate from the same place as they defeacate, and from nowhere else. However, anyone who can go a little further into the complex questions surrounding the whys and wherefores of fish excretion certainly can 'call the shots' - biologically, anyway !
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