MadSci Network: General Biology
Query:

Re: Where do Fish urinate from?

Date: Mon Oct 20 20:27:52 2003
Posted By: Will Higgs, Research Associate
Area of science: General Biology
ID: 1066463189.Gb
Message:

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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