Fish Excretion
Carbon dioxide enters into the bicarbonate equilibrium system, and most is excreted at the gills. Protein digestion yields nitrogenius compounds in addition to carbon dioxide and water. In teleostfishes, these nitrogenous wastes take the form of ammonia, a potentially toxic substance. Thus, teleports are primarily “ammoniotelic.” Despite its toxicity, ammonia has many advantages over urea or uric acid as the chief excretory product of nitrogen metabolism as long as the animal resides in an environment with abundant water. First, the small molecular size and high lipid solubility permits no ionized ammonia (NH3) to dif-fuse easily across the gills. Second, ionized ammonia (NH4+) is exchanged for Na+ at the gills for maintenance of relative alkalinity and internal ion balance. Third, conversion of ammonia to either urea or uric acid requires energy. Thus, in contrast to terrestrial forms, less energy is required to complete nitrogenous compound catabolism and, in tallest fishes, the end products resulting from this catabolism are largely released at the gills rather than the kidney. For example, carp and goldfish (Crassus aerates) excrete six to ten times as much nitrogen at the gills as at the kidney. Of the total nitrogenous excretion,90% is in the form of ammonia and only 10% consists of urea.
Elasmobranches fishes as well as the coelacanth (Latimeria) excrete urea as the primary nitrogenous end product (i.e., are ureotelic). Much of the urea is retained in these marine fishes, giving their body fluids a near isosomotic relationship with their environ-mint. The elasmobranches kidney filters urea from the blood plasma at the glomerulus. Much of the urea is subsequently recovered from the filtrate by active tubular desorption, preventing major “losses” of urea in the urine.
Lungfishes, to varying degrees, possess the biochemical machinery to be either ammoniotelic or ureotelic. For example, the African lung-fish (Protopterus) must sometimes endure extensive droughts. When the aquatic environment is drying up, Protopterus constructs a cocoon of mucus in the bottom mud and estimates there until the water returns. Protopterus is mostly ammoniotelic while aquatic but shifts to completeurotelism while estimating and survives by metabolizing the proteins in its muscles. This shift is made possible by high concentrations of the necessary enzymes for urea production in its liver tissue. Nontoxic urea may accumulate in the blood of estivatingProtopterus to concentrations of 500 m mol/liter after a three-yearestivation (Smith 1961). In contrast, the liver of Australian lungfish, Neoceratodus, possesses only 1% of the concentration of urea-seethe-sizing enzymes found in Protopterus. This finding is in accord with the obligatory aquatic habits of Neoceratodus (which also does not estimate).