Physiology of Finfish and Shellfish (2+1)

2.4.5. Osmoregulatory problem in freshwater fishes

Animals such as invertebrates, fishes, amphibians, reptiles and mammals that live in freshwater are osmoregulators. Osmoregulators face two problems: prevention of water loss from the body and prevention of salts diffusing into the body. Their body fluids (1/3^rd of the concentration of seawater) have a greater concentration than their surrounding environment (hyperosmotic). As a result, they are constantly taking in water by diffusion through their skin and to a much larger extent, through the thin membranes of their gills. Skin has low permeability to water and salt ions, hence very little water is allowed to enter the body or salt to diffuse out through it. To counter the continous inflow of water through gills, freshwater fishes produce a large amount of dilute urine. The kidney has a very large number of well developed glomeruli (often more than 10,000), which are well vascularized and continuously filter excess of water which is eliminated as hypotonic (dilute) urine. The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes and produces copious urine which is 5-12% of the body weight/day. Freshwater fishes drink very little water and take in little water in their food as large amount of water enters the body by osmosis and it is more than necessary for the fish.

A diagram showing the movement of water and ions in freshwater fish.

Freshwater fishes loose salts especially chlorides by passive diffusion through membranes. Some salts are lost through faeces also in urine. Urine also contains some nitrogenous waste as creatine, creatinine, amino acids, urea and a little ammonia. The quantity of salts lost in a day varies in different species. In salmon ( euryhaline species) loss of salts may be upto 17% of the body chlorides while in gold fish, Carassius auratus, it is only 55%. The loss of salts is however, kept at the minimum by active reabsorption in the proximal and distal tubules. Loss of salts is partly compensated through food and by absorption of salt ions from the surrounding water. This is accomplished mainly by the chloride cells in the gills and oral membrane (mouth). Salt ions absorbed by these special cells at the base of the gill lamellae are Li, Ca, Br, Cl, SO₄ and PO₄. The entire mechanism of salt balance is under hormonal control.