Physiology of Finfish and Shellfish (2+1)

2.4.8. Osmoregulatory problem in chondrichthyes or elasmobranchs

The chondrichthyes maintain osmotic concentration of blood nearly at the same level as that of seawater (isoosmotic) and are not in danger of dehydration and do not drink seawater. This is achieved by retaining nitrogen containing compounds, mainly urea and trimethylamine oxide (TMAO) in the blood, which is slightly hyperosmotic to seawater. Consequently, these fishes gain water through osmotic diffusion across the gills and do not drink seawater. The kidney has large glomeruli as in freshwater fishes and urine is formed by filtration. But most of the urea contained in the filtrate is reabsorbed by special segments of the urinary tubule. The gills of chondrichthyes are impermeable to urea so that it is retained in the blood in large quantity to raise the osmotic concentration of blood to a slightly higher level than seawater. It is reported that the average urea content in the blood of chondrichthyes is 1000 – 2000 mg% and that of urine 230 mg%. The osmotic balance in these fishes is maintained by retention of high level of urea in the blood. Whereas, freshwater sharks have a lowered concentration of urea in the blood. Salts are excreted in the faeces and in the urine. Some salts are reabsorbed in the kidney tubules also but gills do not excrete salts. Gills of chondrichthyes do not have special salt excreting cells. They achieve salt balance by secreting a fluid containing higher concentration of Na⁺ and Cl^- ions from a special excretory organ, the rectal gland which is located at the end of the alimentary canal.

(Rectal gland is a structure found in marine elasmobranchs which helps in osmoregulation by excreting NaCl. It consists of a mass of tubules composed of cells very similar in ultrastructure to that of chloride cells of fish gills. In sharks and rays the osmotic concentration is