Physiology of Finfish and Shellfish (2+1)

2.3.4. Freshwater teleosts

Kidneys of type I, II, III are usually seen in freshwater teleosts. The kidneys are completely fused (Type I) in salmon and trouts. Kidneys of type II are seen in the Cyrinidae in which the middle and posterior parts are fused, while the anterior free parts present the head kidney.

In the carps, as Labeo, Cirrhinus and Barbus the trunk kidney (Fig) is broad in the middle and becomes narrow in the posterior part. Kidneys are fused in the posterior region only (Type III) in Cyprinodontidae, Cottidae and Gasterosteidae. In a few species like Mystus, Arius etc the head kidney is completely separate from the rest.

Each kidney has a mesonephric duct (opisthonephric duct) running along its outer border. Both the ducts run upto the posterior end of the kidney and then join to form a common duct as in Mystus. But in Labeo and Cirrhinus they remain separate till they open into the urinary bladder (Fig). The urinary bladder is formed by simple enlargement of the common duct, or it may be distinct sac like on one side of the duct as in Mystus, Barbus. The urinary bladder opens to the exterior by a common urino-genital aperture.

A kidney is composed of a large number of tubules or nephrons, which develop first in the anterior region of the trunk and proceed posteriorly. The collection of nephrons constitutes the kidney. The anterior tubules become functional in early embryonic life and are called pronephric tubules, while those of the hinder region, which become functional later, constitute the mesonephric tubules. During development in fishes a pronephros is replaced by mesonephros (opisthonephros), which opens into the nephric duct called the opisthonephric duct. The tubules of the anterior region of the pronephros usually become degenerate and are converted into a lymphoid organ, while the posterior tubules form the bulk of the functional kidney.