Anatomy of finfish and shellfish

Unit 8 - Urino – Genital System

8.5.1. Structure of Kidney
“Kidney is the organ through which most of the metabolic wastes are excreted”. The kidneys of fishes are reddish brown, soft elongated paired structures lying ventrally to the vertebral column. Though paired, the two kidneys show multiple shapes in different fishes due to fusion at various positions. Ventrally the kidneys are covered by coelomic epithelium. The teleostean kidney may be divided into a “head kidney” and a “trunk kidney” but this differentiation sometimes is not possible by external observations. Anatomically fish kidney is of two types – Pronephric and Mesonephric. In fishes with kidney distinguishable to a head and trunk kidney show the pronephric part in the head region (generally non functional in terms of excretion and mesonephric part in the trunk region. Generally there are no conspicuous differences in shape between the two sexes. “Ogawa” (1961a) has classified the marine teleostean kidney into five (5) configurational classes.

• Type I: The two sides of the kidney are completely fused throughout. No clear distinction between trunk and head kidney. Ex. Clupeidae (Herrings).
• Type 2: The middle and posterior portions only are fused. Clear distinction between head and trunk kidney. Ex. Plotosidae (Marine catfishes), Angulidae (Eels).
• Type 3: Posterior portion only is fused, anterior portion represented by two slender branches, clear distinction between head and trunk kidney. Most marine fishes have this type of kidney. Ex. Bellonidae, Mugilidae, Scombridae, Carringidae and Pleuronectidae.
• Type 4: Extreme posterior portion only is fused and head kidney is not recognizable. Ex. Syngnathidae (Pike fishes).
• Types 5 : The two kidneys are completely separate. Eg: Lophidae (Angler fishes)

Ogawa has observed that all the freshwater teleost species that he examined can be grouped into the first 3 or the 5 groups described above. Examples are as follows.
Type 1 – Salmonidae – Salmons and trouts.
Type 2 – Cyprinidae – Carps and minnows
Type 3 – Cyprinodontidae (Killi fishes), Gastrostidae and Cottidae.
Generally the anterior part of the kidney (Head kidney) is pronephric and possesses lymphoid, haematopoitic internal and chromofin tissue. The nephron is generally absent in this region but when present it is devoid of renal corpusele and glomerulis. In most fishes the pronephrons is traditional and it is taken over by the mesonephros in the later period of the life (Adult stage). In other words the head kidney in majority of fishes is not functional and in some other it is functional only in the early stage of the individual (life). Some teleosts, however, show the extension of posterior kidney (trunk kidney) towards the anterior kidney. In this part of the kidney typical nephrons are present with glomerulli, renal corpuscle, renal tubule and collecting ducts. The variable amount of haematopoitic and pigment cells are distributed among the tubules and vascular spaces in the trunkate region. The two archineptric ducts are always fused. This fusion may occur at the posterior end of the kidney or at some point between the kidney and the urinary papilla. Dilation of the orchinephric duct may found a bladder like enlargement (Urinary bladder) in many bony fishes.