Physiology of Finfish and Shellfish

2.3.1. Introduction

The primary excretory organ in fishes as in other vertebrates is the kidney. In fishes some excretion also takes place in the gills where the waste materials produced during metabolism are given off. Skin is also partly excretory in function serving to remove carbon dioxide. Excretory organs such as kidneys, supplemented by other organs and tissues, are responsible for maintaining the constant internal environment. Or in other words, regulate the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. They accomplish this by a process of selective filtering and processing of the body fluids. Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of water and salt between the body fluids (internal environment) and the surrounding water (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

The excretory organs function in the elimination of wastes, osmotic regulation, ionic regulation and the retention of useful organic molecules.