Physiology of Finfish and Shellfish (2+1)

4.3.1 Digestion

Digestion is the process by which ingested materials are reduced to molecules of small enough size or other appropriate characteristics for absorption, i.e., passage through the gut wall into the blood stream. This generally means that proteins are hydrolyzed to amino acids or to polypeptide chains of a few amino acids, digestible carbohydrates to simple sugars and lipids to fatty acids and glycerol. Materials not absorbed are by definition indigestible and are eventually voided as faeces. Digestibility ranges from 100 percent for glucose to as little as 5 percent for raw starch or 5-15 percent for plant material containing mostly cellulose (plant fiber). Digestibility of most natural proteins and lipids ranges from 80 to 90 percent.

Digestion refers to the process by which ingested food materials get converted into forms of energy which it can utilize for activity, maintenance and growth.

It should be borne in mind that the process of digestion also requires energy. In fishes digestion takes place by 2 processes.

1. Enzyme activity
2. Secretion of acids

Digestion starts when food is taken into the mouth and is completed when faecal matter is excreted from the anus, i.e., during a process of physical and chemical breakdown, ingested food items are reduced to their constituent molecules which are then absorbed in solution through the gut wall into the blood circulatory system.Essentially the food items are broken down into smaller parts which can be absorbed and then reconstituted.

Fishes have evolved variety of morphological and chemical adaptations for the digestion of variety of food taken in. Once the food is taken in the mouth it is pushed into the oesophagus. Oesophagus is a narrow tube which is highly elastic and contains many mucous layers and this function as a lubricating transit for the passage of food material. From the oesophagus the food is pushed into the lower gut. This lower gut is modified into stomach in some fishes. The main function of the stomach is to store ingested food material and this is seen in carnivorous fishes.