Structure of the Digestive System
Irrespective of their main dietary requirements, the digestive systems of fish are very similar. All have a mouth, oesophagus (throat), and areas for the absorption of food components (fore and midgut) and compaction of indigestible waste material (hindgut). In adaptation to diet, the major differences are seen in the structure of the mouth and the teeth, the gill rakers, the pharynx, the stomach (if present) and in the length of the intestine . In carnivorous fish or those with a meat-orientated omnivorous diet there is a definite stomach (foregut) whilst herbivorous or plant-orientated omnivores have no stomach but rely on a much extended midgut area. These two situations are shown below.
Digestive system of a Carnivorous Fish
Digestive system of a Omnivorous Fish
Additionally carnivorous fish have extensions to the upper part of the midgut known as pyloric caecae. Two other internal organs are associated with digestion, namely the liver and the gall bladder, located anteriorly to the stomach or anterior section of the midgut.
Before being digested, the fish food, whether live or in the form of a pellet, has to be caught and positioned before being 'swallowed' - this is the function of the mouth. In carnivorous or predatory fish teeth may be present on the jaws, tongue and inner mouth (buccal cavity) - these teeth do not bite or crush food but simply hold it and prevent escape. Herbivorous fish or those which feed on hard-shelled prey may have pharyngeal teeth to aid in crushing the food before it enters the stomach. The oesophagus is a tube which connects the mouth to the stomach and/or gut - no food absorption occurs here.
For those fish species with a stomach, two areas can be identified - a cardiac area anteriorly (i.e. food enters this part of the stomach from the oesophagus) and the pyloric section posteriorly (where food exits from the stomach into intestinal tract). The stomach wall secretes hydrochloric acid and digestive enzymes which serve to soften and initiate breakdown of food into smaller, absorbable pieces. In stomachless fish, the wall of the anterior midgut secretes digestive enzymes but not hydrochloric acid.
From the stomach the intestine (comprising mid and hindgut) runs to the vent/anus. In carnivorous fish the intestine is relatively short whilst that of herbivorous fish, which tend to lack a stomach, is long and much folded to increase the contact and absorption time. At the anterior end of the midgut a number of blind ending tubes may be present - these are the pyloric caecae. They appear not to be associated with any particular type of diet and the number, if they are present, varies from species to species. Several functions have been attributed to them including increasing the surface area of the intestine for food absorption, specific site of carbohydrates and fat absorption and adding to the digestive functions of the stomach. Caecae are present in salmonids but absent from cyprinid species.
An important source of digestive enzymes is the pancreas. This organ is adjacent to the stomach and can be a discrete or diffuse organ depending on species. In salmonids the pancreas is a diffuse collection of cells interwoven amongst the pyloric caecae. Enzymes are produced by the pancreas only in the presence of food and are involved in the breakdown of all nutrients.
Both the liver and the gall bladder play an important role in digestion. The liver both assimilates the products of digestion into usable forms and aids the digestive process through the production of bile which is stored in the gall bladder.
Bile serves a number of functions in that it (a) helps with emulsification and absorption of lipids/fats, (b) neutralises hydrochloric acid from stomach (and so prevents possible ulceration of the intestine) and (c) ensures that absorbed toxins are returned to the intestine for excretion.The size and fullness of the gall bladder is indicative of feeding status in fish. A large, distended bladder indicates that the fish has not eaten for some time whilst an empty flaccid bladder indicates that the fish has recently eaten a meal. Bile is excreted from the bladder to the intestine via the bile duct.
Emulsification helps to maintain a large surface area from attack by lipases which break fat down (through a process known as Catabolism) to glycerol,fatty acids and glycerides. The basic format is: