As with all animals digestion in fish involves the breakdown of eaten food into its smaller component parts, amino acids, vitamins, fatty acids etc. which can then be used to build up new fish body.Food is brought into the body via the mouth. The tongue of fish is generally very simple, being a thick, horny and immovably pad in the lower jaw which may often be decorated with small teeth. In fish the tongue is not necessary for the manipulation of food as it is in terrestrial animals because the food items remain buoyed up by the water and can me moved threw the mouth adequately by control of the water flow and the placement of the teeth. The teeth of most fish are the fore-runners of vertebrate teeth with an outer layer of enamel and an inner core of dentine. A fish may have teeth at the front of its mouth and along the jaws and in the pharynx as well as on its tongue. The buccal cavity (the empty space in an empty mouth) secretes mucus to aid in the swallowing of food, but there are no special organs involved, no salivary glands, and this mucus is a lubricant only, it contains no digestive enzymes.
Immediately behind the mouth is the pharynx which is the continuation of the tube started at the mouth and in which are found the gill clefts, through which water flows out of the alimentary canal and into the gills. After the pharynx comes the esophagus, a muscular tube that leads to the stomach. It is constructed of two layers of non-striated muscle, one of which is longitudinal and the other circular, strangely in some species of fish the longitudinal muscle is the inner layer while in others the circular muscle is the inner layer.
The gizzard is really a highly muscular modification of the first part of the stomach. Its main purpose is to grind up coarse food items into smaller pieces thus facilitating their later digestion. In those fish which have a gizzard, such as Shad, it is the place where digestion begins because as well as its muscular activity the gizzard also secretes digestive enzymes into the food.
The stomach of fish is less well delineated than it is in the higher vertebrates, and in some cases it is considered to be absent. Where a true stomach is found to exist it is a muscular bag, or tube with a highly acidic internal environment.Acidity of the stomach will change depending on whether it is full or not. Secretion of Hydrochloric acid is stimulated by the stretching or expansion of the stomach walls caused by the presence of food, so the stomach is more acid when it is full that when it is empty. In most fish the pH of the stomach varies between 2 and 4. The main enzymes active in the stomach are Pepsins.
At the hind end of the stomach, before, just at the beginning of the intestines many fish have some thin blind tubes called Pyloric Caeca. Not all fish have them, Wrasses, Pipefish and many Catfish do not have any.The function of these Pyloric Caeca is poorly understood, but they may secrete Trypsin and enzymes active in the intestines, it is also considered likely that they are important in neutralizing the acidity of the chyme (the partially digested food that leaves the stomach) before it reaches the intestines, where the environment is alkaline in contrast to the stomachs acidity. It is possible that the pyloric caeca play a fuller or more complex role in the digestive cycle in some groups of fish than they do in others.
The intestine is a long thin tube with a thin double layer musculature, the outer layer being longitudinal and the inner layer being circular. It is the sight of the final digestion and absorption of the food a fish eats. The rectum is the end of the intestines and through it feces pass out of the fish's body and into the surrounding water. In the lungfish, sharks and rays the rectum opens into the cloaca which also receives wastes (urine) from the kidneys and material from the reproductive organs. In bony fish the rectum reaches the outside environment through the anus, which is normally situated just in front the urinary and reproductive openings.
The pancreas secretes enzymes such as trypsin (attacks proteins), amylases (attack carbohydrates) and lipases (attack fats) into the intestines either through sharing one of the hepatic ducts (those belonging to the liver), or through its own pancreatic duct. The liver a large organ that play various roles in the fishes body, it is the site of glycogen storage, it produces a variety of substances, including enzymes that help with the digestion and it is a major chemical factory producing various hormones as well as numerous other important molecules. The gall bladder is usually found somewhere within the liver, it secretes substances that attack fats and help them to be broken down.
sharon kalunda answered the question on February 25, 2019 at 14:00