Fishes, like other animal species, have intraspecies morphological and physiological diversity, one of which is digestive. In nature there are fishes that are carnivorous, omnivorous, herbivorous, detritivorous and planktivorous, with a digestive tract adapted to their dietary requirements.

In the case of carnivorous fishes their digestive tract is short with a large stomach designed to contain even whole fishes. While the digestive tract of omnivorous fish is larger, and their teeth and digestive tract have some of the traits of both carnivores and herbivores. The stomach is somewhat shorter in omnivores with a primary carnivorous diet, and a more reduced stomach in omnivores with a primary herbivorous diet. Although omnivores can eat plant matter, they cannot digest some types of grains and plants (Figure 1).

Omnivorous fishes are the easiest of all fish to feed, as they eat flake foods as well as live foods, and generally a wide variety of feeds. For that reason, omnivores are an excellent choice for the aquaculture industry.

In fish, the digestive process begins in the mouth and is purely mechanical. Teeth are located in various areas of the oral cavity, and tooth-like structures or gill rakers can also be found on the gill arches, especially in carnivorous fish, although these features may be shared by some omnivores.

The esophagus serves as a transition from the mouth to the rest of the digestive system. The portion of the esophagus, posterior to the sphincter, is continuous with the stomach in gastric fishes or with the intestine in fishes without a stomach. The esophagus is highly elastic in carnivorous fishes and expands to hold prey in the mouth while feeding at a more controlled rate into the stomach. Mucus-secreting esophageal glands are located in the anterior portion of the esophagus to help lubricate the alimentary tract, while digestive juice-secreting glands are located in the posterior part of the sphincter.

The stomach can be divided into the cardiac and pyloric portion. The gastric glands form gastric pits in the cardiac stomach and are the sites of oxynticopeptic cells that produce both HCl and pepsin. Besides the oxynticopeptic cells that secrete pepsin and HCl, the mucosa has two other cell types that play an important role in digestion: the endocrine cells and the mucosal cells. Gastric juice is secreted by the gastric glands to break down structural nutrients and release soluble nutrients, starting protein digestion. The activity of acidic digestive enzymes such as pepsin, gastric lipase, and chitinase is greater at low pH. While the stomach muscles mix and shake the content to facilitate contact between the food bolus and gastric juices. This digestive process is the one that occurs in carnivorous fish, and in omnivores with a carnivorous feeding tendency.

The pyloric cecum, located just behind the stomach, is found in some species. The size and number of ceca is variable among carnivorous species and are usually absent or greatly reduced in omnivores and herbivores. The pyloric caeca are basically extensions of the upper intestine, continuing the digestive process started in the stomach and absorbing nutrients that have already been released from digestion. The mucous cells secrete mucus and certain digestive enzymes that help regulate the digestion of mainly proteins and carbohydrates. The practical result of this folding is to increase the absorption capacity of nutrients.

The small intestine commonly has two parts: the upper anterior part called the small intestine, and the lower part called the large intestine. Histologically, the upper intestine differs from the lower intestine in that the goblet cells decrease and the muscle becomes thinner in the lower intestine. In the case of carnivorous fish, at the intestinal level, there is a greater capacity for digestion of polypeptides to amino acids and less of polysaccharides to monosaccharides, while in omnivores this is more equal.

The pancreas in fishes can be located in one organ or commonly diffuse. Digestive enzymes and bicarbonate enter in ducts connected to the intestinal or cecal lumen. Bicarbonate is produced by the acinar cells of the exocrine pancreas. These secretions enter the ducts connected to the gallbladder and/or directly into the intestinal/caecal lumen. Bicarbonate raises the pH of the intestine to a range in which digestive enzymes secreted in the intestine and pyloric cecum are highly active and protect the brush borders of enterocytes from hydrochloric acid in the stomach. Among the enzymes that differentiate between carnivores and some omnivores, we find gastric and pancreatic chitinase that aids in the digestion of chitin, and can even serve as a defense against microorganisms and parasites in other tissues.

The gallbladder stores bile and then excretes it into the intestinal lumen when food enters the intestine from the stomach in response to nutrients in the digestive tract and perhaps other stimuli. Bile salts in the digestive tract help break down lipids into chylomicrons so they can be taken up by enterocytes.