The digestive system of fish is quite simple when compared to that of the humans. Of course there are many variations between species. A fish’s mouthparts are modified for unique food habits. Some species such as sawhorses that feed on small zooplankton or other tony food sources, have tubular sucking mouthparts. In this case, it would be incapable of eating larger foods. In contrast, some predators capture & swallow prey larger than would seem possible, but special joints enable them to open their mouths to enormous proportions while their aesophagus expands to allow prey to be swallowed whole.
Not all species have teeth, but where they are present they are usually adapted for special functions. For example, predators such as Gar & Pike have teeth that are needlelike, designed to restrain & incapacitate their prey. Predatory catfish have generally small teeth. Like other predators, they do not bite small prey, but inhale them with suction created by the mouth & gill flaps. The teeth of Piranha are used to slash or cut flesh from prey rapidly, the Pacu, a closely related group, have teeth adapted for crushing fruits & nuts. The teeth of Tilapia are pharyngeal, which are almost brush like & help separate minute foods from debris or water.
Anatomists define the stomach as the part of the gut that secretes acid. Hydrochloric acid secreted from stomach glands activates enzymes that digest protein. Tilapia are able to digest algae as their stomach acids rupture the algal cells & release nutrients. The sac-shaped stomachs of predators help to accommodate huge amounts of food. Another stomach adaptation is found in puffer or blowfish, which can inflate their stomachs with air or water rapidly. This protects the puffer from predation by increasing its size. Some species like Prochilodus, some Surgeonfish & others, have a heavily muscularized portion of the stomach which functions like a gizzard in these herbivorus/detritivorous species. Digestion in Cyprinid fish may be impaired, as they have no true stomach.
Partially digested food passes from the stomach into the intestine, where it is digested further & nutrients are absorbed into the body. Bile salts produced by the liver & stored by the gallbladder neutralize stomach acids & emulsify dietary fat. The pancreas secretes enzymes that digest carbohydrates into the intestine. The intestine of herbivorous species is often elongated & is more complicated than that of carnivorous fish. The inner surface of the intestine may be folded or have fingerlike projections that extend into the intestinal cavity. Such structures increase surface area for absorption. Small sacs, called caecae, open into the intestine of many species & also serve digestive or absorptive functions. Foods not completely digested during passage through the intestine leave the body through the anal opening.