A fish's mouthparts are specially modified for unique food habits. Some species such as sawhorses that feed on small zooplankton or other small food sources, have tubular sucking mouthparts. In this case, it would be incapable of eating larger foods. Not all species have teeth, but where they are present they are usually adapted for special functions. Predatory catfish have generally small teeth.
Anatomists define the stomach as the part of the gut that secretes acid.
Hydrochloric acid secreted from stomach glands activates enzymes that digest protein. The sac-shaped stomachs of predators help to accommodate enormous 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. Digestion in Cyprinid fish may be impaired, as they have no true stomach.