Fish Digestion and the Digestive System

How a fish digests food, why many herbivores are stomachless with a long gut while carnivores have a stomach and a short gut, and why that means grazers should be fed little and often.

A fish's digestive system is shaped almost entirely by what it eats. The same basic plan, mouth to vent, is stretched, shortened, and in many species stripped of its stomach altogether depending on diet. Understanding this is practical, not just academic: it explains why a grazing herbivore and a predatory carnivore need to be fed in completely different ways.

The digestive tract

Food enters the mouth and passes down the oesophagus. In species that have one, the stomach stores and begins breaking down food. Beyond it lie the pyloric caeca, finger-like pouches that secrete digestive enzymes and absorb nutrients, and the intestine, where digestion finishes and most absorption happens. The liver and pancreas add enzymes and other chemicals along the way. Sharks and some primitive fish use a spiral intestine, a coiled fold that greatly increases surface area and effective length.

Teeth and stomachless fish

Not all fish have jaw teeth. Cyprinids such as carp and barbs are stomachless, or agastric, with toothless jaws; instead they have pharyngeal teeth deep in the throat that grind food against a bony chewing plate. Stomachlessness is widespread: lampreys, hagfishes, lungfishes and many teleosts have no stomach, with the oesophagus opening directly into the intestine. These fish eat diets that need little storage or no acid pre-digestion.

Gut length follows diet

Animal prey is nutrient-rich and easy to digest, so carnivores get by with a short, simple gut. Plant and algal matter is refractory and nutrient-poor, so herbivores and detritivores have markedly longer, coiled intestines, and they rely more on gut microbes to ferment and break down the food. Omnivores fall in between. The longer gut also dilutes indigestible material such as fibre, sand or exoskeleton over a greater length.