Not all fish have teeth, just as herbivorous fish have no teeth. Fish usually have four feeding habits. Filter-feeding, herbivorous, carnivorous and omnivorous. The digestive organs of fish are divided into mouth, oropharyngeal cavity, esophagus, bone, intestine, rectum and anus.

The digestion of fish food is related to the contraction of gastrointestinal tract, as well as the external water temperature, dissolved oxygen, food intake and physical and chemical properties of food. Some fish are parasitic, such as lamprey in the northeast, and their structures are different from those of ordinary fish.

Extended data

Most fish live in fresh water or seawater, but less than 10% of migratory fish migrate back and forth between fresh water and marine habitats. Those that grow in the ocean but need to breed in fresh water are called upstream migration (such as Chinese sturgeon), and those that grow in fresh water but need to breed in the ocean are called downstream migration (such as eel).

If the fattening and breeding migration occurs between rivers and lakes, it is called semi-migratory fish, usually fattening in lakes and laying eggs in rivers (such as the four major fish). It is very important for the survival of some migratory or semi-migratory fish to keep the migratory passage unblocked.

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