Fish is the oldest vertebrate. Some hybrid offspring with different chromosome numbers still have fertility. They inhabit almost all aquatic environments on earth, from freshwater lakes and rivers to saltwater oceans.

There are two kinds of fish: Annihilis and Jahnoidea. Annihilis includes circadian rhythm and armor, and Jahnoidea includes shield fish, cartilaginous fish and kiwifruit. Most fish live in water all year round and breathe through their gills.

Extended data

Some scientists have found that fish obviously have the ability to learn. Based on this, they think that this learning ability can support a series of complex behaviors of fish. Many kinds of fish can complete extremely complicated navigation behavior by memorizing the idea map.

Some fish know how to observe and remember the past fighting process of potential opponents, so as to take the initiative in the war and strive to win the opponent. Some fish even know how to make and use tools. For example, they know how to pry open the shells of mollusks with incus.

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