Fish (lat. Pisces) are a large group of mobile jawed vertebrates, previously considered a superclass. Fish are characterized by gill respiration at all stages of postembryonic development of the organism[1]. From a cladistic point of view, tetrapods are specialized lobe-finned fishes. However, since the traditional concept of “fish” refers to a paraphyletic group that does not include tetrapods, fish are absent as a taxon in modern taxonomy[2].

Fish live in both salt and fresh water bodies - from cold ocean depressions to mountain streams. Fish play a role in most marine ecosystems as part of food chains. Many types of fish are eaten by people and are therefore of great commercial importance.

Modern fish range in size from 7.9 mm (Paedocypris progenetica[3]) to 20 m (whale shark).

There are 35,768 species of fish known in the world (as of 2020), but new ones are constantly being discovered[4] - about 300-500 species new to science are described every year. About 3,000 species live in Russia[5], including more than 280 species found in fresh waters[6].