The term fish, from the Latin pisces, means a heterogeneous group of organisms
basically aquatic vertebrates, covered with scales and equipped with fins, which breathe
through the gills. With over 32 000 species they cover almost 50% of the total species
of subphylum vertebrata. In the old taxonomies they became a systematic class of
vertebrates, while modern zoology does not recognize[1] the value of category
to fish taxonomic, but a set of evolutionarily convergent or constant external traits, such as
hydrodynamics and the presence of fins, which allow them to move in the fluid element
with particular effectiveness, in addition to mainly gill respiration of oxygen
dissolved. These common data still make various uses of the denomination, even if
in a non-taxonomic field, for example in fish farming and ecology.
The oldest fossil fish is the Haikouichthys lived during the Cambrian 500 million
years ago, related to lampreys. The largest living fish is the whale shark
(Rhincodon typus) which can reach 20 meters in length and weigh up to 34 tons,
the Paedocypris progenetica instead is the smallest living fish, in fact the males reach
i 10 mm and females can be only 7.9 mm long. (source Wikipedia)


(selection 2023)