Fish | Pisces
The term fish, from the Latin pisces, means a heterogeneous group of vertebrate organisms fundamentally aquatic, 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 the vertebrate subphylum.
In the old taxonomies, they were a systematic class of vertebrates, while modern zoology does not recognize the value of the fish as taxonomic category, but a set of evolutionary traits converging or constant, such as hydrodynamics and the presence of fins, which allow them to move 'fluid element with particular effectiveness, in addition to the mainly gill respiration of dissolved oxygen. These common data still make use of the denomination, even if in the 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) that can reach 20 meters in length and weigh up to 34 tons, while the Paedocypris are the smallest living fish, in fact the males reach 10 mm and the females can be only 7.9 mm long.
