2019-6-15 · Fish (plural: fish or fishes) are vertebrates which live in water and respire (get oxygen) with gills.They lack limbs with digits (fingers & toes). This is a definition which does not quite work: some amphibia also live in water and have external gills, but they are not fishes.. Fish used to be a class of vertebrates.Now the term covers five classes of aquatic vertebrates: