What is the nature of fish?

Answer 1

A counter, used in various games., of Fish, A name loosely applied in popular usage to many animals of diverse characteristics, living in the water., An oviparous, vertebrate animal usually having fins and a covering scales or plates. It breathes by means of gills, and lives almost entirely in the water. See Pisces., The twelfth sign of the zodiac; Pisces., The flesh of fish, used as food., A purchase used to fish the anchor., A piece of timber, somewhat in the form of a fish, used to strengthen a mast or yard., To attempt to catch fish; to be employed in taking fish, by any means, as by angling or drawing a net., To seek to obtain by artifice, or indirectly to seek to draw forth; as, to fish for compliments., To catch; to draw out or up; as, to fish up an anchor., To search by raking or sweeping., To try with a fishing rod; to catch fish in; as, to fish a stream., To strengthen (a beam, mast, etc.), or unite end to end (two timbers, railroad rails, etc.) by bolting a plank, timber, or plate to the beam, mast, or timbers, lengthwise on one or both sides. See Fish joint, under Fish, n.

Answer 2

It's strange but true that there's really no such thing as fish! Unlike with mammals and birds, not all the animals we call fish - aquatic, vertebrate animals covered with scales - descend from the same common ancestor. On other hand, if we go back to the most recent common ancestor of everything we now call fish, we find that it was also the ancestor of all four-legged, land vertebrates (tetrapods), which obviously aren't fish at all.

Biologists call this a paraphyletic collection of taxa, in which the case of fish includes hagfish, lampreys, sharks and rays, ray-finned fish, coelacanths and lungfish. Indeed, lungfish and coelacanths are more closely related to the tetrapods (mammals, birds, amphibians etc) than to fish such as ray-finned fish or sharks.