What is a fish anyway?

One discussion that frequently appear around the lunch table at work is wether humans and other tetrapods are fish. New Oxford American Dictionary define the word fish like this:

"...a limbless cold-blooded vertebrate animal with gills and fins and living wholly in water..." 

Clearly humans and other tetrapods in the majority of the cases lack these attributes. However in many cases the boundaries are not as clear. For a couple of examples we have the lobe finned fish, that possess very limb like fins with even many of the bones of tetrapod limbs; we have the axolotl, a neotenic salamander, that carry gills all there life; we have the mudskippers, that live a substantial time of their life on land and as a final example we have many of the large pelagic shark and game fishes that in a sense are warm-blooded (endotherm). This highlights the difficulties in defining whats a fish and that the definition of a fish, which includes sharks, rays and chimeras as well, may be a old and outdated way of defining this diverse group of animals.

So in some cases the morphological attributes cause confusion, but what groups of animals are included under the definition of the word fish then. Considered, as fish, are cartilaginous fish, ray-finned fish, lobe-finned fish (excluding tetrapods) and the lampreys. However in a biological and evolutionary perspective (figure 1) tetrapods are descendants of lobe-finned fish (the reason for our resemblance within the bone set up of the limbs). So in a biological perspective it is silly to exclude tetrapods from the group of fish, a lungfish is in fact more closely related to you than to a common perch (Tip: http://timetree.org/). And to go further we are, together with the lobe-finned fish, more closely related to the common perch (a ray finned fish) than what cartilaginous fish are to the perch. Still the sharks are considered fish while we aren’t.

In a matter of fact fish in a biological sense fish as a group would include basically all vertebrate species.

Figure 1. Rough tree over the vertebrates. Groups of vertebrates qualifying as fish are marked by the pale red field. Tetrapods are excluded.