Perhaps the best definition comes from Tim Berra: "a poikilothermic, aquatic
chordate with appendages (when present) developed as fins, whose chief
respiratory organs are gills and whose body is usually covered with scales."
>So what about fish with no eyes?
>Fish with no Scales?
>What about fish that will drown if in water all the time?
> None of these are technically fish?
Of course, they are. No one said they weren't. What you have just described are
simply anatomic, physiological, behavioral, or ecological ADAPTATIONS.
However, as Dave Neely said:
> The term "fishes" as is generally used, includes members of several different
> and not all of the descendents of some lineages.
What this means is, lampreys, hagfishes, sharks & rays, lungfishes, sturgeons,
gars, and advanced rayfinned fishes evolved along different evolutionary paths,
and are so fundamentally different from each other, that it's not technically
accurate to lump them all together under one name.
A major misconception is that jawless fishes evolved into sharks, then skarks
into armored fishes like sturgeons and bowfin, then armored fishes into higher
teleosts (e.g., minnows, cichlids, gobies, etc.). In truth, these groups all
evolved independently and represent entirely different CLASSES of animals.
Calling all these critters fishes is like calling all animals without backbones
invertebrates. The term is correct, but there are no ancestor-descendant
relationships. A mollusk is to a crustacean as a lamprey is to a minnow.
So, it's okay (for me, at least) to call a lamprey a "fish," as long as we
understand -- and appreciate -- that the form of a "fish" has manifested itself
across different evolutionary lineages.
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