NANFA-- What *is* a fish?

Christopher Scharpf (
Sat, 07 Jun 2003 11:54:32 -0400

Moon asked:

> What exactly IS a fish, anyway? What qualities do ALL fish possess that
> are unique to them, making them fish? I'm sure somebody knows, but that
> somebody isn't me. Every time I think I have it figured out, I think of
> exceptions which derail me.

Moon, you're not the only one derailed!

Okay, a little background here, then your answer:

(Pat Ceas, Dave Neely, etc., correct me if I'm wrong with any of this:)

Systematics is the study of the evolutionary relationships between
organisms, with the ultimate goal of tracing the way in which life has
diversified and changed over time. In short, systematics seeks to
reconstruct genealogies that demonstrate how one species, or one collection
of closely related species, has given rise to another. A genealogy is
considered "good" or "natural" when it forms a monophyletic group -- that
is, a group of organisms derived from a single ancestor that includes all
the descendants of that ancestor. A monophyletic group can be large (the
entire animal kingdom) or small (one or more species that evolved from an
immediate common ancestor). Today, the most widely accepted school of
biological classification is based on the recovery of monophyletic groups.

Theoretically, the various levels of classification -- kingdom, phylum,
class, order, family, and genus, to name just the major ones -- should each
constitute a monophyletic group. The trouble with the animals we
collectively refer to as fish is that they are not contained within any one
monophyletic group. Instead, they are spread out among several

The five groups commonly referred to as fishes -- hagfishes, lampreys,
cartilaginous fishes, lobe-finned fishes, and ray-finned fishes -- have each
followed a separate line of descent, thereby negating the collective term
"fish" as a natural biological unit. Did you know that coelacanths and
lungfishes are more closely related to four-legged animals (including
humans) than they are to other fishes?

Therefore, from an evolutionary perspective, there is no natural biological
unit that can uniquely and correctly be called fish. The word is merely a
term of convenience, used by people from all walks of life (ichthyologists
included) to categorize a group of animals that are superficially similar,
but do not share a common genealogy.

It should not be surprising, then, that ichthyologists sometimes struggle
when challenged to come up with an all-purpose, full-proof definition of the
animals they study. Here are two of the best ones:

A fish is a cold-blooded, aquatic chordate with appendages (when present)
developed as fins, whose chief respiratory organs are gills and whose body
is usually covered with scales.

A fish is an aquatic vertebrate with gills and with limbs in the shape of

Problems with the first definition is that some fishes are warm-blooded
(e.g., tunas), many eel-shaped fishes lack fins, some fishes (e.g.,
lungfishes) do not have gills, and many fishes (e.g., catfishes) are

The glaring problem with the second definition is that one group of fishes
cannot legitimately be included among vertebrates; the eel-like,
slime-covered marine scavengers known as hagfishes contain not a trace of
backbone. Sticklers for systematic accuracy might say calling a hagfish (or
a lamprey, or a shark) a fish is like calling a lion a frog.

Answer your question? :-)

Chris Scharpf
Heart in Huntsville, Body in Baltimore
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