In Defense of Fish

 Fish are great: Here is a phylogeny, an evolutionary history, of fishes. 

Evolutionary groupings are the thing in biology.

All animals in a named group, mammals, for example, have a common ancestor or should.

A little bit ago (okay, a long time ago), analyses of the group that generally were called fish, including sharks, bony fishes, lungfishes, and more, were found not to have a single common ancestor. The same situation also applies to reptiles; there is no common ancestor, and it doesn’t fit a neat evolutionary grouping. It turns out that birds are grouped with dinosaurs—valid and worthy science. But what do we do with the term fish if fish do not exist? An unfortunate carryover is that fish is in the name of many things that are not fish but were once kinda fish, Lungfish, for example. This is a common problem in the English language: Silverfish and starfish are not even pretending to be fish. Do not get me started on seahorses.

Side note: The wordsmiths tell me that I can’t use kinda in written English, just spoken English. I’m supposed to use kind of…well F that.

I’m here to tell you that fish 🐟 still exist. Further, a group of organisms on the planet can be called fish following the evolutionary ancestor rule critical to biology.

What is a fish? To find a group we can call fish, or fishes more properly because we are talking about more than one species, we must exclude sharks, lungfish, the poor maligned coelacanth, and perhaps others. 

Ok, sharks are no longer fish. My brain is having a problem with this, but I’m trying.

What is a fish? Fish have:

Fins?

Scales? 

A swim bladder?

If you have all these then you are a fish. Does this work? No, it doesn’t.

We have to get more specific to make fish a worthy evolutionary group:

Fish have jaws, bones, and paired fins. The fins are rayed. They have one pair of gill openings and one pair of nostrils. They have a swim bladder (well most of them do). Wouldn’t it be great if our fish group had a feature found in no other group? That would satisfy the disorder. Some are too common in many other groups: Jaws and bones. Those will not work. 

 Let’s look at some of the other potential features:

Swim bladder:

Most fish have a swim bladder, but the swim bladder is homologous and leads to other features in groups no longer called fish. Lungs, for example, in Lungfish, and they are no longer in our fish group. Plus, all those terrestrial vertebrates took and modified the swim bladder/lung.

Scales:

Similar to the swim bladder scales, they are observed in other fish-like organisms that we no longer call fish, such as the coelacanth. We may need to get specific with the type of scales. However, that could be dangerous since one kind might lead to another, evolutionarily. In addition, the origin of scales is pre-fish.

There are four types of scales: cosmoid, ganoid, placoid, and elasmoid (sub-tyoes, cycloid and ctenoid). Cosmoid scales in lungfish and ganoid scales, in sturgeons, are related. Placoid scales are in sharks, no longer fish. Maybe the elasmoid scales? But, alas, no. The coelacanth has these types of scales.

Let’s abandon scales.

Gill openings:

Fish, our fish, have one pair of gill openings. Sharks have more (usually 5). Hagfish (no longer fish) have more. Will this work? No, our friends, the lungfish and coelacanth, will mess us up again. 

How about fins:

How could fins work? Sharks have fins, and the coelacanth has fins. But let’s get more specific. Of course, the clue is in the phylogeny above. Our fish have rayed fins. That is, the fish are not fleshy or filled with bony appendages (coelacanth and lungfish) but instead have thin rays of bones that are covered with a thin membrane. Shark fins are not like these at all; for example, shark pectoral fins are much more robust, like airplane wings. It works.

Ray-finned fishes are our new fish. This group is called the Actinopterygii. Again, see the phylogeny (Figure 1). Thus, it is not the bony fishes (Osteichthyes) that are fish, but just the Actinopterygii. The Latin meaning of this term is having rays: Wonderful.

Problems that arise:

What will we do with all the things with fish in the name? And, what do we do with, for example, shark fins, which are not homologous to fish fins yet still carry the name fins? Certainly, we will not clear up the problems with misnaming things in English, adding to the examples above;

Starfish - not a fish. They should be called sea stars.

Cuttlefish - not a fish. Cuttlepods?

Jellyfish - not a fish. They should be called jellies.

Shellfish - not a fish. Call them clams, mussels, crabs, etc.

Silverfish - not a fish. Just rename this one entirely, maybe creepy little bugs?

Seahorse - fish, not a horse. Horsefish? Hmm, I don’t like that much.

The confusion extends beyond fish, of course:

Shipworm, not a worm but a mollusk.

Flying fox, not a fox, but a bat.

Bearcat, not a cat or a bear. These are civets, and they are cat-like. Hmm.

Can we solve the confusion? Nope. But we can push in that direction. Let me propose a few related to sharks, and lobed-finned animals, which, of course, are no longer fish. Rather than the old term, let’s try:

Shark fins = wings.

Pectoral wings, pelvic wings.

Lobed-finned fishes.

Lobes = limbs.
Lobed-finned fishes = Limbed Sarcops (from Sarcopterygii).

Ok, good. All problems solved. Well, maybe not. 

Sources and Further Reading:

Mirande JM. 2017. Combined phylogeny of ray-finned fishes (Actinopterygii) and the use of morphological characters in large-scale analyses. Cladistics 33: 333–350.

Sallan LC. 2014. Major issues in the origins of ray-finned fish (Actibopterygii) biodiversity. Biological Reviews 89: 950–971.

Phylogeny is from:

Yamamoto, Kei & Bloch, Solal & Vernier, Philippe. 2017. New perspective on the regionalization of the anterior forebrain in Osteichthyes. Development, growth & differentiation. 59.