Earlier this week I was working on a webinar presentation, and one of the topics was what makes a fish a fish. This might seem obvious, but it’s really a little trickier than you might think.
If we start with a little taxonomy, fishes are, like us, in the subphylum Vertebrata. From there, they split three ways. The first superclass is the Agnatha (meaning jawless), a primitive group that includes hagfish and lampreys. They’re fish, but they also seem a little like big slimy worms. It’s probably not what you picture when you hear the word “fish.”
The second superclass is the Chondrichthyes (or cartilaginous fishes), which includes the familiar sharks and rays but also the strange and ancient chimaeras, deepwater fish that split off from the shark lineage almost half a billion years ago and haven’t change much since.
The final superclass is the Osteichthyes (jawed fishes), which includes lobe-finned fish and ray-finned fish. There are only seven species of lobe-fins still living (two coelacanths and five lungfish); most have been extinct for millions of years. Today, 99 percent of the roughly 30,000 known fish species worldwide are ray-finned species.
Back to what makes a fish a fish. For starters, you might say it’s not an invertebrate. Considering fishes are in the subphylum Vertebrata, that should be a given. And, you would be correct, mostly. But there’s an exception. Hagfish, despite being classified as a vertebrate, really aren’t.
OK, let’s try this: Fish live in the water. Well, that’s only mostly true too. A few fish — mudskippers, for instance — spend a large portion of their time out of the water. And there are many species that can live outside of water, at least for a while. Who hasn’t seen a walking catfish flopping across the road?
Walking catfish possess specialized gills. This adaptation inhibits gill collapse and the associated loss of surface area which would otherwise inhibit breathing. But they do breathe with gills, so maybe that’s what makes a fish a fish. And, again you’d mostly be correct. Remember those lobe-finned fish I mentioned earlier? Lungfish do indeed have lungs, and locally, our own tarpon can and do utilize atmospheric oxygen.
Alright, how about fish are cold-blooded? Mostly true — but opah, some tunas and lamnid sharks (makos and great whites) are not. How about paired fins? I saw that on the Internet. However, lampreys do not have paired fins. Scales? One word: Catfish! OK, there’s more than that. But let’s face it, we all know catfish.
So, what makes a fish a fish? Well, all fish have a brain and an obvious head region which contains a mouth, teeth or gill rakers and other sensory organs. Beyond that, all bets are off. Rocket science, right?
Whew! Now that that’s taken care of, let’s talk about fish versus fishes. If I caught 29 mangrove snapper, did I catch 29 fish or 29 fishes? I know it really doesn’t matter, but if you want to talk like a scientist, I caught 29 mangrove snapper fish. However, if I caught a Florida Trophy Catch of a bonefish, a permit and a tarpon, then I caught three fishes. The difference is in whether I am talking about one species (fish) versus more than one species (fishes).
So, what did we learn here today? Three things: First, pretty much everything in nature has an exception. Second, if you want to catch fishes, you have to target multiple species. Third, if you got this far, congratulations on being a fish nerd. Welcome to the club.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at email@example.com or 941-764-4346.