Monday October 7, 2024
When it comes to categorizing the natural world, sometimes a seemingly simple question can be shockingly hard to answer. Consider, for example, the following question: what makes a fish a fish? At face value, this may seem easy to answer. One might say it’s an animal that lives in the water and has fins, but of course that doesn’t rule out whales and dolphins. One could add that fish have scales, but this doesn’t hold up to scrutiny either; catfish, eels, paddlefish, and others don’t have scales, and sturgeon have scutes which are somewhat different. One could also offer that they lay eggs in the water, but alas, species like mosquitofish and great white sharks give live birth. Maybe their cold-blooded nature helps clear things up? But no, some species like opah actually maintain a steady body temperature like mammals and birds do. Even adding the qualifier of having gills and respiring underwater doesn’t help, as some species like gar and mudskippers breathe air.
If this is harder than you expected, don’t be disheartened. Naturalists and taxonomists have been trying to clearly define what are and aren’t fish for millenia. Pliny the Elder took a stab at it 2,000 years ago, writing “There are seventy-four species of fishes, exclusive of those that are covered with crusts.” Old Pliny missed the mark by a bit (there are actually around 30,000 species), not to mention he also defined whales and sea cucumbers as fish. But overall, not bad for a first century scientist!
Even with the modern technology available to scientists today, there still is no single set definition of what makes a fish a fish. Examining evolutionary lineage doesn’t necessarily help clear things up either.
For example, African lungfish and other lobe-finned fishes of the group Sarcopterygii represent a separate branch from the evolutionary lineage that ultimately gave rise to the ray-finned fishes (Actinopterygii), which include most modern fish species. These ancient lobe-finned fish share a common ancestor with all four-limbed vertebrates on the planet, so in a sense, humans are just very highly evolved fish.
The ancient evolutionary timeline of fish has allowed these creatures to diverge into an absolutely astounding diversity, with the fish species found on Earth today accounting for over half of all vertebrate species on the planet. This means that counting all the birds, mammals, reptiles, and amphibians together wouldn’t even amount to half the number of fish species on the planet. Despite the challenges inherent in organizing a group this enormous and diverse into neat and tidy categories, taxonomists have put forth their best effort.
Generations of study have led to the classification of fish into three general classes: Agnatha, Chondrichthyes, and Osteichthyes. The meaning of these Greek words give clues as to what types of fish each class includes. Agnatha, for example, means “jawless,” and this ancient group includes species like lamprey and the notoriously slimy hagfish, which have a skull but lack a true jaw and true vertebrae. This group is relatively small, containing a little over 100 species in total. Chondrichthyes, meaning “cartilage fish,” includes the sharks, rays, skates, sawfish, and chimeras. These fish are characterized by cartilaginous skeletons and the lack of a swim bladder, an organ that helps maintain buoyancy in other fish species. This group is an order of magnitude larger than the Agnathans, comprising a little over 1,200 known species. Finally, Osteichthyes, or “bony fish,” is actually a “superclass” that contains everything else. Aside from having skeletons made up primarily of bone tissue, there are not really any other characteristics common to all Osteichthyes simply due to the incredible number and diversity of species this group contains – which amounts to roughly 28,000.
Fish are the perfect example of how challenging the science of taxonomy can be. Evolution is an eternally ongoing process that results in a continuum of biodiversity, and any time a continuous variable is forced into discrete categories, things are bound to get a little messy. Scientists do their best to keep things organized, but it is truly a Sisyphean task, one which must continually be repeated as new data become available. So it turns out if someone asks how one can tell if an animal is a fish, even a fish biologist may have to settle for the answer “I’m not sure, but I know one when I see one.”
Head Image Caption: Collage of different types of fish.
This post was featured in our weekly e-newsletter, the Fish Report. You can subscribe to the Fish Report here.