Clown fish in anemone

Do you know what a fish is?

What’s a fish? Sounds like a simple question, right? Unless we are talking about your dinner, figuring out what a fish actually is, might be a lot harder than you think. After all, a seahorse is a fish but a whale isn’t. So, what the duck is a fish?

We’ve been climbing this tree of life thing for a while now, and we’ve finally reached the vertebrates. I’ve been excited for this episode since we started. Sure, corals are mind-blowingly awesome and octopuses are my absolute favorite animals on the planet, but in broader terms, things really get interesting to me once you reach animals with spines.

Today, we’ll get into the fishes, but we’ll definitely be talking about the tetrapods soon. What would a biodiversity series be without the amphibians, reptiles, birds, and mammals. There’s so much of the animal world left to cover. But for now, let’s stay underwater and dig into the fishes. It’s time to search for Nemo.

What the duck is a fish?

I’ve studied marine ecology, so I’ve heard quite a few definitions of fish. Most of them were arguably wrong.

One professor defined the fish as jawed aquatic vertebrates, which does not make any sense for the simple reasons that hagfish and lampreys exist. Not pretty fish, but fish for sure. And pretty, as we all know, is pretty subjective anyway.

Instead of giving you my definition, I’d like to read you a very cool definition from a book I started reading a long time ago. To be fair, I never finished it, because the teacher gave us a PDF and reading PDF is never fun. Anyway, it’s called The Diversity of Fishes by Helfman et al. and the introduction is already pretty useful for figuring out what a fish is.

“One can define a fish as “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” (Berra 2001, p. xx), or more simply, a fish is an aquatic vertebrate with gills and with limbs in the shape of fins (Nelson 2006). To most biologists, the term “fish” is not so much a taxonomic ranking as a convenient description for aquatic organisms as diverse as hagfishes, lampreys, sharks, rays, lungfishes, sturgeons, gars, and advanced ray-finned fishes.”

But, to make things simple, we’ll just define fishes as aquatic vertebrates with gills and fin-shaped limbs, but still fully aware that exceptions make the rule. Not all fish have fins, as we’ll see quite soon.

Still to complicated? Fish are animals that live in the water and might have gills or fins. Yeah, not perfect, I know. But that’s the best we can do, and why we have to accept that any definition of fish will either be too complicated to understand or to simple to really apply. Deal with it, I guess.

Fish? Fishes?

Before we go any further, there is one more quick thing to clear up: in school, you very likely learned that the plural of fish is, well, fish. That’s correct and incorrect. Yay! Okay, so, what’s the deal? Fish is the singular. We all agree there. Fish is the plural. We also agree there. But fishes can also the plural, and that’s where people get annoyed. When you talk about multiple kinds or species of fishes, you can use fishes.

Let’s say you are describing a coral reef and there is a pair of clownfish in an anemone but they are surrounded by all that the reef fish community has to offer: There are fish in the anemone (two clownfish still are the same species) but fishes over the reef (because there are clownfish and other fishes). If in doubt, just stick with fish.

Okay, let’s go find Nemo, shall we?

Clown fish in anemone

What kinds of fishes are there?

The Vertebrata are divided into two simple groups that are also pretty easy to distinguish: there’s the jawless vertebrates and the jawed vertebrates. Their distinguishing characteristic: drumroll, please! the absence or presence of a jaw. There are four or five or six groups typically counted as fishes, two of which are jawless, the rest jawed.

It’s amazing to me that one of our teachers completely discounted the jawless fishes in their definition of fish without noticing. Well, at least they didn’t include the whales like other definitions did.

Jawless fishes

Let’s start with those jawless ones: The Cyclostomi include the lampreys and the hagfishes, both of which are rather unpleasant to look at.

To make this absolutely clear: Just because I said jawless doesn’t mean these things are toothless. No, the jawless fishes definitely have teeth.

The hagfishes are a little complicated to fit in, because their taxonomy actually underwent a reversal. First, they belonged with the lampreys, but then they were moved out of the vertebrates, because they don’t have vertebrae. Now, they are back with the lampreys, because they apparently lost the vertebrae but evolutionarily had them at some point.

Taxonomy moves around all the time with new things we learn. It’s amazing to me how twenty-year-old definitions are still taught at universities and schools, because teachers reuse their old materials and don’t stay up to date.

Jawed fishes

And then, evolution really pulled a number: gills became jaws. Yes, seriously. That clownfish and you both share this pretty useful anatomical feature.

Oversimplified, the most anterior gill arches developed into the jaw. In the modern fishes with jaws, these primitive jaws added more gill arches to create stronger jaws. The ones with the primitive jaws are the placoderms. I remember that because plaque is removed by the dentist. Placoderm: jaws. I know, not perfect.

The modern fishes are the cartilaginous fishes and the bony fishes—well, and then there’s the sturgeons which are primarily cartilaginous, but also belongs to the bony fishes, because nothing is ever straightforward when it comes to taxonomy. Can we finally talk about sharks now?

Admittedly, it didn’t take us long to get from slimy eel-like things with dental plates to full-on Jaws of the Great White. But, there is a crabton we skimmed or skipped, so let’s continue the overview and talk about all the groups in detail over the next few weeks, okay?

The cartilaginous fishes are typically sub-divided into the Elasmobranchii, so the rays and sharks, and the Holocephali, the chimaeras or ghost sharks. All of which are pretty cool, so I can’t wait to get way too excited about all of them.

The bony fishes, on the other hand, are divided into the lobe-finned fishes, and the ray-finned fishes. The vast majority of fish species belong to the group of the bony fishes, so there is a lot of diversity to be found here. Let’s see where our stupid little Nemo fits in.

Bony fishes, as you’ve probably guessed, have bones. Yeah, I know… not a revelation. Anyway, back to those bony fishes. While the cartilaginous fishes are mostly marine with only one group of freshwater rays, the bony fishes are found in marine and limnic ecosystems.

Early bonefish actually had primitive lungs that were just two sacs on either side of them. In most species, these have turned into swim bladders, but it is also where our lungs are thought to come from. So, we are essentially breathing through swim bladders. Oversimplifying again, of course.

Within the bony fishes, there are two major groups: the lobe-finned fishes which are the coelacanths and the lungfishes, and also where we originate as humans, and then there are the ray-finned fishes, which is where almost anything you’d call a fish belongs.

Just like most of the fish world, Nemo is a ray-finned fish. The Actinopterygii make up more than half of all living vertebrate species.

The Actinpterygii are found in freshwater and marine environments from the deepest deep sea to the mountain lakes. They can survive the heat of thermal vents and the icy cold of the Artic waters. Honestly, I could probably talk about nothing but teleost fishes for the rest of my life and never run out of topics. I won’t do that, of course, as there are way too many way too cool creatures out there and below the waves that are not fishes.

And with that, we’ve continued our overview. So long, and thanks for all the fish!