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narelleart · 3 years

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Cladistics vs “Fish”

I got an ask related to the cladistics thing that probably didn’t need this much detail, but I got excited to talk about this and whoops its a whole big explanation no one asked for. So rather than bombarding the asker with the whole deal here, I figured I’d publish this bit as its own post and just respond to their related question privately.

What I was trying to address with all this is a breakdown of how “fish” is not a valid taxonomic grouping - for it to be accurate, all vertebrates would have to be fish. I tend to just go with that because I think that’s pretty fun, but its probably more correct to say that they way we lump organisms into the group “fishes” is entirely artificial and does not reflect nature. See below for why!

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So cladistics tries to chart evolutionary relationships by shared derived characteristics - unique traits in a group's evolutionary history that can be used to define where they have branched off. A grouping is only taxonomically valid according to cladistics if you pick a branching point and include every organism that would have shared some ancestor where that branch forms. You can't exclude any group that shares that ancestor. (Why birds are reptiles - see below.)

Here's a visual I threw together:

(I made this based on notes from an old class. Apparently the graphic that my notes come from was from “Pough et al. 2013.” May need to open it for best viewing.)

So the thing with "fishes" is that all vertebrates evolved from a common ancestor with what we would call a "fish." Cladistics only considers traits that newly evolve in a lineage to group organisms together, not how many traits are retained from ancestors - but a new trait can be the loss of an old one (why "snakes" fall within the broader group of "lizards" despite perhaps feeling more intuitively primitive - they lost their limbs in more recent evolutionary history).

This is the case with everything that is a vertebrate that we don't consider to be a "fish" - we lost the traits that allowed us to live in the water, which opened up a wider array of body shapes when limbs moved from something to fan around to move forward to something that needed to bear weight.

Up to this point things make sense, but might feel a bit arbitrary, right?

The real problem with "fish" is that, following cladistics, a coelacanth is more related to us than a tuna. And a tuna related more to us than a shark. And a shark more to us than a lamprey.

Let's revisit that graphic.

To really put things in perspective, its important to understand how time is displayed here. Time here travels from left to right, never in the vertical direction. Evolution doesn't stop, and we all evolved from the same original eukaryote, so everything alive today is just as "evolved" as everything else, even if genetic progress isn't apparent. "Living fossils" like horseshoe crabs are just as evolutionarily advanced as humans, they have just evolved differently than us. All currently living organisms’ lineages have spent the same amount of time evolving as every other one. Even though the “fishes” have earlier branches, this isn’t saying our ancestor is a shark, it’s saying what are known as sharks today most recently shared an ancestor with us a very very long time ago - a much longer time ago than our common ancestor with reptiles.

So we can rearrange this graphic like this and it will still be valid:

It looks messier, sure, but it is no less valid because there was no actual hierarchy to the organisms that gave it the previous order. Doing it that way just made it easy to add labels, and over-represented the importance of mammals.

So where does that leave all the "fish"?

They got scattered around the tree. Because they aren't actually very closely related - they share a life strategy that makes certain forms more advantageous, so to us, superficially, they seem very similar. Many traits have independently evolved in multiple lineages because the selection for them in aquatic environments is so strong (convergent evolution), which makes them seem even more similar. But they aren't actually strongly related to each other, which I’ve tried to distinguish in this version by giving each a distinct color group. Each lineage of “fishes” I have listed here has been evolving independently from one another for longer than mammals have even existed.

[Side note, they're so alien! So different from us! This is why fish are so cool!!!]

Calling lungfish, stingrays, and trout all “fish” but not using the same term for koalas and ostriches suggests those first three have a greater degree of similarity to one another than they do. Sure, superficially, they all live in the water and might have some other traits that are vaguely similar. But those traits aren’t exclusive to fish and aren’t inclusive of all “fish”.

Cetaceans are very “fish shaped” and live in water, but they are not fish. Neotenic salamanders live their entire lives in water and retain their gills, but are not fish. Sea snakes live in water and have scales but are not fish.

What makes a fish?

In my ichthyology course, I was taught that there are 5 traits that make an animal a fish, and that each one of them has exceptions:

Aquatic ....except there are fishes that can walk around on land, like mudskippers and walking catfish.

Free-Living ....except there are parasitic species that rely on hosts, such as male angler fish, sea lampreys, and the Candiru

Gill Breathing ....except there are fish that have rudimentary lungs, such as lungfish and walking catfish

Cold-blooded ....except tuna and some sharks are warm-blooded

Have fins, not pentadactyl limbs ....except coelacanths have pentadactyl lobe-fins

So our best definition of what makes a fish isn’t even valid because its full of holes from all the exceptions that exist in nature. The label “fish” ultimately falls to pieces no matter what angle you tackle it from. It is, truly, a superficial similarity that we base the designation on.

Does this invalidate the science of ichthyology, my personal love for “fishes,” or any level of the value of their study?

Nah. It all really doesn’t matter.

What we call the science has no bearing on the studies we do, which aren’t dictated by organisms being “fish” or not in modern science. We have much more specific taxonomic groupings we can use, and so many more species to compare to within them. Would it be more accurate to say I’m studying to become an “Actinopterygii Biologist”? Maybe, but that’s lame and discards the history of the profession.

“Fishes” are the most diverse group of vertebrates, yet they are incredibly understudied. There are so many fish out there for us to discover - and not just in the deep sea! We are still finding fish in shallow freshwater systems, easily accessible habitats. There aren’t enough people studying this massive group to scratch the surface of the knowledge we lack about them.

So who cares if we lump a sparse group of researchers tackling a vast array of organisms together under a label based on an outdated name? The proportion of people studying “fish” to the abundance of these organisms is ridiculous, especially compared to charismatic species like large mammals. So why not use the same term rather than try to divide up an already small group?

Actually, I think few use the label “Ichthyologist” nowadays, but I intend to. Whether a “fish” is a valid group or not, I became a scientist to study fishes. No other taxa were compelling enough for me to dedicate my life to science. I am not going to be just a biologist. I’m going to be an ichthyologist.

#i went off on a personal tangent at the end here whoops #i think cladistics is neat and valid and it will probably be very relevant to my studies #im leaning into evolutionary biology which is badass #so this is the exact kind of relationships il be looking into but on a smaller scale #because i will be studying fish specifically #or actinoptrygiian fishes to be precise #i am actually going to be working within a valid clade #or most likely i mean i wouldnt rule out dabbling into others because theyre all really neat #i need to stop babbling in these notes before i start gushing about being excited to be an ichthyologist whoops

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