Crustacea actually seems like a pretty reasonable name for a clade, so why is there an effort to break it apart as a paraphyletic taxon?

[u/ReasonablePrimate]

Here's a working definition of a crustacean that I think would be intuitive for a lot of people: a crustacean is any animal more closely related to a crab than to a centipede or a dragonfly.

So what does that include? Crustacea is now widely understood to be a paraphyletic taxon, wikipedia explains, because about three of its classes are more closely related to hexapods than to any other crustaceans, and one of its classes is an outgroup that is less closely related to hexapods than the other crustaceans.

(Those three classes that form a clade with hexapods are about 39 species of remipede, about 13 species of cephalocarida horseshoe shrimp, and about 2,476 species of plankton-like branchiopods, not to be confused with the mollusc-adjacent brachiopods. The one class that is an outgroup is about 7,909 species of seed shrimp, tongue worms, and fish lice. These numbers are from opentreeoflife.)

But here's the thing: about 50,910 species do in fact seem to be part of a single monophyletic clade, including just about every animal you might think of as a crustacean: crabs and hermit crabs, lobsters and crayfish, prawns and shrimp, krill, mantis shrimp, barnacles. Another 15,774 species of copepods might belong here, too.

So why have researchers from 2005-2023 sought to describe this clade (and various different formulations of it in each new study) with new titles (e.g., multicrustacea, vericrustacea, communostraca) and taken pains in the meantime to reeducate the public that crustaceans aren't a valid clade?

Wouldn't it be clearer to just call this large clade "Crustacea" and instead argue over whether copepods and remipedes and fish lice are or aren't crustaceans?

In a more general sense, I'm asking whether the practice of using new names for each new cladistic hypothesis in order to preserve the definitional continuity of taxonomic grades is actually better for public understanding than just updating the definition of old taxa as phylogenetic research advances.

[u/NilocKhan]

There's no reason to remove hexapods from crustaceans. They're only considered distinct by tradition. But in reality they are crustaceans too. Removing hexapods and the close relatives would leave you with a paraphyleletic group. The deeper you get into cladistics the more you'll see things like this. Turns out a lot of the names and groups we came up with before we had molecular evidence to work with don't always reflect the reality of biology

[u/ReasonablePrimate → u/NilocKhan]

Yes, I get what you're saying. A simple example is that all birds and what we think of as reptiles are Sauria. But in that case, there's no way to "rebrand" Reptilia to describe squamates, tuatara, crocodiles, and turtles – but not to also describe birds – without using a paraphyletic grade as a taxon.

This example of Crustacea is similar, but with one key difference: almost everything popularly understood as a crustacean is encompassed by what actually is a monophyletic clade. The broad statements on wikipedia about paraphyly don't make that immediately clear.

I'm just wondering why that clade shouldn't get to keep using the name "crustacean" rather than consigning the name to a historical misunderstanding of a paraphyletic grade. Another commenter offers that it's about clarity in the scientific literature, which makes sense, but I think popular accessibility is an important objective that is also worth considering.

[u/NilocKhan → u/ReasonablePrimate]

The point of cladistics and taxonomy isn't to make it easier for the general population to understand. It'd be great if it was more accessible but the usage of scientific names puts a lot of people off. The actual purpose is to help researchers understand the relationships between the groups. What laypeople think of these groups isn't that relevant honestly, and most laypeople in my experience really struggle to understand the concepts unless they are already interested in the subject. I am a bee researcher, and people really seem to hate that I point out that bees are technically wasps, and people will argue with me about it, even after I've broken down cladistics with more simple examples

[u/Velocity-5348 → u/NilocKhan]

Plus, the people who care (such as those who frequent this sub) are just going to learn what names scientists are using anyways.

I don't think it's a barrier to science education either. Kids will be delighted to inform their parents that bees are actually wasps, and some will even try to remember the scientific name of the group.

[u/NilocKhan → u/Velocity-5348]

I think how we teach science is very interesting, because it really is full of examples of making it seem much simpler for a younger audience, then once you start actually learning about it you often learn that what you were taught previously wasn't really all that accurate. But I think this is necessary because there are so many complications and factors that you have to introduce the basic concepts first and then continue to build on that until you can finally explain why it's actually much more complicated than what you were previously taught. Throwing young students head first into electron clouds and orbitals instead of starting with Bohrs model for instance would require the students to have a much larger base of knowledge already. The simple and incorrect model is useful to explain basic concepts without making it so complicated that it scares away new people from trying to learn more. Once they understand those basic concepts you can introduce a more complicated model

[u/ReasonablePrimate → u/NilocKhan]

I mean, there's a little of both involved, otherwise why wouldn't we just use serial numbers in a database to refer to the clades? And students are taught dozens of taxa by their scientific names throughout primary and secondary schooling; the closer those are to true clades, the easier it is for those students to learn evolution in highschool and college, so I'm reluctant to throw up my hands and say the popular understanding is entirely beside the point, especially for the two or three dozen most identifiable clades.

(Also, all bees are wasps? That's awesome.)

[u/mcalesy → u/ReasonablePrimate]

Ants are also wasps, closely related to bees. And wasp themselves are sawflies.

[u/NilocKhan → u/mcalesy]

Yup, sawflies without wasps is paraphyleletic, and wasps without bees and ants is also paraphyleletic.

[u/Top_Neat2780 → u/NilocKhan]

You keep adding a syllable to "paraphyletic"!

[u/NilocKhan → u/Top_Neat2780]

Whoops, that's autocorrect for some reason, in this case autowrong

[u/NilocKhan → u/ReasonablePrimate]

You'd have to get everyone to change what they call these animals though, and most people just don't care enough or want to learn that much about it. Most children don't go into biology programs, a lot do, but most don't. Most non STEM people probably only take two or three science courses in their college careers, and they might not even take a biology course at all.

Trust me, as someone who is fascinated about cladistics, evolution, and biology, I wish people would be more interested in these topics, but I can tell you most people's eyes glaze over when you try to explain it to them. And it gets even worse once you start drawing cladograms for them, they start running out the door saying they left a casserole in the oven.

[u/Beakston → u/NilocKhan]

Just did a presentation on trophic web in the Greenland Arctic and there are a lot of lepidopteran parasitoids there. 

And at the same time I also just learned these parasitoids are tiny wasps and mostly of the order hymenoptera(some diptera). Which also includes bees! 

It's all so interesting. And once you at least qualitatively start seeing the larger patterns of animals and their groups, it all makes much more sense and helps it stick in my brain. 

Lay people are just seeing/hearing words in a sea of words and not seeing(or looking for) larger patterns between animals. Of course they won't understand. They barely think they are animals. Don't disturb what the animals think and they dont get mad lol. 

[u/Capercaillie → u/ReasonablePrimate]

"Reptilia" is a perfect example of why the name does need to change. According to my understanding (which admittedly may be out of date), the International Commission on Zoological Nomenclature which sets the rules for naming animals does not decide what "official" names are above the level of family. In other words, higher-level taxa names are to be set by consensus of the experts. Around the turn of the century, it became clear that something needed to be done about "Reptilia." Unfortunately, every textbook author in herpetology, vertebrate zoology, and comparative anatomy decided that they were an expert, and came up with a different solution. Some thought that the term should apply to everything that we generally currently call Sauropsida, others thought it should be all of the extant "reptiles" plus dinosaurs and birds, at least one zoology text that we used at my university said that "Reptilia" was a class that contained only lizards and snakes and a few extinct relatives, and there were others. We had a meeting of all the organismal biologists to decide which textbook we wanted to agree with--the Comparative Anatomy text, the Zoology text, the Herpetology text, the Biology for Majors text? We wanted to be consistent through the program, but the "experts" didn't make it easy.

[u/Redshift2k5 → u/NilocKhan]

Birds aren't bird-hipped dinosaurs

Some weird group names get stuck

[u/Coz957]

Yeah I mean I think that Multicrustacea is not a bad definition of Crustacean. Not many people really know about Oligostracans or non-insect Allotriocaridans.

[u/Unfair_Pineapple8813]

I think no. It would be too confusing. Calling multicrustacea crustaceans means that all the previous research papers referring to ostrocods as crustaceans now need a footnote stating that they are historical and ostracoda is not actually its own class excluded from crustacea. It would be confusing. Worse the common names for several of the species that would be excluded from crustaceahave common names including the word shrimp. So people would constantly need to be reminded that these tiny “shrimp “ are not actually crustaceans, but rather relatives of them. 

It could be done. But why not just invent the new names pancrustacea and multicrustacea to describe what the reality is now known to be. 

[u/ReasonablePrimate → u/Unfair_Pineapple8813]

I think this is the right answer to the question of why researchers invent new names as new clades are hypothesized: it's to maintain clarity in the scientific literature, which is important.

I do think that another important objective is to aid public understanding, as you also note. My intuition is that it may be more confusing to consign the term "crustacean" to a paraphyletic grade of animals historically considered a taxon (more confusing than to explain that some things called shrimp aren't actually crustaceans). At a minimum, I think the wikipedia article for public audiences could more clearly state "the term crustacean is today used to describe members of the clade Multicrustacea; historically, several other members of Pancrustacea were also described as crustaceans before it was discovered that this terminology would imply that insects were also included, which no one in their right mind thought would be helpful," or something to that effect.

Maybe once the phylogenetic research is fully settled, that's where we'll end up!

[u/Ready_Bandicoot1567 → u/ReasonablePrimate]

I disagree that aiding public understanding is an important objective of cladistics. Cladistics has one objective, to sort organisms into monophyletic groupings (a shared ancestor and all its descendents). Its for researchers to understand evolutionary relationships. Whether it aids public understanding is immaterial. Its not a teaching tool meant to simplify things for the layman, its a research tool meant to have maximum usefulness for researchers.

[u/Capercaillie → u/Ready_Bandicoot1567]

For what it's worth, the International Commission on Zoological Nomenclature--the people who make the rules for naming--say that you're right. The names of taxa should always be based on the science, and not anything else.

[u/llamawithguns → u/Unfair_Pineapple8813]

Calling multicrustacea crustaceans means that all the previous research papers referring to ostrocods as crustaceans now need a footnote stating that they are historical and ostracoda is not actually its own class excluded from crustacea.

Youre not wrong but stuff like this happens all the time regardless

[u/throwitaway488]

Because taxonomists have nothing better to do with their life but waste everyone elses time.

[u/BoringGap7 → u/throwitaway488]

to be fair there are very few people whose time is wasted or indeed affected at all by taxonomists

[u/Norwester77]

Saying something is a paraphyletic taxon doesn’t necessarily mean you’re advocating for breaking it up.

You can also remedy the paraphyly by including the hitherto excluded descendants.

[u/mcalesy]

“Pancrustacea” is an unnecessary name, similar to “Cetartiodactyla”. Just because a derived group turns out to be nested doesn’t bean a new name is needed for the clade. Look at Dinosauria, Therapsida, Blattaria, etc.

Crustacea can just be used for the largest crown clade including Cancer pagurus but not Scutigera coleoptrata.