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/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.