Does the word "shellfish" annoy you since shellfish aren't fish? Does it bother you when people call orcas "killer whales" when they're dolphins? Because it bothers me even though I only have a high school level understanding of Biology I wanted to know if it was just me...
Hmm I think I used to, but now not so much, and I’ll tell you why.
Common or colloquial names vary so much within and between localities and languages that we shouldn’t expect the same kind of stringency we hold to real scientific names and groupings. The point of a name is to convey information, and in certain contexts an informal and not necessarily biologically accurate common name is suitable to convey relevant information to a wide group of people within a certain locality. Bird common names are particularly bad, for example an american blackbird is not closely related to the eurasian blackbird at all, and in addition there are about 26 (not necessarily related) species called blackbirds found in the Americas. However, if you are sitting in your garden in the UK and you hear a blackbird, you don’t need to differentiate between that and the 26 American birds to know that what you are hearing is Turdus merula. Common names are perfectly acceptable in the right context.
Now, shellfish is a handy and historically well established culinary term for basically any edible marine invertebrate. I don’t think it would be necessary to have to start saying bivalve chowder, linguine with marine invertebrates, or decapod tempura just for the sake of scientific accuracy. The term is specific to English too - in latin based languages such as French, Spanish, and Italian etc., the same group of animals are referred to under the umbrella term of “Fruits of the Sea”. We know they are not fruits, and I’m sure (or I at least hope) that most people know that “shellfish” are not actually fish, however, as the title of my favourite podcast goes, there’s No Such Thing As A Fish - this is because the group of animals that we would call fish, is a paraphyletic group - which in terms of biological semantics, doesn’t exist.
Basically, a paraphyletic group is a group of organisms including the latest common ancestor, but not including all descendants. Below in yellow are the groups that we would typically refer to as “fish”, however this excludes amphibians, and other land vertebrates etc., which are nested in the fish family tree. In fact, humans are more closely related to ray finned fish (such as salmon etc.) than ray finned fish are to sharks, yet the term fish removes this information.
The proper, monophyletic groupings (ancestor and all descendants), which retains such information are displayed below for contrast, but you don’t need to say that you are going Osteichthyes-ing when you are going on a fishing trip.
We basically use the word fish to refer to non air-breathing marine vertebrates with that share general habitats and ecologies, which is a useful word to have. For example don’t need to have a different, scientifically accurate term for overfishing for each fishy group, that would weaken the meaning of terminology for the action of overfishing, and make conservation policy and public outreach more difficult. Overfishing as a word is easy to understand, and in this context, it gets the job done, whether you are a biologist, a policy maker, a fisherman, or an average joe.
SO scientifically, even the word fish to begin with is problematic! But such semantics aren’t necessary for everyday life, and thus the word fish still has value. It’s widespread usage is simply historical leftover from when the word fish basically meant anything living in the sea (shellfish, starfish, jellyfish) - even the word dolphin comes from the latin for fish with a womb, which leads me onto your next example…
And guess what, there’s no such thing as a dolphin - yes, it is yet again another paraphyletic group. The common term dolphin excludes porpoises and other small toothed whales which are nested within classical dolphin groups, i.e. the superfamily, Delphinoidea.
But, like fish, dolphin is still a handy term to refer to a specific type of cetacean, so it’s not going to stop being used.
The important thing to remember is that all dolphins are whales. There are two major sub orders within Cetacea, the Mysticeti, or baleen whales such as humpback, blue, grey, minke etc. - i.e what we would typically think of as whales. However, there are also the Odontoceti, the toothed whales, which includes sperm whales, beaked whales, river dolphins, oceanic dolphins, porpoises, beluga whales, and narwhals. If the term whale is understood not to include dolphins then it becomes a paraphyletic group. Even though an Orca is part of the oceanic Dolphin family Delphinidae (which also includes bottle nosed dolphins, common dolphins etc.), it is still technically a whale. ADDITIONALLY the name killer whale may be due to a mistranslation of their 18th century Spanish name, asesina-ballenas which literally translates as whale killer as indeed, Orcas will hunt baleen whales.
Anyway the point is, at the end of the day, if the right information is conveyed by a common or informal name within the context of day to day life, scientific semantics are unnecessary. Lol, following that logic to the extreme would mean that the name seahorse is wrong. Of course it would be cool if people knew more about cetacean taxonomy, or took an interest in marine invertebrates, but I don’t think that enforcing correct nomenclature is central to doing that. Most of the time these terms are simply just the name for a thing, disassociated from any greater meaning - I would still use the words shellfish in a restaurant, or the word starfish or jellyfish etc. and I am currently studying marine invertebrates!
And hey, then next time those terms come up in conversation you could always use that opportunity to crack open a few fun facts about how orcas are part of the dolphin family, and that all dolphins are whales, or that the prawn and clam on your plate are not related to each other, or to that can of tuna in your cupboard.