What Animals Are Shellfish?

What Animals Are Shellfish?

Most animals are not shellfish.

Animals that are not shellfish include: vertebrates, arthropods, flatworms, squids, cephalopods, and lobsters.

Of the approximately 37 phyla of animals, only four are shellfish: mollusks (snails and clams), echinoderms (starfish), brachiopods (lamp shells) and bryozoans (moss animals).

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So what is a shellfish? A shellfish is an animal that has a shell.

Why do we consider these four types of animals to be more "shellfish" than the other 33?

No one knows.

My friend at a seafood restaurant was complaining about the high price of tuna. I was trying to explain that there are other kinds of fish besides tuna, which led to the question "What are shellfish?"

I've always thought shellfish were animals without backbones. But my friend insisted they must be animals with shells on their backs. This seemed like an unfortunate design flaw, so I checked. The term "shellfish" turns out to be both vague and imprecise.

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The Wikipedia article on shellfish starts by saying "Shellfish is a culinary term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms." The phrase "culinary term" signals that we're in a zone where things have been defined by usage rather than biology.

Next I looked up the Wikipedia article on molluscs, which says "The molluscs compose the large phylum Mollusca (/məˈlʌskə/; from Latin mollis soft) of invertebrate animals." So far it looks like we're just going around in circles here. 

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I remember when I first heard the term "shellfish." I was a kid, and I took it literally: it meant fish with shells. At first this seemed unremarkable; if there were fish in shells, then there would obviously be fish in shells. But as I thought about it more, I realized that most fish did not have shells. What's more, I didn't know of any other animals that had shells. If a shellfish was an animal with a shell, did that mean shellfish were the only animals with shells?