02 July 2014

Why there is "no such thing as a fish"

I have been listening to podcasts of QI's "No Such Thing as a Fish," each of which is introduced with the statements "No, seriously.  It's in The Oxford Dictionary* of Underwater Life.  It says it right there in the first paragraph: there's no such thing as a fish."

I wondered about the logic of the statement, which maddeningly was never discussed in any of the podcasts I had heard.  Finally a web search led me to the QI episode above, during which Stephen Fry quotes the eminent Stephen Jay Gould's assertion that there is no such thing as a fish. (q.v. for explanation) (addendum: or read Sylvia's notes in the Comment section).

While I'm on the subject of podcasts, I can also recommend QI's series of "International Factballs" produced to coincide with the World Cup.  Lots of tidbits of "things you wouldn't know" in those broadcasts.

*There is actually no book with that title.  Oxford does, however, publish an encyclopedia of underwater life (see the comment thread below).


  1. Though I do love QI, they tend to play a little fast and loose sometimes... (Which I totally forgive; Stephen Fry can lie to me all day long.)

    The "no such thing as fish" is the kind of mess that you get trying to reconcile colloquial speech with scientific knowledge - kind of like how the word "theory" gets mangled in evolution debates, but more complicated. Taxonomy wants to name entire groups - one common ancestor and all it's descendants (these are "monophyletic groups", as opposed to paraphyletic groups [groups that have an ancestor and some but not all of its descendants], and polyphyletic groups [groups that several descendant groups but not their most recent common ancestor]). From that perspective, all the things we colloquially call "fish" are not a proper group - if we went back to the common ancestor of all fish, and then looked at that common ancestor's descendants, we'd get quite a lot of things that aren't "fish", including humans.

    But there are monophyletic groups of fishy things. Actinopterygii, the ray-finned fishes, constitute most proper fishy things one would think of - everything from goldfish to tuna. Sharks, rays and other cartilaginous fishes are part of another monophyletic group, Chondrichthyes. And then there are a couple weirdos like hagfish and lampreys, and things like the coelocanth, which is part of Sarcopterygii, the lobe-finned fishes, of which Tetrapoda (and thus humans) are also a part.

    So really, colloquial "fish" is like, 90% a real thing - it's *mostly* Actinopterygii. There definitely is such a thing as fish, it's just that if you want to speak scientifically, you either have to add some qualifiers or accept that essentially all vertebrates are fish.

  2. I see a pseudo quote attributed to Stephen Jay Gould, but I can't find where the statement was ever recorded or published. This is pretty much word play about science words vs. colloquial words. Depending on how you define "fish", all tetrapods including us are fish. We retain features that first evolved in fish. We often make a show of our education by saying that cetaceans aren't fish, but that's just a superficial (superfishal?) understanding of the relatedness of species and their evolutionary history.

  3. I also can't confirm there's any such publication as, "The Oxford Dictionary of Underwater Life". The only hits that come up are related to the QI show and its echoes.

    1. I believe they are referring to the "Oxford ENCYCLOPEDIA of Underwater Life."


      One of the elves will probably show up here to confirm.

    2. Your elf has arrived & her university has online access to this book! :-)

      In the book, "The Encyclopedia of Underwater Life" (edited by Andrew Campbell and John Dawes) there is a chapter called "Fish, what is a?". Under the section header "Classes of Fish?" subsection "Basic Parameters" starts with the relevant part (which boils down to what Anderov said):

      "Incredible as it may sound, there is no such thing as a “fish.” The concept is merely a convenient umbrella term to describe an aquatic vertebrate that is not a mammal, a turtle, or anything else. There are five quite separate groups (classes) of fishes now alive – plus three extinct ones – not at all closely related to one another. Lumping these together under the term “fishes” is like lumping all flying vertebrates – namely, bats (mammals), birds, and even the flying lizard – under the single heading “birds,” just because they all fly. The relationship between a lamprey and a shark is no closer than that between a salamander and a camel.

      However, the fact that “fish” has become hallowed by usage over the centuries as a descriptive term dictates that, for convenience's sake, it will be used here. It is worth remembering, however, that employing this term to describe the five different living groups is equivalent to referring to all other vertebrates as tetrapods (four-legged animals), even if some have subsequently lost or modified their legs."

    3. Way more than three extinct groups of "fish", as there's been quite a bit of paraphyly and polyphyly uncovered within jawless and basal jawed fish in more recent work.

  4. A relevant QI forum thread is here -


  5. It had bothered me as well. I satisfied my curiousity with this telegraph article:

    "No such thing as a fish? This really means that unlike mammals and birds, not all the creatures we call fish today descend from the same common ancestor. Or put another way, if we go back to most recent common ancestor of everything we now call fish (including the incredibly primitive lungfish and hagfish), we find that they also were the ancestor of all four-legged land vertebrates, which obviously aren’t fish at all. So, it’s a term to use with caution. After all, in the 16th century, seals, whales, crocodiles and hippos were called fish and cuttlefish, starfish, crayfish, jellyfish and shellfish still are."

  6. As far as I'm concerned, fish are like vegetables--neither are scientific terms, but both have dietary meaning.

    My advisor, on the other hand, likes to quip that, if he were still Catholic, he would feel perfectly fine eating hamburgers during Lent on the basis that "fish" could be considered a monophyletic group equivalent to Craniata or Vertebrata.

    1. Your advisor might enjoy serving capybara to his guests:

      "The Church, by the way, also classified another semi-aquatic rodent, the capybara, as a fish for dietary purposes. The critter, the largest rodent in the world, is commonly eaten during Lent in Venezuela. “It’s delicious,” one restaurant owner told the New York Sun in 2005. “I know it’s a rat, but it tastes really good.”"


    2. I'm sure he would. Apparently that entire group of rodents is delicious.