Argonauts float!

Argonauts are odd animals. They rather resemble a nautilus, but they aren’t particularly closely related to them; their closest cephalopod relatives are the octopuses. Females have a thin shell and scoot about in the water column, but the poor males are all dwarfs, rarely seen, with no shell.

What is the shell for? It seems to be a chamber for holding a bubble of air that the animals use to maintain neutral buoyancy. I’m a little surprised that this was a surprise, though — the analogy to the chambered nautilus is obvious, and all the photos and videos I’ve seen of them suspended in midwater suggested that they were maintaining neutral buoyancy somehow.


  1. godlesswoman says

    Here is another video that explains the new discovery and shows their shell even clearer. These are really amazing animals. I have actually spent the last several hours finding out everything that I could about them.

    One funny fact I learned was that researchers originally thought that the male hectocotylus (the arm of the male argonaut that includes his penis) was a parasitic worm when they found it inside the females.

  2. gre says

    Does anyone know what the “black dot” between the eyes (where the two “crests” meet) is? Interesting creature.

  3. Anri says

    Maybe it was just the general look of the beastie (and I know it’s a depressing level of personification), but I could almost hear a little voice saying “Let go of me!” during the first segment of the film.

    Super cute.

  4. hznfrst says

    What a cool little creature! Truth remains stranger than fiction – unless the ficion is religion, of course.

  5. dashukta says

    The first I ever heard of these critters was from Jules Vern’s 20,000 Leagues Under the Sea,, where they float on the surface and use their paddle-like arms as sails…
    Yeah, Vern didn’t get much of his biology right.

  6. davegodfrey says


    Its the argonaut’s mouth. You can see the siphon just below it. The “crests” are the two modified arms that secrete the shell.

  7. Matthew Gill says

    @ #5 gre
    It appears the be the mouth. I could be wrong, but if it’s an octopus then that black spot is right where the mouth would go.

  8. says

    Does this really prove that the *primary* role of the shell is holding air? It still might be mainly an eggcase, and the animal just collects air to balance the additional weight of the shell itself, i.e. something which she neither needed nor could do without a shell.

    Ralf Muschall

  9. David Marjanović says

    The argonaut is, in fact, a species of octopus, and that shell is partially an egg case (it is only found in a female argonaut)!

    This is one reason why the shell was thought not to be for buoyancy.

    The other is that they hadn’t been observed alive till recently.

    Jules Vern