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"Argonaut" Octopus Sucks Air Into Shell As Ballast

Posted by timothy on from the 8-legs-good dept.

audiovideodisco writes "Even among octopuses, the Argonaut must be one of the coolest. It gets its nickname — 'paper nautilus' — from the fragile shell the female assembles around herself after mating with the tiny male (whose tentacle/penis breaks off and remains in the female). For millennia, people have wondered what the shell was for; Aristotle thought the octopus used it as a boat and its tentacles as oars and sails. Now scientists who managed to study Argonauts in the wild confirm a different hypothesis: that the octopus sucks air into its shell and uses it for ballast as it weaves its way through the ocean like a tiny submarine. The researchers' beautiful video and photographs show just how the Argonaut pulls off this trick. The regular (non-paper) nautilus also uses its shell for ballast, but the distant relationship between it and all octopuses suggests this is a case of convergent evolution."

6 of 72 comments (clear)

  1. That's not ballast. by jcr · · Score: 5, Informative

    Ballast is weight that counteracts buoyancy. By introducing air into its shell, the animal is adding buoyancy.

    -jcr

    --
    The only title of honor that a tyrant can grant is "Enemy of the State."
    1. Re:That's not ballast. by Rene+S.+Hollan · · Score: 2, Informative

      Nope.

      The argonaut traps air, and then forcefully descends to depth. So long as it has not reached the appropriate depth, it has to keep thrusting itself downward with it's jet, but once there, it is neutrally buoyant and no further expenditure of energy is required.

      It if can't get deep enough, then ultimately it will tire and the buoyancy will bring it to the surface again.

      --
      In Liberty, Rene
    2. Re:That's not ballast. by Daniel_Staal · · Score: 5, Informative

      As the pressure increases with depth, the volume of the air will decrease as it is squeezed into a smaller space. Buoyancy is determined by density, which is mass per unit volume. Mass is staying the same, but volume is decreasing.

      Above a certain depth, they will be be positively buoyant, and rise. Below that depth, they will be negatively buoyant, and sink. They gather enough air to be neutral at a certain depth, and stay there. The more air they gather the lower that depth is. If they can't get deep enough, they will tend to rise back to the surface (unless they vent air).

      The article is right.

      --
      'Sensible' is a curse word.
    3. Re:That's not ballast. by pixelpusher220 · · Score: 2, Informative

      From the comments on TFA: As the Argonaut decends, the volume of air decreases under the increased water pressure. That causes the air to be less buoyant. So with more air, the air pocket maintains its buoyancy force for deeper dives. The Argonaut still has to 'force' its way down to the depth of neutral buoyancy though.

      Also from the comments, the Nautilus traps more air and has a hard shell so they can resist the water pressures more than the Argonauts. This allows the Nautilus to dive to deeper depths than the Argonaut.

      --
      People in cars cause accidents....accidents in cars cause people :-D
  2. Re:This isn't news. by SQLGuru · · Score: 2, Informative

    The news is that we now understand how and why they do it.

  3. Re:Debated for millenia? by ivandavidoff · · Score: 4, Informative

    FTFA: No less a thinker than Aristotle put forward a hypothesis.

    That was about 2.35 millenniums ago.