The polyp-fish is a soft-bodied sea-creature about a foot long. It has in its head an organ that forms an inflated air-space filled with gas, which makes the head appear hard and round. The rest of its body appears uninflated, and its skin is smooth. It has no fins along its body, aside from its tail, and it moves by swimming through the water. It is slow and eats shellfish

Could this fish balance horizontally in the water under realistic hydrostatics?

  • 1
    $\begingroup$ Well, a submarine can. I'd say a fish can. I don't really feel like that qualifies as an answer, though. $\endgroup$
    Commented Nov 12, 2021 at 12:13
  • $\begingroup$ @KEY_ABRADE Submarines have air roughly throughout their hulls $\endgroup$ Commented Nov 12, 2021 at 12:25
  • $\begingroup$ worth considering where it gets the gas to inflate it. $\endgroup$
    – John
    Commented Nov 13, 2021 at 1:09

1 Answer 1


Yes, it can.

As long as the head where the air inflated organ is located has also a corresponding "ballast organ" which can be made heavier on demand, to compensate for the positive buoyancy created by the air, the overall equilibrium of the body won't change and it will be able to stay horizontal.


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