pfd (personal flotation device).

I know you stay up late at night wondering how nautiluses and cuttlefish stay at the depth they want to be at in the ocean. How do they keep from sinking to the bottom, being crushed by the pressure (cue Queen song here), or floating up to the top and being eaten by ever so many predators? It turns out that nautiluses and cuttlefish have the ability to adjust their neutral buoyancy (i.e. they don't sink, and they don't have to swim to stay at a certain depth). They achieve this by having a mixture of gas and sea water inside of them that they can adjust. This is all due to a very cool organ known as the siphuncle that uses active transport and the balance of water and gas in the animal to regulate its buoyancy. This means they use hardly any energy maintaining the position they want to be at in the ocean. Nautiluses can go to lower depths in the day to avoid predators (down to 600 meters!) and then come back up to more shallow levels at night to feed. (Info taken from "An Introduction to the Invertebrates, 2nd Edition by Janet Moore, 2006, Cambridge University Press.)

So, everyone: I'm trading in my water wings for a siphuncle next time I go swimming.