[Hannah LS (OP)]

John asks:

I know giant sea turtles mostly live under the ocean, but their shells must weigh a ton - so how do they get to the surface to take a breath?

What do you think?

[evan_au]

Many marine animals have close to the same density as water, so they neither sink nor float, but can move up and down in the water column with little expenditure of energy.
- Some fish have a swim bladder which fills with gas to control buoyancy
- Turtles have lungs which they fill with air, and controls buoyancy

But there are exceptions:
- The Portuguese Man o' War, with it's balloon of gas is one that stays on the surface.
- The starfish is a bottom-feeder, so it would be denser than water..