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If we are in water and stop swimming we would float on the surface, to be able to sink during a considerable amount of time (for scuba diving for example) we need to add ballast weight to counteract the buoyancy.

However, red sliders get to float or sink at their will without swimming at all, just staying still, how is that physically possible?

With the air in their lungs... they regulate buoyancy by adjusting residual lung volume. Pretty cool article here:

http://www.bioone.org/doi/abs/10.1655/07-050.1

steve wrote:With the air in their lungs... they regulate buoyancy by adjusting residual lung volume. Pretty cool article here:

http://www.bioone.org/doi/abs/10.1655/07-050.1

Wow, very interesting. I already thought it had to be something like that but I couldn't find an article.

It seems curious to me this fact I read: 'The shells of chelonians are negatively buoyant, and the tendency to sink is opposed by air in the lungs'.
Following that statement we could assume that turtles tend to sink, but they fight against sinking by enlarging their lungs with air... so, why when a turtle is dead they are found floating? When they die they obviously don't have control of their bodies, so it should be natural for them to sink instead of floating.

My guess is that either might happen when they die. Perhaps bacteria is producing gas inside the body?

steve wrote:My guess is that either might happen when they die. Perhaps bacteria is producing gas inside the body?

Yes, that’s the only explanation I can think of, lungs and other organs are probably filled with some gas or air so their body float.