The shell of some sea turtles allows for different levels of buoyancy for juveniles and adults by changing shape.
"Sea turtles sometimes swim on the surface; Jeanette Wyneken tells me that the flared, V-shaped bottom is characteristic of buoyant baby sea turtles, which are obligatory surface swimmers. With maturity and the shift to submerged swimming, the hull shape changes to one more characteristic of submarines." (Vogel 2003:116)
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