Aquatic turtles

Turtle shells come in different shapes and sizes based on the adaptations of the individual species. Land-dwelling turtles tend to have more domed shells, which appear to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells which allow them to cut though the water. Sea turtles in particular have streamlined shells which reduce drag and increase stability in the open ocean. Some turtle species have ridged, lumped, or spiked shells which provide extra protection from predators and camouflage against patterned backgrounds. The humps of a tortoise shell may tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the lead edge of the plastron is thickened; it is used for butting and ramming during combat.[22]

Shells can also vary in flexibility. In tortoises, the plastron and its extensions lock the sides of the carapace together, giving it even greater crushing resistance. Some species, such as box turtles, lack the extensions and instead have the carapace bones fully fused or ankylosed together, creating a single unit. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. Softshell turtles have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, which instead consists of thick connective tissue covered in leathery skin.[23]

Jackson (2002) suggested that the turtle shell can function as a pH buffer. To endure through anoxic conditions, such as winter periods beneath ice or within anoxic mud at the bottom of ponds, turtles utilize two general physiological mechanisms: their shell releases carbonate buffers and uptakes lactic acid.[24]