The turtles' backbone and ribs are all fused to the carapace. This is the largest reptile on earth today by weight. Its shell makes it an oddball, for instead of a hard carapace on its back, the leatherback has five bony ridges under leathery skin which characterizes its name. Large leatherbacks, like some oceanic fish, retain waste heat from muscle activity during swimming due to their large volume to surface area ratio and blood circulation system. Therefore, they are more active in colder waters than other turtles.
Sea turtles spend nearly their whole lives in the water.
They have characteristics that are very different from turtles found on land or in fresh water. All sea turtles have rather flat, streamlined carapaces that move easily through the water. Instead of stumpy feet with claws for walking, they have flippers that cannot be pulled into their shells. In the water, these flippers help turtles maneuver as they feed and swim quickly to avoid predators, such as sharks. On land, however, the flippers are not very useful, and the turtles' large size and weight are awkward.