The leatherback sea turtle, sometimes called the lute turtle or leathery turtle, is the largest and heaviest of all living turtles; sometimes exceeding 2.2m and 700kg. It is the only living species in the genus Dermochelys and family Dermochelyidae and is belived to be related to the now extinct Archelon (a prehistoric turtle from the Late Cretaceous which measured up to 14 feet). Surprisingly, the Leatherback Sea turtle is the fastest reptile on the planet, clocking in at up to 22mph/36kph. Leatherbacks are also the most widely distributed sea turtle, inhabiting anywhere from alaska to new zealand to west africa!
The reason they can inhabit such a diverse aray of places, including very cold water, is due to their high activity rate. Leatherbacks only spend an estimated 0.1% of the day resting -meaning the other 99.9% is spent traversing the open ocean. This constant swimming creates lots of endothermic heat which, when paired with very high circulation systems in their fat, allows them to maintain a very high temperature in comparison with the surrounding water. It also allows them to acheive other impressive feats - being the deepest diving reptile at up to 1280m.
Leatherback Sea Turtles' jaws are very delicate and would be damaged by anything other than soft-bodied animals. This is why they almost exclusively live off a diet of jellyfish - occassionally supplemented by squid and other cephalopods. Due to them primarily eating jellyfish, it is estimated that over 1/2 of Leatherbacks have consumed plastic mistakenly, due to it being almost indistinguishable from their favourite source of food - unfortunately, not many turtles actually survive. Because they are pelagic hunters, travelling great distances, encounters with plastic might not be as rare of you'd imagine.
Because of human activity and increasing deposits of plastic being fed into the ocean, The leatherback sea turtles population is in rapid decline. Luckily lots of conservation efforts have and are being implemented globally to combat this decrease in population, and protection of their nesting beaches has been a big help in the fight to bring them back from the brink of extinction!