Osteological differences in the humerus of loggerhead and green turtles.

Limbs are the principal locomotory structures in vertebrate body, closely related to the habitat use and locomotion strategies of the species. Sea turtles rely almost entirely on their fore flippers for aquatic locomotion. The humerus is the most crucial bone structure connecting the scapulocoracoid and flipper, and studies thereof are used to understand turtles' locomotion mechanisms and swimming strategies. To define the morphological traits of the humerus of loggerhead (Caretta caretta) and green (Chelonia mydas) turtles and analyze interspecific differences, sea turtle carcasses were sampled from strandings and bycatch in Korean waters within their northern range limit. The length and width of the carapace, plastron, and 15 parts of the left humerus from the carcasses were measured and analyzed. The humerus length and width were positively linearly correlated with the straight carapace length of both sea turtle species, indicating that the carapace size of sea turtles can be reasonably estimated based on the size of the humerus. Additionally, significant interspecific differences were identified in 12 humeral segments, with loggerhead turtles having a humerus that was longer overall but thinner toward the shoulder than green turtles. These differences may be related to their migration patterns, as loggerhead turtles, which migrate long distances, have elongated humeri that are wider toward the tip, which may reflect their efficient use of ocean currents for swimming instead of relying on flapping. In contrast, the green turtle, which is a more vigorous swimmer, has a short but thick humerus that can withstand the stress on the shoulder caused by flapping. These findings demonstrate that bone structure and function differ among even closely related species, depending on their habitat and the environmental exploitation strategies employed.