CONSIDERATIONS ON THE MIGRATION OF THE LEATHERBACK TURTLE, DERMOCHELYS CORIACEA (L.), IN THE JAPANESE AND ADJACENT WATERS

Abstract

The leatherback turtle, Dermochelys coriacea (L.), is indeed a peculiar species among the living members of the order Chelonia. Not only it attains the greatest size, but it lacks the bony carapace intimately united with vertebrae and ribs that is commonly seen in other chelonians. The animal has been adapted, it is considered, to the pelagic life in the sea for a much longer time of phylogenetic evolution and more perfectly than other species of sea turtles, and consequently its bony plates have been markedly reduced (VoLKER 1914). Owing to this peculiarity in carapace structure, a classification system was once widely circulated, in which the present species constituted, together with a few of its extinct ancestral forms, the suborder Athecae against the Thecophora including all other kinds of turtles and tortoises. The leatherback turtle is widely distributed in tropical and subtropical seas throughout the world, and is occasionally transported to temperate regions by ocean currents. In tropical and subtropical seas, the species is said to be but rarely seen as compared with other sea turtles such as the loggerhead, Caretta, the green turtle, Chelonia, and the hawksbill, Eretmochelys. This seems, however, to be a superficial phenomenon. Since the leatherback prefers offshore waters to shallow bays or lagoons, there may be less opportunity of observing it (SMITH 1954). If this is true, the leatherback is expected to have more chance to be carried to higher latitudes by offshore-flowing ocean currents. In fact, there are as seen later a considerable number of records of occurrences of this species along the coasts of the Japanese Islands which are mostly situated in the temperate region and are washed by two strong warm currents, namely the Kuroshio originated in the tropical seas and its branch, the Tsushima current.