Distinction of two morphotypes of Turritopsis nutricula medusae (Cnidaria, Hydrozoa, Anthomedusae) in Japan, with reference to their different abilities to revert to the hydroid stage and their distinct geographical distributions

Abstract

By field sampling and laboratory rearing during 1976-2003, Turritopsis nutricula has been collected from almost the entire coast of Japan, from Okinawa Prefecture to Hokkaido, including offshore islands such as the Nansei Is., Tsushima I., and the Ogasawara Is. Combining all previous records of this species in Japanese waters since 1909 with recent records and observations, two morphotypes of medusae can be distinguished by umbrellar size and the number and arrangement of tentacles. The previously known type, here termed the "large type", has many tentacles, up to 196, in two or more rows. It is distributed in northern Japan (Hokkaido and N Honshu) and in the Seto Inland Sea. The "small type", a newly recognized morphotype in the present paper, has fewer than 82 tentacles in only one row and occurs in southern Japan (S Honshu, Kyushu, and the Nansei Islands). Life history studies have demonstrated rejuvenation in both morphotypes, although not as complete as the process in Mediterranean specimens, which are morphologically similar to the small type. In Japanese waters, the small type can revert to the hydroid more frequently than the large type. Rejuvenation of the small type over two generations was observed for the first time, and indefinite rejuvenation may be possible under optimal culture conditions.