Abstract
A living mature female medusa of Melicertum octocostatum (M. Sars) (Leptomedusae, Melicertidae) was collected in the Sea of Japan off the coast of Yamagata Pref., northern Japan in June, 2003. This specimen serves as the basis for the first description of this developmental stage of this species in Japan. The morphology, cnidome on the tentacles and oral lips (atrichous isorhizas and microbasic mastigophores), and egg size (mostly 104 μm in diameter) of this medusa are described, and occurrence records of 23 other medusan specimens of this species from the same area in Yamagata Prefecture are given. In comparison with 7 other described specimens from two localities in northern Japan (Hokkaido and Aomori Pref.), the present specimen is the largest (15.1 mm in both diameter and umbrellar height) and has the most numerous tentacles (at least 80 large tentacles and a nearly equal number of small tentacles). Two individuals of a parasitic trematode were found in the mesogloea of the female medusa.
