Abstract
A commensal hydroid Monobrachium parasitum MERESCHKOWSKY, 1877 associated with a new host bivalve, Cadella lubrica (GOULD, 1861), is described based on five colonies collected from the bottom of the sea off Yubetsu, Hokkaido, Japan taken at a depth of about 55 m. Every colony shows an undescribed colony form by the presence of a plate-like hydrorhiza with one stolonial extension. This is not only the southernmost distributional record of this species in the Sea of Okhotsk but also the first record in Japanese waters.
