A new dicyemid from Opisthoteuthis depressa (Mollusca: Cephalopoda: Octopoda).

Abstract A new species of dicyemid mesozoan is described from Opisthoteuthis depressa Iijima and Ikeda, 1895, collected from Suruga Bay in Japan. Dicyemennea umbraculum n. sp. is a medium species that reaches approximately 3,500 μm in length. This species attaches to the surface of the renal appendages. The vermiform stages are characterized as having 22 or 23 peripheral cells, a disc-shaped calotte, and an axial cell that extends to the propolar cells. An anterior abortive axial cell is absent in vermiform embryos. Infusoriform embryos consist of 37 cells; a single nucleus is present in each urn cell, and the refringent bodies are solid. This is the first description of a dicyemid from Opisthoteuthis.