Cucumaria frondosa japonica is the largest of the Cucumaria species on our coast. It has the typical Cucumaria shape, but has a grey to black body. The tentacles and mouth region can be quite colourful, with areas of white or red. When dredged or disturbed it resembles a grey football. It is gigantic compared to other species of Cucumaria on this coast. It can attain a length of 30 cm and longer, depending on its state of relaxation. The contractile tube feet are in five rows. The 10 equal-sized tentacles are black and bushy when extended.
Skin ossicles: quite variable and usually cannot be found in large specimens; in small specimens, elongate perforated plates with large holes, sharp bumps on the surface and scalloped margins. In the introvert, the ossicles are larger and more complex.
Similar SpeciesCucumaria frondosa japonica resembles Cucumaria miniata in general body form, but the former is usually larger and darker. A juvenile C. f. japonica might be confused with C. miniata; however, the latter is usually orange. Cucumaria frondosa is a common sea cucumber on the east coast of North America. Cucumaria japonica is the closely related species in Japanese waters of the northwestern Pacific. Some consider japonica to be a subspecies of frondosa, but there is presently no agreement. The specimens on this coast seem to be intermediate in many characters, hence I have used the subspecies designation. However, Dr V. Levin from Russia has looked at our specimens and has concluded that they are not C. f. japonica and suggests there may be more than one species. Until the status of this species is resolved, I shall use the name C. f. japonica to indicate its close relationship to C. frondosa and C. japonica.