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Rathbun, 1932

C. japonicus differs from the early known Chionoecetes opilio O. Fabricius in the following particulars.
The bathymetric range of C. japonicus is more or less than 2000 meters, while that of C. opilio (in Japanese waters) is nearly 200 meters. The fresh colour of C. japonicus is uniformly deep red, that of C. opilio is light copper brown.
In C. japonicus, the branchial regions of carapace are extremely swollen, and in consequence, the posterior slope of carapace appears very steep. In C. opilio, the branchial regions are rather flattish and the posterior slope is not so steep.
In both species, the lateral and posterior transverse ridges of branchial regions are set with low protuberances, each of which is mounted with a cluster of granules. These two ridges meet at the postero-lateral corner of the carapace at an angle of about 60°, where a strong process is found in C. japonicus, but no such process in C. opilio, the process being replaced by a cluster of granules.
The posterior margin of carapace of this genus is marked with two granulated ridges; in C. japonicus these two ridges meet near the postero-lateral angle, in Chionoecetes opilio, however, they run parallel onto the antero-lateral portion.
The anterior pleopod of male of C. japonicus is provided with long inwardly curved apical process, that of C. opilio, the apical process is short and beak-like and curved rectangularly inward. (Sakai, 1978)

Type locality: off Sawazaki, Sado-gashima, Japan.
Range: Japan - off Sado-gashima, off Toyama Bay, Noto Peninsula, Mikuni, Tsuruga, off Tottori, Matsue, off Sendai, Onahama, and off Inubo-zaki (Sakai, 1976a), Yamagata Prefecture (Suzuki S., 1979), off Tottori and Kamaishi (Miyake, 1983), Kushiro, Hiroo and Rebun, Hokkaido (Komai et al., 1992), Soya Strait (Takeda & Miyauchi, 1992); 480-2300 m.

Chionoecetes japonicus