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(de Haan, 1837)

The body and appendages are densely covered with soft tomentum; the carapace behind the orbital region is triangular in outline, smooth and glabrous beneath the tomentum. The gastric region markedly convex, having two obtuse tubercles in the median line, the anterior of which being very indistinct; there is also a small low tubercle on each protogastric region, but it becomes sometimes indistinct. The cardiac and intestinal regions are slightly convex, the latter being mounted with a low tubercle. The branchial regions are less convex, having a strong forwardly-pointed spine at the junction of the antero-lateral and postero-lateral borders.
The pseudorostral spines are shorter than half the length of the carapace; they are usually not widely divergent, although the angle between them is individually variable. The preocular tooth almost obscure; postocular cup with no accessory process on the superior margin. The basal antennal segment slender, armed distally with an obscure tooth and also with an obtuse one on the outer border near the base. A small tubercle just outside the green gland probably corresponds to the infraorbital lobe. There is a prominent tubercle on the pterygostomial region followed by a smaller one.
Chelipeds are unarmed and thickly covered with tomentum, but the fingers are naked in both sexes. In full-grown male, the fingers gape near the base, the movable finger being proximally armed with a stout tooth; in the female (and also in young male) they leave no hiatus and are uniformly denticulated throughout their whole length. Ambulatory legs are thickly covered with tomentum, the tip of dactylus being only naked and sharply curved.
Abdomen of male consists of seven distinct segments, while that of female consists of five segments, the 4th to sixth being fused together, the suture line between these segments can be faintly traced in the median convex part. (Sakai, 1938: 279)

Type locality: Japan.
Range: Red Sea - Eritrea (Griffin & Tranter, 1974); Mozambique Channel (Griffin, 1974); India - Orissa (Alcock, 1895a); Sri Lanka (Alcock, 1895a); Andamans (Alcock, 1895a); Burma - Tavoy (Alcock, 1895a); Japan - (de Haan, 1837), Kagoshima (Ortmann, 1893), Kagoshima Bay (Stimpson, 1907), Shimoda (Sakai, 1935), Tateyama Bay, Ise Bay, Gobo and Nagasaki (Sakai, 1938a), Sagami Bay (Sakai, 1965b), Tokyo Bay, Sagami Bay, Izu Peninsula, Mikawa Bay, Wagu, Kii Nagashima, Kii Minabe, Tosa Bay, and Nagasaki (Sakai, 1976a), Amakusa (Yamaguchi et al., 1976), Nuzumigaseki, Yamagata Prefecture (Suzuki S., 1979), Tanabe Bay (Miyake, 1983), Nagasaki (Griffin & Tranter, 1986a); Korea - Korea Strait (Miers, 1879b); East China Sea (Takeda & Miyake, 1969e, Griffin & Tranter, 1986a); Taiwan - Hsiak'unhsen and Tingch'ieting (Lin, 1949); Taiwan Strait (Griffin & Tranter, 1986a); China - Hong Kong and Northern China Sea (Stimpson, 1907), Hong Kong (Shen, 1931), Shaotsun, Pingtan and Xiamen (Shen, 1940b), Guangdong (Dai & Yang, 1991); Malaysia - Terengganu (Lanchester, 1902); Singapore (Nobili, 1903c); South China Sea - Namoa Island (Griffin & Tranter, 1986a); Philippines - north of Lubang Island (Serène & Vadon, 1981); Indonesia - Noordwachter Island (de Man, 1887d), Java Sea, north of Sulawesi, Ambon Bay, and Kepulauan Aru (Griffin & Tranter, 1986a); Arafura Sea (Miers, 1886); Australia - off Double Island Point, Queensland (Rathbun, 1918b); 12-180 m.

Hyastenus diacanthus