Red crab - Red crab

This crab belongs to the family Grapsidae, class Crustacea, order Decapoda, phylum Arthropoda. It is endemic to the Far East, found from the Tohoku region of Japan to Okinawa, the Korean Peninsula, northern China, and Taiwan. It has a square shape with a shell about 4 cm wide, with no notches on the sides. The carapace is dark blue-green, the calyx and front edge are yellow or yellowish brown, and the claws are crimson-red. Young individuals are uniformly yellowish brown. They are often found on banks and marshes near coasts and river mouths, but can also be seen in upstream areas that are not affected by seawater. They are active from late April to early November, but their breeding season is in the summer. A female with a carapace width of 2 cm can carry about 15,000 eggs, and a female with a carapace width of 3 cm can carry about 55,000 eggs. After carrying the eggs for about a month, on the night of a full moon or new moon, at high tide, they are partially immersed in seawater and open and close their abdomen vigorously to hatch the zoea larvae. Such dramatic habits are mainly seen on the Pacific coast; on the Sea of ​​Japan coast, where there is almost no tidal difference, no clear living behaviors caused by tidal differences are observed.

[Masamichi Takeda]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

節足動物門甲殻綱十脚(じっきゃく)目イワガニ科に属するカニ。日本の東北地方から沖縄、朝鮮半島、中国北部、台湾に分布する極東地域の固有種。甲幅4センチメートルほどの四角形で、側縁には切れ込みがない。背甲は暗青緑色で、額(がく)や前側縁が黄色ないし黄褐色、はさみが紅赤色。若い個体では一様に黄褐色。海岸や河口近くの土手や湿地に多いが、海水の影響がない上流域でもみられる。4月下旬から11月上旬まで活動するが、繁殖期は夏である。抱卵数は、甲幅2センチメートルの雌で約1万5000粒、3センチメートルの雌で約5万5000粒に上る。約1か月間抱卵したのち、満月か新月の夜、満潮の時刻になかば海水につかって腹部を激しく開閉してゾエア幼生を孵化(ふか)させる。このような劇的な習性は主として太平洋側でみられるものであって、干満の差がほとんどない日本海側では潮汐差による明瞭(めいりょう)な生活行動はみられない。

［武田正倫］

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