Land-dwelling hermit crab

This terrestrial hermit crab belongs to the order Decapoda, class Crustacea, and family Paguridae. It is widely distributed in tropical and subtropical areas of the Pacific Ocean south of Amami Oshima and the Indian Ocean, and those found in Japan are designated as a national natural monument. It does not enter the sea except in summer when releasing its larvae. This species has a carapace length of about 3.5 cm, which is well calcified and hard. The cephalothorax is long and slender, with only the gill area swelling. The first antennal stalk is extremely long, and the baleen is flattened on both sides with rounded tips. The left side of the claw is larger. The last leg of the male has a short protrusion on the base segment, which is almost the same length on both sides. Females have three abdominal appendages only on the left side, but males have no abdominal appendages.

The closely related Nakigo hermit crab C. rugosus has a file-like projection on the upper edge of the palm of its left claw, and makes a sound by rubbing it with the ridge on the lower edge of the phalange of the first walking leg. It is widely distributed from Amami Oshima southwards, and is active not only at night but also during the day. The purple land hermit crab C. purpureus , which is found in abundance on the Ogasawara Islands, is slightly larger and purple than the land hermit crab, and prefers to use the shells of the African snail. The Sakishima land hermit crab C. perlatus , which is smaller and reddish brown than the land hermit crab, lives on the Ogasawara Islands and Yaeyama Islands, but is also common in Micronesia.

[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)目オカヤドカリ科に属する陸生ヤドカリ。奄美(あまみ)大島以南の太平洋、インド洋の熱帯、亜熱帯地域に広く分布し、日本産のものは国の天然記念物に指定されている。夏に幼生を海に放つとき以外は海中に入らない。甲長3.5センチメートルほどの種で、甲はよく石灰化して硬い。頭胸甲は細長く、鰓域(さいいき)のみが膨れている。第1触角柄(へい)は著しく長く、ひげは左右に扁平(へんぺい)で先端が丸い。はさみ脚(あし)は左側が大きい。雄の最後の脚の底節には短い突起があり、左右ほぼ同長。雌では左側にのみ三つの腹肢(ふくし)があるが、雄には腹肢がない。

　近縁のナキオカヤドカリC. rugosusは左側のはさみ脚の掌部(しょうぶ)上縁にやすり状突起があり、第1歩脚の指節下縁にある稜(りょう)でこすって発音する。奄美大島以南に広く分布し、夜間だけでなく昼間も活動する。小笠原(おがさわら)諸島に多産するムラサキオカヤドカリC. purpureusはオカヤドカリよりすこし大形で紫色を帯び、アフリカマイマイの殻を好んで利用する。オカヤドカリより小形で赤褐色のサキシマオカヤドカリC. perlatusは、小笠原諸島と八重山(やえやま)列島にすむがミクロネシアに多い。

［武田正倫］

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