Shamisengai - Shamisengai

A general term for animals in the Lingulidae family of the Brachiopoda class of the Tentaculata phylum, and one of these. A small animal that lives in the sand and mud of shallow seas, it was named after the shape of a pair of rectangular shells supported at the end of a fleshy rod-like stalk, which resembles a shamisen. It is also known as Mekaja (female crowned person).

The Lingula jaspidea is found in inland bays from Mutsu Bay in Aomori Prefecture to the Ariake Sea in Kyushu, where it lives by digging holes in the organically rich mud of the intertidal zone. Its shell is yellowish-green and about 3 cm long, with a fleshy stalk 4-5 cm long. It resembles bivalve mollusks, but the two shells are located dorsoventrally on the body and have no ligaments. The shells are made of calcium phosphate and are symmetrical. The body consists of internal organs wrapped in a mantle and arms that extend in a complex shape. A row of tentacles grows on the arms, forming a crown of tentacles that surrounds the mouth. It has nephric ducts and a closed vascular system, and its blood is colorless. It has an equilibria. It is dioecious and fertilizes externally.

The larvae also have two shells and swim in the sea. The shell, which was originally circular, changes to an elliptical shape, and when a fleshy stalk protrudes from the rear, they settle on the seabed. Adults position themselves in a vertical hole about 20 centimeters deep, with their shell on top and their fleshy stalk on the bottom, and the tip of the fleshy stalk, which has a lump of mud attached to it, acts as an anchor. When the tide is high, the fleshy stalk extends until the tip of the shell reaches the seabed, and seawater is taken in by ciliary movement into the open shell, where it feeds and exchanges gases at the same time. At low tide, the fleshy stalk retracts and dives into the hole. When an adult is dug up and placed on the mud, it rubs its two shells together to dive into the mud.

In addition to L. unguis, about 10 extant species of the genus L. have been recorded from Japan, including L. unguis , a green L. with a shell length of about 4 cm. Some species are edible, but in recent years, most species have become rare due to reclamation and pollution of the sandy and muddy bottoms of coasts. Many fossil species of the L. family have been known since the Ordovician period of the Paleozoic era, and many of them are considered standard stones. Extant species are famous as "living fossils" because the shape of their shells has hardly changed.

[Mawatari Shunsuke]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

触手動物門腕足綱無関節目シャミセンガイ科Lingulidaeの動物の総称、およびそのなかの1種。浅海の砂泥中にすむ小動物で、肉質棒状の肉茎の先に長方形の1対の殻を支える形を三味線に見立てて名づけられた。メカジャ（女冠者）の別名もある。

　シャミセンガイLingula jaspideaは、青森県陸奥(むつ)湾から九州の有明(ありあけ)海にかけての内湾に産し、潮間帯の有機質に富む泥に穴を掘ってすむ。殻は黄緑色で長さ約3センチメートル、肉茎は長さ4～5センチメートル。軟体動物の二枚貝類に似ているが、2枚の殻は体の背腹に位置し、靭帯(じんたい)もない。また、殻はリン酸石灰でできていて、各殻は左右対称である。体は外套膜(がいとうまく)に包まれた内臓と、複雑な形に伸びた腕とからなる。腕には触手が列生し、口を囲む触手冠を形成する。腎管(じんかん)と閉鎖血管系を有し、血液は無色。平衡器をもつ。雌雄異体で体外受精を行う。

　幼生も殻を2枚もち、海中を遊泳する。初め円形であった殻が楕円(だえん)形に変わり、後部から肉茎が突出するころ、海底へ定着する。成体は深さ20センチメートルほどの垂直の穴の中に、殻を上に肉茎を下にして定位し、泥の塊を付着した肉茎の先端が錨(いかり)の役目を果たす。潮が満ちているときは、殻の先端が海底面に届くくらいに肉茎を伸長し、開いた殻の中へ繊毛運動で海水を取り込んで、餌(えさ)をとると同時にガス交換を行う。干潮時には肉茎を縮めて穴の中へ潜っている。掘り出した成体を泥の上へ置くと、2枚の殻をこすり合わせる運動を行って泥の中へ潜る。

　日本からはシャミセンガイのほかに、殻長4センチメートルほどのミドリシャミセンガイL. unguisなど現生シャミセンガイ属約10種が記録されている。食用となる種もあるが、近年、海岸砂泥底の干拓や汚染が進み、ほとんどの種がまれになった。シャミセンガイ科は、古生代オルドビス紀以来多くの化石種が知られ、標準化石とされているものも多い。現生種はその殻の形がほとんど変わっていないため「生きている化石」として有名である。

［馬渡峻輔］

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