Itaboya - Itaboya

This colonial ascidian belongs to the order Wallea of ​​the class Urichordata of the phylum Protochordata and the family Botryllidae. The colony is membranous and about 3 mm thick. It spreads by covering other objects, and can sometimes reach a diameter of about 10 cm. The common integument is soft, agaric, and almost colorless and transparent. The body color of the zooids, which are densely embedded in the integument, can be seen through the integument, so the colony appears purple-brown or light brown when live. The body length of the zooids is about 3 mm or less. There are 10 to 11 gill slit rows. The ovaries are located posterior to the testes and contain eggs with a diameter of less than 0.1 mm. The eggs enter the brood pouch formed by the invagination of the parent's body wall immediately after ovulation and continue to receive nutrients from the vascular system of the colony even after the parent zooid degenerates. About 5 weeks after ovulation, they grow into embryos with a diameter of about 1 mm and swim out of the colony as caudate larvae. The larvae that reach the bottom spread the colony by budding. It is commonly found in the intertidal and subtidal zones along the coast of Japan. There are closely related species such as Botryloides simodensis , which are difficult to distinguish from this species in terms of colony and adult zoon structure.

[Teruaki Nishikawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

原索動物門尾索綱壁性目ボトリルス科に属する群体ボヤ。群体は膜状で厚さ3ミリメートル程度。他物を覆って広がり、ときに直径10センチメートルほどになる。共同外皮は柔らかい寒天質で、ほぼ無色透明。外皮にぎっしりと埋まり込む個虫の体色が透けてみえるため、群体は生時、紫褐色あるいは淡褐色を呈する。個虫の体長は約3ミリメートル以内。鰓孔列(さいこうれつ)数は10～11。卵巣は精巣の後背方にあり、直径0.1ミリメートル以下の卵を含む。親の体壁が陥入してできた育嚢(いくのう)に入り込んだ排卵直後の卵は、親個虫の退化後も群体の血管系から養分を供給され続け、排卵後約5週間で直径1ミリメートルほどの胚(はい)に成長し、有尾幼生として群体外に泳ぎ出る。着底した幼生は出芽により群体を広げる。日本の沿岸の潮間帯から潮下帯に普通にみられる。群体や成体個虫の構造が本種と区別しにくいボトリロイデス・シモデンシスB. simodensisなどの近似種がある。

［西川輝昭］

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