Bonerimushi - Bonerimushi

A marine animal belonging to the family Bonellidae of the class Echinocoelida of the phylum Annelida. It is found from Chiba Prefecture southward, and digs holes in dead coral reefs and lives there. The female is dark green and about 2 cm long, and has a snout that is about 2 to 3 times longer than its body length, which protrudes from the top of its head and is bifurcated at the tip. It has a pair of abdominal setae. On the other hand, the male is very small, about 1 mm long, without a snout, mouth, or blood vessels, and lives parasitically in the female's pharynx. The dorsal surface is thickly covered with cuticle, and the ventral surface is densely covered with cilia. It has one large spermatheca, and sperm pass through tubules to fertilize eggs in the female's uterus. Small males that are only useful for reproduction in this way are called co-males. The fertilized eggs develop into larvae, which attach to the proboscis of the female parent and grow, gradually becoming males, and then live a free-floating life without attaching to the proboscis, and metamorphose into large-bodied females. In an experiment, when larvae attached to the female's proboscis were separated and reared, individuals with intermediate characteristics between male and female were produced to various degrees, depending on the location and length of time they were attached. In other words, the longer they were attached to the proboscis, the stronger the male characteristics of the individuals produced, so it is thought that the female's proboscis secretes a substance that turns the larvae into males.

[Minoru Imajima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

環形動物門ユムシ綱ボネリムシ科に属する海産動物。千葉県以南に分布し、死んだサンゴ礁に穴を掘ってその中にすむ。雌は体長約2センチメートルの濃緑色で、頭の頂端から体長の約2、3倍の長さの吻(ふん)を出し、その先端は二またに分かれている。1対の腹剛毛がある。一方、雄は非常に小さく、体長1ミリメートルぐらいで吻も口も血管もなく、雌の咽頭(いんとう)中に寄生している。背面はクチクラが厚く、腹面には繊毛が密生している。1個の大きい貯精嚢(ちょせいのう)があり、精子は細管を通って雌の子宮内の卵を受精させる。このように生殖に役だつだけの小さい雄を補雄とよんでいる。受精卵から幼生になり、その幼生が雌の親の吻について成長すると、しだいに雄になっていき、吻につかないで自由に浮遊生活をして変態すると大きな体の雌になる。実験的に雌の吻についた幼生を途中で離して飼育すると、付着していた場所や時間の長さなどによって、いろいろな程度の雄と雌の中間の性質をもった個体ができる。つまり、吻についていた時間が長いほど雄の性質の強い個体ができることから、雌の吻からは幼生を雄にする物質が出るのであろうと考えられている。

［今島　実］

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