芦田川におけるイトメTylorrhynchus heterochaetusの生殖群泳について

芦田川河口域にはイトメのほか5種類の多毛環虫類が分布しており,筆者らはそれらの生態について研究調査を行なっているが,そのうち,1968,69年にわたりイトメ(生殖型個体はバチまたは日本パロロと云われる)の生殖群泳に関する生態について行なった調査の結果,次のような知見を得た.

1) イトメは,河口(本学箕島実験所前,st15)より約6.6km上流のst.4の辺りを上限とし,河口より約1.9km上流のst.13付近を下限とする約4.7kmの区間にわたって分布する.
2) バチの生殖群泳は毎年旧暦10月および11月の3,4,5日即ち新月大潮の頃の各3日間に出現する.
3) 上記の出現時期には,満潮後30～50分頃から群泳が始まりやがて退潮に乗って流下するが,群泳は約2時間(第3日は1時間余)で終焉する.また,st.7(河口より約5.5km上流)におけるバチの採集量は第1日が最も多く,第2日は大幅に減り第3日は極めて少なかった.
4) 群泳時間の後半は退潮の流れが激しくなり,バチはすべて濡筋に収斂されるのでそこでは急激な増加がみられる反面,川の浅所では群泳個体は殆どみられなくなる.
5) 群泳の初期は雌の割合が著しく少ないが,次第に増加し終期には雄を上廻るようになった.1晩の全採集個体についてみた性比は,ほぼ雌1:雄2であった.
6) バチの大きさは,始めは雌雄とも小形のものが多いが,大形のものが漸次その数を増すようになり,特に終期にはその割合が著しく大きくなる.
7) バチの体色は游出後の時間経過に従って変化し,雌の場合は橙黄色から薄緑色になり,さらに青緑色になると放卵がみられ,雄は淡紅白色から乳白色に変り放精する.この変化は体壁が破れ易くなる変化とともに,群泳出現の第1日から第3日にかけ順次にその多くの個体にみられる現象でもあり,これらからバチの多くは游出後群泳を繰返し3日目に産卵するものと推定される.
8) 孕卵数は個体による変異が大きく,53.5×103～714.5×103を示した.

As a part of the ecological studies on benthic animals in the estuary zone of the Ashida River, some researches on polychaete worms, especially on Tylorrhynchus heterochaetus, have been carried out since 1968. In those, we specialized on the swarming of epitocous individuals of the above mentioned species (viz., the so-called Japanese Palolo or "Bachi" in Japanese). The results of these researches can be summarized as follows:
(1) Actocous individuals (viz., "home" in Japanese) were found in the area from st. 4 shown in Fig. 1 (about 6.6 km up from the river mouth) to st. 13 (about 1.9 km up from the river mouth), and they were densely distributed from st. 5 to st. 7.
(2) The swarming of Japanese Palolo in this district took place in a definite period for three days following the new moon (that is from 3rd to 5th of October and November in the lunar calendar).
(3) In each day of that period, the swarming began at the time of 30-50 minutes after high water and disappeared about two hours later when while they were swept downwards. The amount of the swarming individuals collected at st. 7 each day was the highest in the first day decreasing remarkably the next day and was very faint the last day.
(4) As the downstream current became too swift in the latter half of the swarming time, the individuals were concentrated to a water-route, their number increased markedly there while they almost disappeared in the other areas.
(5) In the early stage of the swarming time, the ratio of female amongst the collected individuals was very low (i.e. 17%) but increased gradually and exceeded one-half at the end. The sex ratio of female to male was about 1:2 for a whole group of individuals collected in one night.
(6) In the early stage, individuals of small size were numerous in both sexes but later on large ones appeared more and more, until finally in the latest stage most of them were very large individuals.
(7) The body color of the Japanese Palolo changes with the elapse of time in both sexes. Change of coloration of individual occurs from orange yellow to faint green, further to blue green in the females, and from orange white to milky white in the males. The discharge of ova and sperm was observed after the occurrence of such a change of coloration. This change as well as the fragility of body wall are the phenomena commonly observed in the majority of Palolo within three days of the swarming. Then, from the above mentioned facts, it can be presumed that the majority of Palolo in this district spawn in the third night after the repeated swarming in succeeding night.
(8) The individual variation in egg number was not small, ranging between 53.5×103～714.5×103.