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.