Comparison of the life cycles of two sympatric estuarine polychaetes, Hediste diadroma and H. atoka (Polychaeta: Nereididae), in the Nanakita River estuary, northeastern Japan

The life cycles of two species of estuarine polychaetes, Hediste diadroma and H. atoka, were investigated using field surveys and colonization experiments in the Nanakita River estuary, Japan. Lactate dehydrogenase (LDH) allozyme banding patterns in starch gel electrophoresis were used to distinguish between the species. We found differences in the life cycle, reproductive ecology, and distribution of the two polychaetes. H. diadroma exhibited conspicuous reproductive swarming behavior during the spring tide in April to early May. After hatching, the trochophore larvae may be transported out to sea and return to the river estuary at the =3-setiger stage. By late July, juveniles settled into brackish water, where they completed the 1-year life span. In contrast, H. atoka never exhibited this conspicuous reproductive swarming behavior. The reproductive period of this species spanned most of the year, except winter. H. atoka juveniles hatched mainly as 3-setiger nectochaetes, which immediately became benthic. The juveniles actively dispersed during the 4-to 7-setiger stage, although they could also do so at any later stage. The life span of H. atoka was about 6 months. In small river estuaries, river flooding may flush sediment and benthic animals out to sea and create vacant habitats for colonization by polychaetes. In the Nanakita River estuary, H. atoka maintained a large and stable population in Gamo Lagoon, located 250m upstream from the river mouth, whereas H. diadroma was rarely found there. Swimming excursions of juvenile H. atoka at any stage in the life cycle may ensure its distribution to new vacant habitats, and its life-cycle characteristics may allow large populations to build rapidly. As a result, H. atoka was able to maintain dominance in both the river and the lagoon in the Nanakita River estuary.