Life history and dispersal pattern of the polychaete worm Neanthes japonica (Izuka) in the Nanakita River Estuary, Miyagi Prefecture

Abstract

The breeding, dispersal, recruitment and life span of Neanthes japonica were investigated in the tidal flats of the Nanakita River estuary. Dispersal was examined by collecting larvae in water and assessing the recruitment of worms into settling buckets filled with air-dried azoic sediment. The results indicated that the spawning season extended from May to November with two peaks of breeding activity in late-spring and autumn. Migration and recruitment of N. japonica occurred mainly in larval stages, especially the 4- and 5- setiger nectochaete stages, in May-June and from September to October, respectively. From April to August the larger juveniles also migrated and colonized the buckets, while in autumn the juveniles with peristomial width of more than 0.75 mm, which were found in the surrounding sediments, rarely colonized. Analysis of the size structure of the population and rearing experiments suggested that N. japoncia had about a half-year life cycle with terminal reproduction. In this study, all female worms examined had eggs of a diameter greater than 180 μm, suggesting that the description of the life cycle of N. japonica in the Nanakita River estuary relates to individuals of the large-egg type. This paper may be the first detailed study focused on the life cycle of a large-egg type.