Vertical distribution and migration of planktonic polychaete larvae in Onagawa Bay, north-eastern Japan

Abstract

The planktonic larvae of polychaetes are one of the most numerous and diverse groups in coastal zooplankton; however, little is known about their larval dynamics and the factors that affect their vertical distribution. We investigated the vertical distribution and migration of planktonic polychaete larvae in Onagawa Bay, north-eastern Japan, particularly focusing on the dominant spionid larvae. In total, 14 families of planktonic polychaete larvae and 14 species or genera of spionid larvae were identified during our study. Their density greatly fluctuated according to season and depth, with the polychaete larvae generally distributed in the lower layers of the water column. Furthermore, trends in vertical distribution of spionid larvae varied between species. In winter and spring, larvae of Polydora onagawaensis were the most prevalent, with a wide range in vertical distribution. In summer and autumn, larvae of Pseudopolydora achaeta and Prionospio spp. were the most prevalent spionid larvae and were primarily distributed in the lower layers of the water column. Trends in larval vertical distribution varied as a result of differences in adult habitat; these variations would enable the larvae to efficiently recruit into their appropriate adult habitats. Spionid larvae did not show diel vertical migration. Larvae of two spionid taxa, Pseudopolydora achaeta and Prionospio spp., exhibited tidal vertical migration, with larvae appearing to avoid dispersal by moving to slower-flowing deeper water during flood and ebb tides. Although many previous studies assume that, because of their limited swimming capacity, polychaete larvae are passively dispersed within the water column, this study indicates that polychaete larvae can control their vertical distribution to some extent, and this smallscale vertical migration may be important as a retention mechanism for polychaete larvae.