H.,�Sato-Okoshi,�W.,�Nishitani,�G.�andEndo,�Y.�2014.�Verticaldistributionandmigrationofplanktonicpolychaete� larvae in Onagawa Bay, north-eastern Japan. Memoirs of Museum Victoria�71:�1-9. 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 focusingonthedominantspionidlarvae.�Intotal,�14�familiesofplanktonicpolychaetelarvaeand�14�speciesorgeneraof� spionidlarvaewereidentifiedduringourstudy.�Theirdensitygreatlyfluctuatedaccordingtoseasonanddepth,�withthe� polychaetelarvaegenerallydistributedinthelowerlayersofthewatercolumn.�Furthermore,�trendsinverticaldistribution� ofspionidlarvaevariedbetweenspecies.�Inwinterandspring,�larvaeofPolydora onagawaensis were the most prevalent, withawiderangeinverticaldistribution.�Insummerandautumn,�larvaeofPseudopolydora 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 efficientlyrecruitintotheirappropriateadulthabitats.�Spionidlarvaedidnotshowdielverticalmigration.�Larvaeoftwo� spionid taxa, Pseudopolydora achaeta and Prionospio spp., exhibited tidal vertical migration, with larvae appearing to avoiddispersalbymovingtoslower-flowingdeeperwaterduringfloodandebbtides.�Althoughmanypreviousstudies� 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 small- scale vertical migration may be important as a retention mechanism for polychaete larvae.