Long-Term Observations on Larval Recruitment Processes of Bivalve Assemblages on Temperate Tidal Flats

Abstract

Six years of data concerning larval recruitment processes,collected from1990to 1995,indicate that there are remarkable seasonal and annual fluctuations in the densities of different life stages of two species of bivalve mollusc(Ruditapes philippinarum and Musculista senhousia)that are common and abundant on Japanese tidal flats.Based on the results of cohort analyses for the two species,we estimated densities of planktonic larvae,new settlers,and recruits by cohort.Then,in order to identify the factors that are vital for determining the dynamics of the benthic populations,we examined the relationships be tween densities of planktonic larvae and new settlers,between the density of new settlers and recruitment success/failure,and between densities of new settlers and recruits.The present study indicates that recruit ment success/failure depends on a matching/mismatching with unpredictable environmental disturbances on the tidal flats.Furthermore,the density of recruits increases with that of new settlers for cohorts that succeed in recruiting,even though density effects were not detectable statistically between the two.Taking into account the results of the present study and our earlier reported findings,we conclude that remarkable seasonal and annual fluctuations in the densities of benthic populations can be caused by a few cohorts that have huge densities of planktonic larvae and new settlers and that subsequently manage to survive and succeed in recruiting.