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Populations of Alitta (=Neanthes) succinea (Frey and Leuckart, 1847) collected from two closely situated locations on the Romanian coast of the Black Sea were analysed from March 2007 to May 2008 in order to elucidate their population dynamics and genetics. The mean density was 655 ind. m −2 , presenting two peaks, the first in June 2007 with 950 ind. m −2 and the second in November 2007 with 1633 ind. m −2. The mean biomass over entire period of survey was 4.33 g dry weight (DW) m −2 , with a minimum of 1.15 g DW m −2 in September 2007 and a maximum of 8.08 g DW m −2 in June 2007. The decrease of density and biomass in winter is determined by the mortality of adults, whereas the increase in numerical abundance in late spring-early summer and especially in the autumn can be explained by the recruitment of juveniles. The analysis of the size frequency histograms indicated the existence of two recruitment periods: the first in May-June and the second in September-November. From June 2007 to May 2008, the annual secondary production, mean biomass and the production/biomass ratio (P/B) for the entire population were estimated to be 5.66 g DW m −2 year −1 , 4.22 g DW m −2 and 1.34 year −1 , respectively. Extracts for isozyme analysis were prepared from 23 individuals from the polluted Danube-Black Sea Canal (population C) and 25 individuals from relatively pristine area situated at only 2 km distance (population A). A genetic analysis was conducted with 29 DNA markers on 10 individuals from population A and 10 individuals from population C. The value of Nei's genetic index was h = 0.1873 for population A, h = 0.2099 for population C and h = 0.2172 for the total, while the gene flow was Nm = 1.12. The results showed that the two sampled populations are genetically close and could be considered as a single population and genetic pool.

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