Population changes and reproduction of the alien polychaete, Streblospio gynobranchiata, living in 5, 15 and 30 m depths of the south Caspian Sea (Noor coast) was studied seasonally during 2005. Density and biomass of S. gynobranchiata increased with increasing depth, total organic matter and decreased with sand percentage of the substrate. Maximum density was 10311.11 ± 1596.4 ind m−2 and biomass 2320 ± 359.19 mg m−2 were observed in 30 m depth in winter. The maximum and minimum densities and biomass were obtained in winter and summer, respectively. Mature females were observed in all seasons, but reproduction peaks were observed in summer and autumn.
