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 22 and biomass 2320 + 359.19 mg m 22 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.

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For studying community structure and biodiversity of macrofauna, seasonal samplings were carried out along three different depths (5, 15 and 30 m) in four transects (12 stations) at Noor coast during 2005. Four higher taxa were determined. Polychaeta was the dominant group comprising 96.61 of the total individuals, followed by Bivalvia 1.35%, Oligochaeta 1.15% and Amphipoda 0.87%. Of the six encountered species, the polychaete, Streblospio gynobranchiata accounted for 84.95 of the total population. Also, Tubificoides fraseri was observed in the Caspian Sea for the first time. During this study, density of macrofauna increased with depths, total organic matter and percentage of silt–clay. The highest density was obtained in winter and the lowest was observed in summer. Maximum diversity, richness, and evenness were obtained, 0.91, 1.05 and 0.88, respectively. Also, multivariate analysis separated differences of density between seasons and depths.

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