First record of Nais variabilis Piguet, 1906 (Annelida: Oligochaeta: Naididae), and its spatial and temporal distribution along Cheshmeh kileh and Sardabroud estuaries in South Caspian Sea, Iran

Nais variabilis is cosmopolitan oligochaete worm and is common to fresh and brackish water habitats. This species has not been reported from Iran freshwater fauna and South Caspian Sea before and it was found while investigating limnology of Cheshmehkileh and Sardabroud rivers and estuaries alongside the Iranian coasts. Nais variabilis was collected bimonthly from November 2014 to September 2015 using a Van Veen grab (0.03 m2) and Surber (0.1 m2, 0.2 mm-mesh size) with three replicates at 6 stations. Results of temporal distribution showed that the highest and lowest density and biomass of this species were in November (118.8 ±79.8 ind/m2 and 0.075 ± 0.034 gr/m2) and in September (26.5 ±9.8 ind/m2 and 0.015 ± 0.005 gr/m2) respectively which were significantly different (P<0.05). Among sampling stations, freshwater stations (S1) within the river showed higher density and biomass (139.4 ± 83.3 ind/m2 and 0.08 ± 0.029 gr/m2) than brackish stations (S2) (21 ±10.6 ind/m2 and 0.015 ± 0.006 gr/m2) and marine stations (S3) (0 ±0 ind/m2 and 0 ± 0 gr/m2) which were significantly different (P<0.05). Density and biomass of this species in Cheshmehkileh River and estuary was more than Sardabroud. A significant correlation (P<0.05) between the density and biomass of N. variabilis and environmental variables were found.