en Ecology Ecology پژوهشي Orginal research papers <p>The cosmopolitan oligochaete worm, <em>Nais elinguis,</em> is common to fresh and brackish water habitats. This species was found while investigating the limnology of two rivers alongside the Iranian coasts and has not been reported in the Iranian freshwater fauna and Caspian Sea before. <em>N. elinguis</em> was collected bimonthly from Cheshmehkileh and Sardabrood estuaries using a Van Veen grab (0.03 m²) and Surber (0.1 m², 0.2 mm-mesh size) with three replicates from 6 stations from November 2014 through September 2015. The results of temporal distribution showed that the highest and lowest density and biomass of this species were in January (242.2<span dir="RTL">&plusmn;</span>84.45 ind m^-2 and 0.105<span dir="RTL">&plusmn;</span>0.035 g m^-2) and in September (33.35<span dir="RTL">&plusmn;</span>23.5 ind m^-2 and 0.010<span dir="RTL">&plusmn;</span>0.006 g m^-2) respectively which were significantly different (<em>p</em><0.05). Among the sampling stations,&nbsp; station (S1) within the river with freshwater showed higher density and biomass (273.5<span dir="RTL">&plusmn;</span>87.35 ind m^-2 and 0.116<span dir="RTL">&plusmn;</span>0.036 g m^-2) than riverine brackish station (S2) within the mouth (39.8<span dir="RTL">&plusmn;</span>22.6 ind m^-2 and 0.013<span dir="RTL">&plusmn;</span>0.006 g m^-2) and marine brackish station (S3) (0<span dir="RTL">&plusmn;</span>0 ind m^-2 and 0<span dir="RTL">&plusmn;</span>0 g m^-2) which were significantly different (<em>p</em><0.05). Density and biomass of this species in Cheshmehkileh River and estuary were more than that in Sardabrood. A significant correlation (<em>p</em><0.05) between the density and biomass of <em>N. elinguis</em> with environmental variables was found.</p> Cheshmehkileh, Sardabrood, Estuaries, Nais elinguis, First record. 996 1013 http://jifro.ir/browse.php?a_code=A-10-2237-1&slc_lang=en&sid=1 M. Tavol Koteri meysamtavoli@yahoo.com 100319475328460031070 100319475328460031070 No Coldwater Fishes Research Center M.R. Fatemi reza_fatemi@hotmail.com 100319475328460031071 100319475328460031071 No Islamic Azad University R. Mousavi Nadushan Mousavi.nadushan@gmail.com 100319475328460031072 100319475328460031072 Yes Islamic Azad University M. Khoda Bakhshi mkhbakhshi@yahoo.com 100319475328460031073 100319475328460031073 No Shahid Beheshti University