en Ecology Ecology پژوهشي Orginal research papers <span style="font-size:11pt"><span style="line-height:107%"><span style="text-autospace:none"><span style="font-family:Calibri,sans-serif"><span lang="TR" new="" roman="" style="font-family:" times=""><span style="color:black">Polychaetes are the most abundant taxon in benthic communities. The polychaete assemblages on the Iranian Caspian Sea coast were studied during the summer of 2016 and winter of 2017 to examine the spatial-temporal variability in diversity and community structure, as well as their relationships with the main environmental variables. As a representative of the study area, fifteen stations at 1, 3, and 5 m depths were chosen.<i> Capitella capitata</i> was reported for the first time in the Caspian Sea. Four species were found: <i>Streblospio gynobranchiata</i>, <i>Alitta succinea</i>, <i>Hypania invalida</i>, and</span></span><i><span lang="EN-GB" new="" roman="" style="font-family:" times=""><span style="color:black"> Capitella capitata</span></span></i><span lang="TR" new="" roman="" style="font-family:" times=""><span style="color:black">.&nbsp; The eastern transect (Gorgan Bay) had the highest total abundance of polychaetes (</span></span><span lang="TR" new="" roman="" style="font-family:" times=""><span style="color:black">15146 </span></span><span lang="TR" new="" roman="" style="font-family:" times=""><span style="color:black">ind.m²) with increasing in the abundance of <i>S. gynobranchiata</i>. Furthermore, <i>S. gynobranchiata</i> appears to be well-distributed in this sea and plays the most prominent role in the community structure. <i>A. succinea</i>, <i>S. gynobranchiata</i>, and <i>C. capitata</i> showed an increasing trend from the western to the eastern parts of the shoreline. Changes in polychaetes abundance can be attributed to changes in the sediment type, with western transects containing more sand and eastern transects comprising silt-loam. Total polychaete abundance was correlated with salinity, water temperature, depth, dissolved oxygen, Total Organic Matter and sediment characteristics in the summer and winter seasons, according to the Canonical correlation analysis.<i> S. gynobranchiata</i> and <i>C. capitata</i>, as invasive species, may have altered the current polychaete community distribution, structure, and diversity in the southeast Caspian Sea. </span></span><span lang="EN" new="" roman="" style="font-family:" times=""><span style="color:black">The disappearance of some species and the decrease in species richness could be due to the success of </span></span><i><span lang="TR" new="" roman="" style="font-family:" times=""><span style="color:black">S. gynobranchiata</span></span></i><span lang="EN" new="" roman="" style="font-family:" times=""><span style="color:black"> in the competition for resources and space.</span></span> <span lang="EN" new="" roman="" style="font-family:" times=""><span style="color:black">Other conditions such as environmental factors and pollutants can also affect the presence of sensitive and tolerant organisms.</span></span><span dir="RTL" lang="FA" new="" roman="" style="font-family:" times=""><span style="color:black"></span></span></span></span></span></span> Capitella capitata, Community diversity, Environmental parameters, Sediment analysis, Streblospio gynobranchiata 44 65 http://jifro.ir/browse.php?a_code=A-10-221-1&slc_lang=en&sid=1 O. Rowshan biomariner.1@gmail.com 100319475328460047112 100319475328460047112 No 1Department of Marine Biology, Faculty of Marine Science and Oceanography, Khorramshahr University of Marine Science and Technology, Khorramshahr, 669, Iran. N. Sakhaei nsakhaee@yahoo.com 100319475328460047113 100319475328460047113 Yes Khorramshahr University of Marine Science and Technology, Khorramshahr H. Zolgharnein hzolgharnein@gmail.com 100319475328460047114 100319475328460047114 No Khorramshahr University of Marine Science and Technology, Khorramshahr H. Paknezhad paknejadi570@gmail.com 100319475328460047115 100319475328460047115 No 3Fisheries Department, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran M. Taheri mehrshadtaheri@yahoo.com 100319475328460047116 100319475328460047116 No 4Hormoz Strait Oceanography Center, Iranian National Institute for Oceanography and Atmospheric Science, Iran