49 * E-mail: amrollahi@hormozgan.ac.ir Biological Structure and the Effect of Temperature on Embryonic Stage of the Spirobranchus kraussii (Baird, 1865) (Annelida: Serpulidae) in Bandar Abbas in the Persian Gulf Lavajoo, Fatemeh 1 ; Amrollahi Biuki, Narges 1* Dep. of Marine Biology, Faculty of Marine Sciences, University of Hormozgan, Bandar Abbas, IR Iran. Received: February 2015 Accepted: August 2015 © 2015 Journal of the Persian Gulf. All rights reserved. Abstract A Serpulidaepolychaetae, Spirobranchuskraussii, was newly recorded in the Persian Gulf, Iran. It is the first record of Serpulidae species in Iran. Serpulidaeforms dense aggregations in rocky shores in Bandar Abbas. This research investigated the biological structure and the effect of temperature on embryonic stages of S. kraussii. Sampling was conducted with a quadrant of 10×10 cm 2 , in rocky shores from January to May. The largest specimen of S. kraussiimeasured to a total body length of 36.4 mm, body width of 2.3 mm, total tube length of 48mm, and body wet weight of 0.11mg. Outer surface of tube is white and triangular in cross-section, inner surface colored blue or violet. Body color in ventral surface of thorax is dark violet, blue and pink. The sex ratio in S. kraussii was 1:2 and 1:0.81 in winter and spring respectively. Length-weight relationship was exponential and growth seems to be allometric for S. kraussii. A linear relationship was found between total length and other parameters. Correlation coefficients between parameters were positive and significant (p<0.05). The specimen of S. kraussii in this study was larger than other specimens from different regions. The highest fertilization value and embryonic developmentwere observed at 28 ºC while at 34 ºC lowest fertilization occured and embryonic stage did not develop and cells denatured. The smallest eggs are produced at 34 ºC, whereas the largest are produced at 28 ºC. Results of this study suggest that temperature is an effective environmental factor which influences S. kraussiilife cycle, offsprings and survival of larvae. Keywords: Spirobranchuskraussii; Life cycle; Persian Gulf. 1. Introduction The Family Serpulidae (Rafinesque, 1815), is a member of fouling communities and resident polychaetes attached to hard substrata from intertidal to abyssal (5200 m) zone (Rouse and Pleijel, 2001; Ten Hove and Kupriyanova, 2009; Kupriyanova and Nishi, 2010; Kupriyanova et al., 2011). Serpulidae are easily recognizable by several characters such as, their calcareous tube, operculum, colorful radiolar Journal of the Persian Gulf (Marine Science)/Vol. 6/No. 22/December 2015/11/49-60