In East-Asian countries,the ark shell Scapharca broughtonii is one of the most commercially important shellfish resources.In China,however,the wild resources of S.broughtonii have experienced a dramatic decline in abundance for the past decades due to overfishing,declining water quality,and loss of habitat.To develop management strategies and optimize the hatchery-based seed production,the reproductive cycle and seasonal variations in biochemical composition of S.broughtonii from the coast of Shandong Peninsula were investigated from October 2010 to September 2011 in relation to environmental factors.The specimens were collected monthly,with temperature and chlorophyll a concentration in seawater column recorded synchronically.Standard histological techniques were used to analyse the gonadal development,and the seasonal cycles of the biochemical composition of separate organs of mantle,adductor muscle,gonad-visceral mass and foot were examined.Histological analysis showed a unimodal gametogenesis cycle with one spawning season(June-August)during the year.The gonads of both sexes showed synchronical development,enhancing the probability of fertilization.The gametogenesis of the ark shell initiated simultaneously with temperature and chlorophyll a concentration ascending.Mean oocyte diameter increased gradually during ovarian maturation,reaching the maximum(41.80 μm)ahead of spawning.The sex ratio of female to male was approximately 1:1,indicating that there was no difference in the quantities of both sexes inhabiting this area.Although the lipid content in the gonad was relatively low compared with those of protein and carbohydrate,it displayed a seasonal pattern coincident with the gonadal development of the ark shell,showing an evidence that it is a major component of bivalve oocytes.A significant increase in the glycogen content of all tissues during the gametogenesis suggested carbohydrates play the most important role in this species.Except for the adductor muscle,two significant decreases in the protein content were observed during the spawning peak and winter,which implied protein not only supports reproduction,but also meets the energy demand after glycogen was depleted.