Studies On Reproductive Biology And Artificial Breeding Technology Of Scapharca Broughtonii

The ark shell Scapharca broughtonii is one of the most commercially importantshellfish resources in East-Asian. In China, however, the natural resources ofS.broughtonii have experienced a dramatic decline in abundance for the past decadesdue to overfishing, declining quality of water, and habitat destruction. To developmanagement strategies and optimize the hatchery-based seed production, thereproductive biology and artificial breeding technology are essential to be investigatedand improved.Seasonal variations in gonadal development and biochemical composition of S.broughtonii were investigated in relation to environmental factors from October2010to September2011along the coast of Shandong Peninsula. The specimens werecollected monthly, with temperature and chlorophyll a concentration in seawatercolumn recorded synchronically. Standard histological techniques were used toanalyse the gonadal development, and the seasonal cycles of the biochemicalcomposition of separate organs of mantle, adductor muscle, gonadal-visceral massand foot were examined. In this paper, effects of stocking densities and diets ongrowth and survival of pelagic larvae of S.broughtonii were also studied in twoexperiments by rearing the larvae at four densities and by feeding the larvae with fouralgal diets, respectively. In addition, the effect of various substrates on the settlementof S. broughtonii larvae was evaluated in order to search suitable collectors forefficient hatchery culture. The major results of the paper are listed as follows:1Studies on reproductive biology of the ark shell Scapharca broughtoniiHistological analysis showed a unimodal gametogenesis cycle with onespawning season (June-August) during the year. The gonads of both sexes showedsynchronical development, enhancing the probability of fertilization. Thegametogenesis of the ark shell initiated simultaneously with temperature andchlorophyll a concentration ascending. Mean oocyte diameter increased gradually during ovarian maturation, reaching the maximum (41.80μm) ahead of spawning.The sex ratio of female to male was approximately1:1, indicating that there was nodifference in the quantities of both sexes inhabiting this area. Although the lipidcontent in the gonad was relatively low compared with those of protein andcarbohydrate, it displayed a seasonal pattern coincident with the gonadal developmentof the ark shell, showing an evidence that lipid is a major component of bivalveoocytes. The glycogen content of all tissues had a significant increase during thegametogenesis, suggesting that carbohydrates play the most important role in thisspecies. Except for the adductor muscle, two significant decreases in the proteincontent were observed during the spawning peak and winter, which implied proteinnot only supported reproduction, but also met the energy demand after glycogendepleted.2Effects of stocking densities and diets on growth and survival of pelagic larvaeof S.broughtoniiResults of the feeding experiment indicated that larvae fed bi-algal diets hadlarger mean shell length and specific growth rate (SGR) than that fed single-algaldiets, with no significant difference among survivals of these treatments except forlarvae fed Chlorella sp.. When feeding the bi-algal diet of Isochrysis galbana andChlorella sp., optimal larval growth and survival were observed. For stocking densitytreatments, larval growth and gross SGR were found to significantly increase withincreasing stocking density. Statistical differences in mean shell length, SGR, andsurvival at different stocking densities appeared from day10, and the density of4larvae ml-1proved to be optimal for larval growth. Results of the present experimentwith different stocking densities indicated that it was feasible to set10larvae ml-1forrearing the early pelagic larvae and then decrease the stocking density to4larvae ml-1during the post larval culture. In addition, according to the data of SGR, larval growthcan be divided to two phases, including the accelerated growth stage (day1-9) and thedecelerated growth stage (day9-15).3The effects of various substrata on the settlement of S. broughtonii larvaeThe settlement of larvae on four kinds of substrates were compared in this experiment. Results showed that larval settlement of various collectors were in theorder of red coir ropes> polyethylene meshes> plastic sheets> cotton cloth. Thenumbers of larvae on polyethylene meshes and red coir ropes were not affected by thecollector orientation (i.e. vertical or horizontal setting). The numbers of spat on thevertically positioned cotton cloth increased with increasing water depth. However,the opposite tendency was observed on both collectors of red coir ropes andpolyethylene meshes. In addition, collectors of rough surface contour attractedsignificantly more larvae than that of smooth surface contour.