Studies On Development Of Gonads, Embryos And Youngs Of Phaxas Attenuatus (Bivalvia, Mollusca)

The structure of reproductive system and the development of gonads, embryos, youngs ofPhaxas attenuatus were studied, using histological and transmission electron microscopy. Thesedates can provide reference basis for the study of reproductive biology and provide certainguidance for artificial seedling technique of Phaxas attenuatus. The results are as follows:The male and female gonads were located in visceral cavity. The female reproductive systemwas composed of vasa efferentia, gonaduct, follicle. Vasa efferentia was a large tubular structure,which was composed of epithelial cells. Tubular gonaduct was formed by the branch of vasaefferentia. It was composed of ciliated epithelial cells and reproductive epithelial cells. Folliclewas swollen end of gonaduct. It was the place of oogenesis, which was composed of epithelialcells and ovogonium. The male reproductive system was similar to female reproductive system. Itwas also composed of vasa efferentia, gonaduct, follicle. Follicle was the place ofspermatogenesis. it was composed of epithelial cells and spermatogonia. However, inreproduction season, follicle of male reproductive system appeared reproductive zone.Reproductive zone was formd by the epithelial cells of vasa efferentia extending into follicle. Itcould temporarily store sperm and sand the sperm to the vasa efferentia.Gonadal development cycle of Phaxas attenuatus was one year. The process of gonadaldevelopment was divided into five period: proliferating stage, growth stage, mature stage,discharge stage, resting stage. Mature stage was May and June. Gonadal index was to the largestin June. A few individuals began to spawn in late June. In early July, part of the individualbegan to spawn. When it was to late July, most individuals had spawned. The gonad afterspawning was similar to that of growth stage. However, the spawned gonad had little matureovum compared the gonad of growth stage. The gonad would restore to mature stage in earlyAugust and reach next discharge stage. We could proceed the artificial oxytocin and artificialseedlings from June to September.The process of oogenesis was divided into primary ovogonia, secondary ovogonia, yolksynthesis early oocytes, yolk synthesis interim oocytes, yolk synthesis later oocytes, matureovum. Primary ovogonia were located in the basement membrane of follicles, which had theability of rapid proliferation. They displayed an large, ovoid nucleus and shallow dyeingcytoplasm. Secondary ovogonia stopped proliferation and began to move to follicle cativy.They displayed1-2nucleolis and less dyed red cytoplasm.Their sizes were much larger than that of the primary ovogonia. Yolk synthesis early oocytes were derived by mitosis prophasefrom secondary ovogonia. They were ovoid, with the germ-vesicle forming. A half of cellyolk synthesis interim oocytes had move to the follicle cativy and the long axis was parallelwith follicular wall. Oocytes rapid increased and moved to the follicle cativy. Theconnections with the oocyte and follicle wall became smaller. It would form a egg handle.When oocyte was completely separated with follicle wall and dropped into follicle cativy, theovum was full maturity.The process of spermatogenesis consisted of primary spermatogonia, secondaryspermatogonia, primary spermatocytes, secondary spermatocytes, spermatids, mature sperm.Primary spermatogonia were pale, disperse and large cells. Secondary spermatogonia were darkand clustered cells which appeared smaller than primary spermatogonia. Primary spermatocytesconsisted of leptotene stage, zygotene stage, pachytene stage, diplotene and diakinesis stages.Leptotene spermatocytes lacking of synaptonemal complex compared with pachytene andzygotene. Secondary spermatocytes were rarely observed. During spermatid differentiation,proacrosomal vesicles fused to a large discoidal acrosomal vesicle. It migrated along the plasmamembrane towards the presumptive anterior pole of the nucleus in early stage spermatid. Duringintermediate stage spermatid, the acrosomal vesicle was invaginated basally and subacrosomaloccupied the invagination. A thin moderate electronic density membrane was formed between theacrosomal vesicle base and nuclear membranes; In late stage spermatid, the invagination wasdeeper than the height of the acrosome vesicle. The cone-shaped acrosomal complex was formed.A special handle-shape structure called acrosomal handle was formed to link cone-shapedacrosomal and nuclear membranes. A light region linking to the nuclear membranes was observedin the acrosomal handle. In mature sperm, acrosomal handle elongated and the invagination wasabout four times as deep as the height of the dense electron density region of acrosome.In development of embryos and youngs, the process of fertilization undergone the stage ofsperm entering into the egg, first polar body stage, second polar body stage, stage of male andfemale pronucleus formation, stage of male and female pronucleus fussion. At the watertemperature24-25degrees, embryos developed D shaped larvae stage in24h after fertilization. Itundergone the merogenesis stage, blastosphere stage, gastrula stage and trochophore stage. At Dshaped larvae stage, pouch stomach, shell gland, embryos shell, bowel had appeared and youngsbegan to have feeding behavior. At the water temperature22-25degrees, larvae grew to the(0.156±0.013)mm×(0.198±0.011)mm at9d after fertilization. Eyespot, foot, and gill appeared atthis stage. When larvae grew to the (0.190±0.014)mm×(0.245±0.016)mm at12d after fertilization,it developed the prostrate young stage and foot became the main motor organs instead of velum.When larvae grew to the (0.210±0.013)mm×(0.260±0.011)mm after14days development oflarvae, it entered the attached-betty stage. Outlet pipe, incurrent canal and chilidium began toappear at attached-betty stage.