Observation On The Gonadal, Gametogenesis And Embryonic Development Of Hyriopsis Schlegelii

Developmental and reproductive biology of bivalve are researched deeply all over the world. However a prelmiary study on developmental biology of Hyriopsis schlegelii, such as the process of gonadal development, embryonic development and gemetogenesis, remains unknown up to date. As a kind of representative species of freshwater pearl mussels, H.schlegelii have huge commercial values. The results not only enrich basic knowledge of developmental biology and cytological information, but also have important referenced significance in regard to artificial breed, conservation of germplasm resources and breeding of new variety of freshwater pearl mussel. We observed the process of the first sexual maturity development, gonadal development and gametogenesis of the shell by morphological and historical examination. Besides, the morphology of sperm and egg and embryonic development of H.schlegelii were investigated by continuous observation and in vitro culture. The assessment can be briefly summarized as follows:1).Gondal developmentChanges in the gonad development of H.schlegelii. from one month old to five years old, were examined on the basis of histological observation. We observed the monthly changes of the gonad before the first sexual maturity development and it maturated in the third year, about 36 month. The gonad consisted of follicle,genital canal and gonoduct. According to the cytological characteristics and the relative amounts of the developing sexual cells in the follicle, the phases of gonad could be divided into five stages both male and female, i.e., proliferating stage, growing stage, maturing stage, spawning stage, resting stage. In addition, the main breeding season started from early April to middle July. There were two spawning peaks in a year, i.e., the May and early October. Temperature was the most important factor for influencing the maturation of the gonad and the spawning period. The gonadal development of H. schlegelii has only one cycle in a year.2).GametogenesisIn follicular epithelium primordial germ cells develop into oogonia by mitotic division. Oogonia swell and thus form early oocytes, which fall into the follicular cavity. In follicular cavity early oocytes swell further and increase in size and finally develop mature. On the basis of observation, the development of male germ cells of H. schlegelii can be divided into five stages as follows: spermatogonium stage, primary spermatocyte stage, secondary spermatvcyte stage, spermatid stage and spermatozoan stage. And spermatogonia were the biggest cells in the testis. Besides, several free spermatid morulae were observed.3). Embryonic developmentWe found that fertilization eggs can develop to gastrula periods successfully in vitro culture by the balanced salt solution (BSS) mixed by ourselves. The results showed that mature eggs is rotundity and the diameter is about 199.81±37.89μm. Mature spermatozoon, about 41.04±4.16μm long, belongs to primitive type, consisting of head, middle piece and flagellum tail. The shape of head like bullet and flagellum tail is threadlike. Characteristics of embryonic development and process that the fertilized eggs developed into mature glochidia after the cleavage and gastrula periods were introduced. When the water temperature ranged between 18-23℃,the hatching time of the embryonic development took about 12 days. The optimum temperature range of the embryonic development was 20-30℃. As observed, the glochidia of this species were hooked and half-ellipse in shape. There were relations between different stages of the embryonic development and the character of the outer gill of the female.