偏顶蛤的性腺发育和生殖周期

Gonadal development and the reproductive cycle of Modiolus modiolus

为了探讨偏顶蛤的繁殖规律,自2013年10月至2014年9月,每月采集大连海区野生偏顶蛤（Modiolus modiolus）样品1次,测定采样点水温,采用组织学方法分析其性腺的季节变化。结果表明,偏顶蛤性腺发育以1年为1个周期,可分为增殖期、生长期、成熟期、排放期、休止期5个时期。偏顶蛤卵子的发生可分为卵原细胞期、生长初期、生长中期、生长后期和成熟期。雄性生殖细胞的发生经历了精原细胞、初级精母细胞、次级精母细胞、精细胞和精子等时期,精原细胞分为A型和B型2种类型。该海区偏顶蛤性腺指数和肥满度最低值均出现在1月,分别为17.54%和18.23%;二者最高值均出现在8月,分别为32.25%和38.43%。偏顶蛤繁殖期为6—11月,繁殖期水温在16.8～22.1℃;雌雄性腺发育基本同步,雄性发育略快,雌雄异体,二者性比接近1∶1。本研究为偏顶蛤的天然资源保护及大规模人工增养殖的开展提供了基础资料。

The wild Modiolus modiolus resource has experienced a dramatic decline in abundance over the past few decades because of overfishing, declining seawater quality, and a lack of proper protection. The reproductive cycle and gonadal development of M. modiolus from the coast of Heishijiao, Dalian city were investigated from October 2013 to September 2014 to develop management strategies and optimize hatchery-based seed production. Specimens were collected monthly, and seawater column temperatures were recorded simultaneously. Shell size, total weight, tissue weight, and gonad weight of M. modiolus were measured to calculate famess and the gonadosomatic index, and histological techniques were used to analyze the seasonal gonadal cycle. The histological analysis showed a unimodal gametogenic cycle with one spawning season （August--October） annually. Although shellfish gonadal development stages have no unified standard, M. modiolus gonadal development was divided into five stages according to previous studies： the pro- liferating stage, growing stage, maturing stage, spawning stage, and resting stage. Oogenesis was divided into the oogo- nial stage, early growth stage, middle growth stage, late growth stage, and mature stage, according to the oocyte devel- opmental characteristics and the proportions of different oocyte developmental stages in the follicle. Spermatogenesis of M. modiolus included five stages of spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, and spermatozoon. Spermatogonia were morphologically divided into types A and B. Type A spermatogonia distributed in all developmental stages, and their nuclear chromatin was arranged loosely. Type B spermatogonia occurred during the proliferating stage, and the nuclear chromatin was concentrated. Thus, types A and B spermatogonia were considered differentiated cells and stem cells, respectively. The peak gonadosomatic index of 32.25% and fatness of 38.43% were observed in January, and the maximum values for both occurred in August. The breeding season was from June to No- vember, when the average seawater temperature was 16.8-22.1 ＊C. M. modiolus gonads matured by the end of May, but they did not spawn until August of the rainy season, indicating that the mature gonads of the parental shellfish only provided a material basis for reproduction and that the reproductive process requires particular conditions for induction, such as temperature and salinity. The developmental rate of the male gonad was slightly faster than that of the female. Gonochorism existed, and the female to male ratio was 1 ： 1. These results provide basic information about M. modiolus reproduction to help with natural resources protection, and will aid in developing a large-scale artificial breeding program.