盐度对沙蜇有性繁殖阶段早期发育的影响

近几十年来,沙蜇的频繁暴发给东亚海域的海洋生态系统带来了广泛影响。在秋季,沙蜇成熟的雌雄水母体在沿岸水域聚集产卵,有性繁殖产生的受精卵发育成新的底栖螅状体,为螅状体种群数量进行补充。河口浅滩海域为沙蜇的繁育地,沿岸盐度较低,在秋季降雨期盐度多变,较低、多变的盐度可能对沙蜇有性繁殖阶段的早期发育产生重要作用,从而影响螅状体种群数量的补充。实验设置了4种不同盐度(15、20、25、30)试验组,在不同盐度下对沙蜇受精卵进行培养,探讨盐度对沙蜇早期发育过程中受精卵、浮浪幼虫发育以及早期螅状体生长及存活的影响。试验结果:沙蜇受精卵胚胎发育的适宜盐度为20,发育基本与盐度25、30同步,盐度15受精卵细胞发育迟缓,发育率显著降低;浮浪幼虫发育适宜盐度为20和25,两组浮浪幼虫附着变态率高于盐度15、30,盐度15时浮浪幼虫活力明显降低、发育迟缓,浮浪幼虫在盐度15时水中存活时间较长可达8 d,但附着时间集中在培养后的3、4天,与其他组相同;早期螅状体幼体适宜盐度为20、25、30,早期螅状体存活率、相对增长率及特定生长率均显著高于盐度15,三组间差异不显著。结果表明,盐度显著影响沙蜇有性繁殖阶段的早期发育,随着受精卵至螅状体的发育生长,其对盐度的适应范围逐步扩大。

Effects of salinity on early development in sexual reproduction stage of the scyphozoan Nemopilema nomurai Scyphozoa Rhizostomeae

In the past few decades, Nemopilema nomurai underwent blooms over many years that have affected ecosystems in East Asian waters. In the fall, male and female N. nomurai medusa gather to spawn in coastal seawaters. The egg develops into a tiny larva known as planula that swims to the seabed and transforms into the polyp stage. New polyps formed by sexual reproduction replenish the polyp population, and after sexual reproduction, the mature medusa die during the autumn season. The polyps survive in the winter to produce the following year''s progeny. Therefore, the number of polyps serves as the basis for medusoid stage (free-swimming) blooms. The Yangtze river''s offshore area is a principal breeding location of N. nomurai, where its pelagic stages appear earliest in the Yellow Sea and East China Sea. Additionally, the northern inner coast of the Liaodong Bay (LDB) is another breeding place, and as the season progresses, N. nomurai in this area grow and mature, expanding their area of distribution to the whole LDB or are advected to the southern Liaodong peninsula. In these coastal waters, the spawning of N. nomurai coincides in August-September with the summer monsoon season, where extreme rainfall subject fertilized egg embryo development, planula survival and settlement, new polyp survival and growth. In this study, laboratory experiments were conducted to determine how prolonged exposure to salinity (15, 25, 20, and 30) affected the development of the fertilized egg, settlement of planula, and the development of metamorphosed new polyps. During the embryonic development stage, salinity 20 fertilized eggs developed the highest percentage of planula, where the time to develop planula was synchronized with salinities of 25 and 30. Salinity 15 fertilized eggs were stunted, and their embryo development rate decreased significantly. In the planula settlement stage, planulae showed the highest settlement rates in environments with salinities of 20 and 25. At salinity 15, the larvae''s swimming ability in water was significantly reduced, and the larvae developed slowly while the settlement rate decreased significantly. A salinity of 15 significantly increased the planktonic larval duration, however, the settlement time was found to be concentrated 3 or 4 days following culture, which was the same as the other groups. During the settled polyp stage, 4 tentacle polyps after settlement were observed to have the highest survival rate, relative growth rate and specific growth rate in environments with salinities of 20, 25 and 30. At a salinity of 15, the predation ability of the 4 tentacle polyps decreased, and the survival rate, relative growth rate and specific growth rate decreased significantly. This study demonstrates that salinity has a significant effect on the survival and development of the fertilized egg, development and settlement of planula and its survival, as well as the development and growth of the 4 tentacle polyps. In regard to the development and growth of the fertilized egg to the polyp stage, the adaptive range of salinity was found to further increase. Sexual reproduction of N. nomurai occurs in coastal waters, and its sexual reproduction behavior has a certain adaptability to less saline water. Accordingly, when the water is highly saline (20-30), it is more favorable for sexual reproduction. Therefore, high salinity has a positive effect on the supplementation of N. nomurai to the polypoid population via sexual reproduction. However, when salinity in sea environments is too low (15), the development process in the sexual reproduction of N. nomurai is slowed, reducing the development rate and adversely affecting the supplementation of sexual reproduction to the polyp population.