黄条鰤胚胎发育和早期仔鱼生长的温盐适应特性

The ecological and physiological responses of embryonic development and early larval growth of Seriola aureovittata to temperature and salinity

中文关键词: 黄条鰤  胚胎发育  仔鱼生长  水温  盐度

英文关键词: Seriola aureovittata  Embryonic development  Larval growth  Temperature  Salinity

中文摘要:

      采用实验生态学、形态测度和分子生物学的方法，研究了温度、盐度对大洋性经济鱼类黄条鰤(Seriola aureovittata)胚胎孵化率、初孵仔鱼畸形率、内源性营养吸收利用、生长基因表达、存活指数(SAI)和饥饿不可逆点(PNR)的影响，并对早期仔鱼活力进行了评价。结果显示，在最适水温20~22 ℃条件下，胚胎孵化率最高，达75%~81%，且初孵仔鱼畸形率低于6.7%，胚胎发育的温度系数Q10值最接近2，且初孵仔鱼全长和卵黄囊体积最大。受精卵在盐度＞30时漂浮在水面，而在盐度为20~25时悬浮在水中，在盐度为10~15时下沉于水底部。受精卵胚胎发育的最适盐度范围为30~35，胚胎孵化率达79%~80%，初孵仔鱼畸形率低于6.0%。在4个不同温度条件下(18、20、22、24 ℃)初孵仔鱼卵黄囊吸收利用速率随着温度的升高而加快。不同盐度条件下，初孵仔鱼的SAI值表明，盐度为30~35时，仔鱼的SAI值较高且峰值出现在盐度为30组，而盐度为10组仔鱼SAI值最低。在水温为20~22 ℃时，6 d仔鱼的初次摄食率最高(78%)，PNR出现在7~8 d。初孵仔鱼在水温为20~24 ℃、盐度为30~35条件下，IGF-1 mRNA表达水平显著高于其他实验组。饥饿条件下，IGF-1 mRNA在饥饿后第2天显著升高，其后在第3~4天显著下降，但仍保持较高表达水平，随着饥饿的进行继续下降至显著低于开口期表达水平。本研究明确了黄条鰤受精卵孵化的最适温度为20~22 ℃、最适盐度为30~35，并建立了初孵仔鱼活力评价的指标，研究结果可为建立规范化的黄条鰤胚胎孵化和苗种培育技术提供依据。

英文摘要:

      Yellowtail kingfish, Seriola aureovittata, is a long-distance migratory oceanic species belonging to the Carangidae family of Perciformes, which has a global distribution and inhabits temperate and subtropical marine waters. S. aureovittata is large in size, has a fast growth rate, and is highly favored by international consumers owing to its excellent flesh taste, nutritional quality, and economic value. Furthermore, it is a promising candidate for the global farming industry and is particularly suitable for rapidly developed open ocean aquaculture in China. Currently, yellowtail kingfish aquaculture occurs in over 10 countries including Japan, Australia, New Zealand, South Africa, Chile, Greece, Holland, USA, Mexico, and China. In 2017, a great breakthrough in seedling production of S. aureovittata was achieved, and currently juveniles are mass-produced in China by combining the “engineering pond” and “land based indoor tanks” modes, which led to the rapid development of the Seriola fish farming industry in China. Nowadays, Seriola species are farmed in Liaoning Province, Fujian Province, and Shandong Province of China, and the combined annual farming yield is approximately 500 tons. However, we found that during seedling production of S. aureovittata, especially at the early larval growth stage, the hatching rate of eggs was variable among different spawning batches, and the survival of early larvae was low especially when the larvae reached 8~10 d post hatching. Occasionally, the high total death rate was attributed to the sudden “sinking death” of larvae, which may have been caused by stress as a result of changes in environmental factors. Therefore, it is necessary to determine the ecological and physiological effects of environmental factors, especially temperature and salinity fluctuations, on the early life stages of S. aureovittata under artificial breeding conditions. In the present study, the effects of two key environmental factors, temperature and salinity, on embryonic development and early larval growth of S. aureovittata were investigated using experimental ecology, morphological measurements, and molecular methods under laboratory conditions. The indexes including hatching rate of eggs, deformation rate of newly hatched larvae, absorption of yolk sac, IGF-1 gene expression, survival index (SAI), and point of no return (PNR) were determined. Moreover, the vitality of newly hatched larvae was tested and evaluated. The results showed that the highest hatching rates of 75%~81% were obtained under temperatures of 20~22 ℃, and the deformation rates of newly hatched larvae were lower than 6.7%. In addition, according to the Q10 calculation, the most appropriate water temperature range for embryonic development of S. aureovittata was confirmed to be 20~22 ℃. Meanwhile, the total length and yolk sac volume of newly hatched larvae of yellowtail kingfish hatched from the 20℃ and 22 ℃ groups were larger than those in the other temperature groups. Regarding salinity, the fertilized eggs floated on the water surface when salinity was over 30 ‰, were suspended in the water when salinity was between 20~25, and sank to the bottom of the container when salinity was lower than 15. The optimum salinity range for embryonic development of S. aureovittata was therefore determined to be 30~35, when hatching rates were between 79%~80%, and the deformation rate of newly hatched larvae was 6%. The yolk sac absorption by newly hatched larvae was measured under four temperatures (18 ℃, 20 ℃, 22 ℃, and 24 ℃). It was found that the absorption and exhaustion speed of the yolk sac increased with temperature, and the yolk sac was exhausted at 7 d post hatching at 18 ℃, whereas the time decreased to 6 d, 5 d, and 4 d at 20 ℃, 22 ℃, and 24 ℃, respectively. The highest SAI value for newly hatched larvae was observed at a salinity of 30, whereas the lowest was observed at a salinity of 10, which was consistent with the hatching results of embryos under different salinities. The highest first feeding rate of newly hatched larvae was observed at 6 d post hatching, and the PNR appeared between 7 d and 8 d post hatching at culture temperatures ranging from 20~22 ℃. IGF-1 mRNA levels in newly hatched larvae from different temperatures and salinities were detected, and significantly higher expression levels were found at temperatures of 20~24 ℃ and salinities of 30~35. Under continuous starvation conditions, the IGF-1 mRNA in larvae significantly increased at 2 d post mouth open and decreased at 3 d and 4 d, although expression levels remained relatively high, and then continually decreased to a significantly lower level as starvation continued. Results from the present study provide basic knowledge and useful tools for the construction of standardized technological methods for optimal embryonic hatching and seedling production of S. aureovittata.