Effects of salinity on fertilization, embryonic development, larval growth, ingestion and metamorphosis of Heliocidaris crassispina

Heliocidaris crassispina is one of the most important economic sea urchin species in the southern China. Its breeding season is from April to June, with high temperature and rainy, and the salinity varies greatly in the sea area. In order to determine the appropriate salinity for the early development of H. crassispina, this study was conducted to investigate the effects of different salinity (15, 20, 25, 30, 35, and 40) on the fertilization, incubation, embryonic development, larval growth, ingestion and metamorphosis of H. crassispina under laboratory conditions. The main results are as follows. The fertilization rate was above 95% under the salinity range of 20-35, but there was no fertilization with a salinity of 15 or 40. The hatching rate under salinity 30 was 89.77%, which was significantly higher than other groups, the lowest hatching rate was 46.65% under salinity 20 treatment. The embryonic developed fastest with salinity 30 exposing, 92.61% of fertilized eggs developed to the 4-cell stage in 1 h after fertilization, and all developed into the multicellular stage in 2.5 h, and 68.86% of the 2-arm larvae appeared in 27 h, which were significantly higher than other groups. Moreover, with salinity 30 treatment, the larvae develop fastest and best, and were significantly higher than other five groups, 91.67% of 4-arm larvae developed to the 8-arm larvae stage I on the 5th day, and all developed into the 8-arm larvae on the 7th day, of which 16.67% have developed to stage Ⅲ, and 83.33% of the larvae have developed to stage Ⅳ on the 13th day. The larvae with salinity 25 and 35 followed, 83.33% and 75% of stage Ⅰ larvae appeared on the 5th day, and some larvae could develop to stage Ⅳ on the 13th day, but there was a difference in individual development. Under salinity 20 or 40 condition, the larvae only developed to 8-arm larva stage Ⅰ on 13 days; further more under salinity 15 exposing, the larval development was inhibited and stayed at 4-arm larvae stage and died after 7 days. The metamorphosis rate of 8-arm stage Ⅳ larvae with salinity range of 15-40 in benthic diatoms is higher than 21.43% after 72 hours, and the salinity 30 is the highest with 54.71%. The larval post-oral and antero-lateral arms grew fastest under salinity 35 treatment on the 5th days (830.50 and 694.50 μm, respectively), which were significantly higher than other groups. But the stomach area was largest under salinity 30 with 23048.25 μm² , which was significantly higher than other five groups. And the feeding rate of 4-arm larvae and 8-arm larvae were highest under salinity 30 exposing (2780 and 3286 cell/ind·h, respectively), which were significantly higher than other groups. The study suggested that salinity has a significant impact on the growth and development of H. crassispina in early developmental stage. The optimum salinity was 30, which H. crassispina has fastest growth and best development. The suitable salinity range was 25 to 35, which H. crassispina can maintain normal growth and development. The results may provide basic data for the seed breeding technology of H. crassispina.