| 1. Comparative study on juvenile growth of hybrids between Chinese and Japanese stocks of sea cucumberIntraspecific hybridization was investigated using two stocks (C and J) of the sea cucumber (Apostichopus japonicus). Stock C is sampled from Lidao, Shandong province, China. Stock J is derived from a red type of A. japonicus which was introduced from Oita, Japan. The fertilization rate, larval survival rate, growth and heterosis at larval and juvenile stages were compared between two reciprocal hybrid crosses (C♀×J♂, CJ and J♀×C♂, JC) and two parental groups (C♀×C♂, CC and J♀×J♂, JJ). The results indicated that the rates of fertilization and larval survival of hybrid crosses were inferior to those of parental groups, but not showing statistically marked significant difference (p > 0.05). The growth performance at larval and juvenile stages of JC cross was superferior to the other three groups, showing marked heterosis in the characteristics of body length and weight growth. The heterosis of the JC cross ranged from 4.60% to 6.90% for larval body length, from 8.39% to 18.85% for juvenile body length, and from 23.21% to 49.77% for juvenile body weight. In contrast, the CJ cross did not show positive heterosis until 160 days passed after fertilization. The different heterosis between the two reciprocal hybrid crosses might be related to the maternal effects, sex linkage and cytoplasmic inheritance.2. Effect of salinity and temperature on salinity tolerance of the sea cucumberSea cucumber A. japonicus (Selenka) juveniles acclimated in different environmental conditions (23, 25 and 27°C combined with 25, 30 and 35‰) were examined for salinity tolerance by increasing and decreasing salinity at a rate of 2‰h-1. They were also tested for the LS50 (median lethal salinity) when transferred directly into a series of higher salinity (32-46‰) and lower salinity (9-25‰). The CSMax (critical salinity maximum), CSMin (critical salinity minimum), USTL (upper salinity tolerance limit) and LSTL (lower salinity tolerance) were positively correlated to salinity, but negatively correlated to temperature. A. japonicus exhibited CSMax 6.2-10.0‰higher than the values of USTL, and exhibited CSMin 5.5-8.5‰lower than the values of LSTL, indicating that gradual changes in salinity result in the wide range of salinity tolerance, but abrupt changes in salinity result in the narrow range in the sea cucumber. Two-way ANOVA showed that salinity and temperature had a significant effect on 50% CSMax, 50% CSMin, USTL and LSTL (p < 0.001). The information obtained in this study will be valuable for the development of sea cucumber aquaculture industry.3. The effects of salinity and temperature on the growth and survival rates of juvenile sea cucumberThe growth and survival of juvenile sea cucumber A. japonicus was measured in different environmental conditions (21, 23, 25 and 27°C combined with 28, 31, 34 and 37‰). Thirty individual sea cucumbers were used in each combined condition. The results showed that juvenile sea cucumbers have the highest survival between temperatures of 21 and 25°C, and between salinities 28-31‰. The best growth was obtained between temperatures of 21 and 23°C, with little difference among various salinities, indicating that the growth of juvenile sea cucumber was mainly affected by the temperature. Survival and growth coincide best at around 21°C to 23°C and 28‰to 31‰. The results demonstrated the optimum conditions for production and the information obtained in this study will be valuable for the development of sea cucumber aquaculture industry.
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