Abstract:The sea cucumber Apostichopus japonicus is one of the most important aquaculture species in China. In recent years the sustainable development of sea cucumber culture has been hindered in China due to issues such as the depletion of natural resources, the genetic degradation, and frequent outbreaks of diseases. Hybridization has been an effective solution to these issues and improving the breeding in aquaculture. Therefore in this study we investigated the temperature and salinity tolerance in the inbred and hybrid offspring of Chinese and Korean sea cucumber populations. Four hybridized combinations were created with complete dual cross, including C(♀)×C(♂), K(♀)×K(♂), K(♀)×C(♂), and C(♀)×K(♂). Four groups of sea cucumber were acclimated at 16℃ and 31 psu for 7 days. The temperature tolerance was estimated in two ways: transferring the subjects directly into high-temperature seawater (27℃, 28℃, 29℃, 30℃, and 31℃) or raising the temperature gradually at 1℃/h. In the salinity tolerance trials, the salinity of acclimated seawater was increased or decreased at 2 psu/h to determine the SSMin (survival salinity minimum), CSMin (critical salinity minimum), SSMax (survival salinity maximum), and CSMax (critical salinity maximum), respectively. The sea cucumbers were also transferred directly into a series of high salinity (36, 38, 40, 42 and 44 psu) and low salinity (21, 19, 17, 15, 13, and 11 psu) to measure the USTL (upper salinity tolerance limit), and LSTL (lower salinity tolerance limit) respectively. It was found that when the seawater temperature increased gradually, the STMax (survival temperature maximum) of C(♀)×K(♂) and C(♀)×C(♂) group were significantly higher than K(♀)×K(♂) group, the 50%CTMax (50% critical temperature maximum) of C(♀)×K(♂) and C(♀)×C(♂) group were significantly higher than other two groups (P˂0.05). When the seawater temperature increased abruptly, the LT50 (median lethal temperature) of both hybrid groups were higher than that of the Korean population, but lower than that of the Chinese population. One-way analysis of variance showed that when the seawater salinity increased gradually, the 50%CSMax (50% critical salinity maximum) of C(♀)×K(♂) group was significantly higher than K(♀)×K(♂) group (P˂0.05); the 50% CSMin (50% critical salinity minimum) of K(♀)×C(♂) group was significantly lower than K(♀)×K(♂) group (P˂0.05). When the seawater salinity decreased abruptly, the LSTL (lower salinity tolerance limit) of K(♀)×C(♂) group was significantly lower than K(♀)×K(♂) group (P˂0.05). These results suggested that the intra-species hybrids might have moderate heterosis in heat and salinity tolerance. Therefore crossbreeding may effectively improve the stress resistance of A. japonicus.
