Effects of ionizing irradiation of γ ray on mortality and growth of one year-old Pacific oysters （Crassotrea gigas） were studied. Irradiation was conducted at 4 ℃ and without sea water, and the survival, shell length, shell width, whole wet weight and condition index of irradiated oysters were measured. A biphasic dose response effect of γ irradiation on Pacific oyster with high stimulatory response dose range to 100 Gy, and strong hormesis effects had been observed. Gamma ray induced mortality happened within 90 days after irradiation, and the relation between dosage （D） and mortality at 90 days could be described by the equation of M90=0.0041D2-0.1816D （R2=0.9958） with an LD50/90 of 135 Gy, indicating that the Pacific oyster has strong resistance to γ irradiation. Growth in shell length, shell width and whole wet weight of Pacific oyster were also enhanced by 60 Gy of γ ray. The condition index was also enhanced by γ irradiation in the dosage range from 40 to 100 Gy, with the maximum stimulatory response of 2.5 fold of control value at the 40 Gy dosage, suggesting that the accelerating effects of γ ray on growth of the Pacific oyster led to increase in dry meat. Results of this study showed that ionizing irradiation might be useful in the improvement of production traits in shellfish.
A strategy to improve aquaculture mollusks in genetics, which involve the theory, method, material and application system, is very important for the sustainable development of the marine molluscan breeding and industry. Selection and hybridization as the traditional way have proved effective in the genetic improvement of mariculture mollusks. The growth had increased significantly by selective breeding in oysters as well as the resisitance to MSX and QX disease. A frame chart of genetic improvement of mollusks had been derived from the breeding application of abalone and scallop. The systematic breeding with selection and hybridization has a substantial progress based on the family establishment in Haliotis discus hannai, H. diversicolor, Argopecten irradians, Chlamys farreri and Pinctada martensii. The first self-fertilized family of bay scallop had been established in the 1999 and then a series of family lines with different traits have been succeed and formed the basic element of genetic breeding. The hybridization of Pacific abalone between diffeent populations with a large genetic distance has showed a remarkable traits. The hybrid of Pacific abalone has applied to improve the traits of growth and resistance in the aquaculture and 98% seed used in the farming is hybrid. The hybridization among populations has used in other mariculture mollusks such as zhikong scallop, bay scallop, pearl oyster and small abalone. Hybridization among different populations and systematic and massive selection are the dominant approaches in the genetic improvement of mollusks. So far in China the material system for molluscan genetics and breeding is more considerable as well as the basic theory of heritability, heterosis, inbreeding depression, genetic-environment interaction for the genetic improvement. The different lines have been produced by self-fertilization, inbreeding and crossing and some self-fertilized lines of bay scallop have been bred to F4 successively. The color traits heritable are useful as the marker of selec
