| The native zhikong scallop (C. farreri) is one of the most important species cultured in North China. The annual production accounted for four-fifths of the total scallop production in China before the bay scallops (Argopecten irradians) and Japanese scallops (Patinopecten yessoensis) were introduced. Successive heavy mortalities since 1996 have been the major limiting factor in the development of this industry. There are several suspected causes of such massive mortality, such as: prevalence of virus and rickettsia-like pathogenic microorganisms, deterioration of zhikong scallops' germplasm, high stock densities which exceed carrying capacity of sea areas, deterioration of water quality etc. This paper was to study potential causes for mortality of cultured zhikong scallops, with a focus on three aspects: stocking density, parasites, and genetics.Wild seeds were deployed and cultured in suspended cages at high, medium, low densities to experience mortality at Jiao Nan (JN), Peng Lai (PL), Yan Tai (YT) in Shandong province from May to September of 2000. Samples were collected before, at the peak of, and after mortality, and survival rates (death rates) were recorded at each sampling. Massive mortality occurred at all three sites during the period of middle of July to early August, and reached 90%, 89% and 85% at JN, PL and YT, respectively. The final cumulative mortalities of layers with different density treatments showed no significant differences, while at the beginning and peak of mortalities, death rates of layers at low densities were obviously lower than those at medium and high densities. Scallops kept growing in shell height (SH) and whole wet weight (WW) during mortalities. High density inhibited increasing of SHand WW, and the differences increased with time. The density used by farmers (i.e., the medium density in this research) is also too high for scallops. GH (Instantaneous growth rates of shell height) during mortality episode decreased gradually, while GW abnormally went up at the interval of T0-T1, which implied a period of fast fattening just before mortality. Losses were highly species and age specific, preferentially affecting two-year old or older native zhikong scallops (C. farreri) which were matured for spawning and 53.7±6.1mm in shell height, 22.2±8.4g in whole wet weight. Samples were dissected, fixed and made into 5μm slices. The following is results by light microscopy. A lot of T. sp. with diameter of 30~50μm were observed in mantle cavities and between gills with 0~66.7% prevalence, ranging from 0 to116 per slide. Abundant secretions were observed around various tissues, especially in scallops at PL and YT, and at the peak of mortality and after mortality. The prevalence ranged from 3.3% to 33.3% with 12% on average. A lot of exotic microorganisms and tissue fragments existed in the secretions. Huge invaders with sizes of 100μm~1mm were also found in these slides which had higher prevalence at PL and YT. It's concluded that these scallops were at the peak of spawning during mortality. But it needs further study that stress from spawning contributed to the death of scallops. No obvious pathogen was identified in soft tissues of the 180 samples examined by light microscopy. But there may be other pathogens which were too small to be seen through light microscopy. The growth condition and histology results implied there was no starvation stress on these scallops.Genetic diversity was studied among 140 zhikong scallops from five populations which were Dalian wild (DL), Korea wild (K), hybrid population from China wild × Japan wild (CJ), samples from JN and from PL, using RAPD with 7 primers. There were total 37 markers amplified from scallops with 30 being polymorphic. The average heterozygosities were: DL, 0.3360; K, 0.3249; CJ, 0.3539; JN, 0.3157; PL, 0.2862. Those of cultured populations were a little lower than wild and hybrid populations, which indicated that genetic diversity of culturing population declined for some degree. Genetic distances (D) among th...
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