Summer mortality in the Pacific oyster Crassostrea gigas, overview of 3-year results of the cooperative "MOREST" Project

Summer mortality of C. gigas oyster is not yet clearly understood in literature. Complex interactions are suggested between environment, oysters, and pathogens. Such multifactorial event led us to organize a research network (including genetics, physiology, immunology, pathology, ecotoxicology, and environment) collaborating on the same natural or hatchery biological material. This strategy allowed to progressively classify importance of the different factors involved in the phenomena by coupling field and experimental studies. A temperature over 19 degree C is the first condition in France. This temperature is associated to C. gigas reproductive period. Defense responses are lowered during this reproductive stage contrary to sterile triploids that are mainly resistant, corroborating that reproduction is also a risky biological stage. A genetic component evidenced by divergent selection in two generations was confirmed the third year. Sensitive (S) and resistant (R) oysters in the same area or in controlled experimental conditions showed a difference in reproductive strategy faced to similar food conditions. High trophic conditions increased the oyster reproductive effort and so the risky phase as shown by a negative scope for growth and a higher sensitivity to bacterial challenges. However, some stresses were necessary to induce mortality when temperature and reproduction were in the risky window. A simple transfer of oyster induced mortality in these conditions. Sediment proximity appeared also as a detrimental factor, because oysters at 10 cm from sediment died contrary to oysters at 70 cm. Different strains and species of vibrios were isolated from moribund oysters and determined by molecular techniques. Their virulence tested by injection in the muscle appeared variable according to strains and species. Effect of environmental factors on their virulence expression is also one among the key points under study.