长牡蛎(Crassostrea gigas)代谢率的季节变化及其与夏季死亡关系的探讨

SEASONAL VARIATION IN METABOLIC RATE OF PACIFIC OYSTER, CRASSOSTREA GIGAS AND ITS IMPLICATION TO SUMMER MORTALITY

2002年6月—2003年9月,在中国北方重要养殖海区桑沟湾,采用呼吸瓶法现场研究了两种不同规格长牡蛎(Crassostrea gigas)的耗氧率、排氨率及氧氮比的季节变化,从代谢水平探讨了长牡蛎夏季死亡的原因。结果表明,长牡蛎的耗氧率、排氨率以及氧氮比均具有明显的季节变化(P<0.001)。三龄牡蛎的个体耗氧率7月份最高[2.86mgO2/(ind·h)],1月份最低[0.07mgO2/(ind·h)];个体排氨率8月份最高[6.45μmol NH4-N/(ind·h)],1月份最低[0.47μmolNH4-N/(ind·h)];一龄牡蛎个体耗氧率和排氨率的变化范围分别为0.01—1.38mgO2/(ind·h)和0.04—1.66μmol NH4-N/(ind·h)。三龄个体单位个体耗氧率、排氨率高于一龄牡蛎,而单位软体干重的耗氧率、排氨率则小于一龄个体。一龄个体的氧氮比变化范围为16.7—58.0,10月份最高,一月份最低;三龄个体的氧氮比变化范围为8.7—64.0,6—7月份较高平均为52.7,8月份产卵后迅速下降到14.7,氧氮比的剧降表明该阶段长牡蛎代谢消耗了大量的蛋白质,使其体质脆弱,可能是引起三龄牡蛎夏季大量死亡的原因之一。

Data of oxygen consumption rates （ Ro）, ammonia excretion rates （RN） and 0 ： N ratios of Pacific oyster Crassostrea gigas were measured seasonally from June, 2002 to September, 2003, under the ambient conditions of temperature, pH, and salinity in Sanggou Bay, an important mariculture field in north China. Different sized oysters, namely 1-year oysters （0.12-0.53g/ind, DW） and 3-year oysters （0.84-2.34g/ind, DW） were collected from integrated cultivation areas of bivalve and kelp in the bay monthly or bimonthly, depending on water temperature. The Ro and RN were determined using 6.4 liter respiration chambers, each chamber contains 4-6 3-year or 8-12 1-year individual oysters. The chambers were filled with field seawater and submerged in the sea by tuck net to keep the temperature. The chambers were taken out from seawater after 2-4h depending on field water temperature. In order to record stable oxygen consumption and ammonia excretion rates, oysters were acclimated in the chambers until their valves opened （approximately 30 min）, and then close the chambers with rubber plugs. When the experiments were conducted, oxygen concentration in the chambers must be kept in high level above 60 % of the initial concentration, to avoid any negative impact on oysters in normal physiological function. There were significant seasonal variations in Ro and RN for both 1-year and 3-year oysters. According to S-N- K＇s test, the highest Ro in 3-year oysters occurred in July with an average of 2.66mgO2/（ind·h） or 1.54mgO2/ （gDW·h） ; the lowest Ro occurred in January with an average of 0.07mgO2/（ind·h） or 0.08mgO2/（gDW· h）. The maximum and minimum Ro of 1-year oysters also occurred in July and January varied from 0.01-1.38mgO2/ （ind·h） or 0.03-2.9002/（gDW·h）. The least ammonium excretion occurred in January for both 1-year and 3-year oysters in average of 0.04μmol NH4-N/（ ind·h） or 0.40μmol NH4·N/（ gDW· h） for 1-year oysters and 0.47μmol NH4-N/（ind·h） or 0.61μmol NH4-N/（gDW·h） for 3-year oysters, respectively. The highest RN of 1- year and 3-year oysters occurred in July and August at 1.66μmol NH4-N/（ ind·h） or 2.90μmol NH4·N/（ gDW· h） and 6.45μmol NH4-N/（ind· h） or 5.24μmol NH4·N/（gDW· h）, respectively. Per single individual Ro and RN in 3-year oysters were higher than those in 1-year oysters, while the per unit dry issue weight Ro and RN in 3-year oysters were lower than 1-year oysters. The O ： N ratios of both 1-year oysters.The O：N ratios of both 1-year and 3-year oysters showed significant difference as well. The O： N in 1-year oysters ranged between 16.7 and 58.0, the minimum and maximum were recorded in January and October, respectively. The O：N in 3-year oysters changed from 8.7 to 64, higher in June--July, and lower in August in average of 14.7 at the period of spawning. Low O ： N ratio （below 20） is used as an indicator of nutritional stress for marine bivalves as it shows increased reliance on protein as a catabolic substrate rather than carbohydrate and lipid. The lower O ： N ratio in 3-year oysters during spawning period, was indicative of a greater demand for dietary nitrogen than carbon in this period, and had relatively high rates of protein catabolism to meet the energy consumption for spawning and resisting harsh environment, such as high temperature or abnormal salinity, which would impose a supplemen- tary stress and increase the risk of summer mortality. It was believed to be one main reason that resulted in mass summer mortality of 3-year oysters.