A physiological and biochemical approach to selecting the ideal diet for Ostrea edulis (L.) broodstock conditioning (part A)

To select the best microalgae for Ostrea edulis conditioning, four single species diets were fed to batches of broodstock, which were then compared using physiological and biochemical approaches. Ingestion and absorption were first studied, showing that Chaetoceros gracilis and Skeletonema marinoi were efficiently ingested (4.9-5.3 mg g-1 h-1) and absorbed (1.9-2.5 mg g-1 h-1) while Tetraselmis suecica led to the lowest physiological responses (0.36 and 0.12 mg g-1 h-1 respectively). When O. edulis were fed any microalgae except T-ISO with only 79 mg g-1, gonads accumulated carbohydrates from 116 to 141 mg g-1 and the extent of accumulation depended on the species supplied in the order T. suecica < C. gracilis < S. marinoi. When oysters were fed either of the diatoms (C. gracilis or S. marinoi), an efficient transfer of 20:5(n-3) to all tissues, including the gonads, was recorded while no enrichment in 22:6(n-3) occurred in all tissues (polar fraction) when oysters were fed T-ISO. In contrast (22:5(n-6)), a characteristic fatty acid of T-ISO accumulated in all tissues, confirming its allocation despite particularly low ingestion (1.66 mg g-1 h-1) and absorption (0.32 mg g-1 h-1). When oysters were fed C. gracilis or S. marinoi an efficient transfer of cholesterol and campesterol from diatoms to the gonads was observed, while no sterol accumulation occurred in the gonad when fed T. suecica. Because of low ingestion, absorption and poor biochemical compounds' transfer, T. suecica is not recommended for O. edulis conditioning. T-ISO also exhibited low physiological performances but due to a specific 22:6(n-3) enrichment in the gonad neutral fraction (16.1%), its potential role in reproduction should be considered. Chaetoceros gracilis is highly recommended for O. edulis broodstock while a source of DHA other than that provided by T-ISO should be found due to its poor absorption. Because a mixed diet has been shown to be more efficient for O. edulis broodstock conditioning, complementary trials dealing with the effects of other species rich in 22:6(n-3) such as Rhodomonas salina or Pavlova lutheri should be performed.