Adult European flat oysters, Ostrea edulis L. 3 receiving a food supplement of the unicellular alga Tetraselmis suecica (Kylin) Butch, during laboratory conditioning for spawning produced more broods of larvae than oysters receiving only the natural phytoplankton in the seawater supply. Many of the broods from adults receiving this extra food were liberated sooner, and these larvae grew at an enhanced rate and provided greater spat yields than the broods from the control stock. The viability of larvae, which was significantly correlated with their lipid, and in particular neutral lipid, proportion at the time of liberation, declined as the length of the stock-conditioning period prior to female spawnings increased. This may be related to the decline in the dry meat weight and the condition factor of the adult oysters during the experiments irrespective of the feeding regime. The extra algal ration provided a check to the decline in dry meat weight of the food-supplemented adults. I N T R O D U C T I O NA technique for conditioning Ostrea edulis to produce viable larvae in the laboratory has been described by Walne (1966). This procedure included supplementing the natural food in the sea-water supply to the breeding stock with cultured unicellular algae, but the effect of the extra food ration on the vitality of the larvae subsequently released was not determined. This paper reports the results of two experiments which determined the value of providing oysters with an algal supplement during the conditioning process. Variations in the viability and biochemical composition of newly liberated larvae of the different broods produced have been investigated. METHODS Procedure for the conditioning of the breeding stockTwo experiments were made, one in the spring of 1970 and the other in the spring of 1971, using 4-to 5-year-old 65-75 mm diameter commercial-grade oysters from the Helford River, Cornwall. In Experiment 1, 50 oysters were held on a i-25-cm-mesh polythene frame, 10 cm off * Present address: ICI Laboratory, Brixham, Devon.

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A total of 113 broods of Ostrea edulis L. larvae from a closely-controlled hatchery conditioning regime, from specific broodstock treatments, and from brooding oysters taken from a wild population in Poole Harbour, was analysed for fatty acid composition. Major differences in polyunsaturated fatty acids (PUFAs) were detected as the year proceeded, with increased levels of 20:5 (n-3) and 22:6 (n-3) from late June. Significant decreases in the PUFA content of both phospholipids and neutral lipids accompanied development from fertilized eggs to immediately pre-liberation larvae, but there was some evidence that the 20:4 (n-6) and 22:6 (n-3) content of phospholipids was protected. Fatty acid composition and content of newly liberated larvae were influenced by conditions during broodstock fattening and early gametogenesis (phospholipids) and by conditions during the later stages of oocyte development (neutral lipids).