EFFECT OF FOOD-SUPPLY ON OYSTER SPATFALL

The amount, concentration and type of algae diet supplied to larvae of the European oyster (Ostrea edulis L.) and the Pacific oyster (Crasso strea gigas Thunberg) prior to and during spatfall (setting) were vari ed. Both oyster species gave lower spatfalls when feeding was disconti nued immediately prior to settlement. In comparison with control larva e, that were fed a normal diet throughout, 47.7% (range 19-78%) of O. edulis set if the larvae had grown to 295-325 mu m (mean shell length) before feeding was stopped but 8.8% (3-16%) if they had only grown to 275-295 mu m. Mean spatfall of C. gigas was 10.6% (2-20%) compared wi th fed controls and, in contrast with O. edulis, was not related to si ze of larvae, in the range 262-312 mu m, at the time that feeding was withheld. The proportionally greater spatfall from larger larvae was r elated to higher lipid reserves in these larvae, compared with smaller larvae of this species and all C. gigas larvae. O. edulis spat which set in the absence of food contained less lipid and initially grew mor e slowly than spat which set from fed larvae. Between 95 and 100% of t he larvae of both oyster species survived without food for 2 days, but only 20-30% after 6 days. Subsequent spatfall on resumption of feedin g was lower than in controls that were fed throughout. This was relate d to loss in organic weight of surviving unfed larvae of 0.320.79 mu g over 6 days (15-25% of initial weight when feeding was stopped). This loss was sustained by preferential utilisation of lipid reserves in O . edulis (53-61% of organic weight lost) and protein in C. gigas (abou t 70% of organic weight lost) during the unfed period. The food requir ements for maximum spatfall were estimated as 0.4 mu g (organic weight of algae).larva(-1) day(-1) for O. edulis and 0.6 mu g.larva(-1).day( -1) for C. gigas. With this amount of food per larva available, spatfa ll at a food concentration as low as 0.035 mg (organic weight of algae ).l(-1) was similar to that at the control concentration of 1.0 mg.l(- 1). Spatfall was also affected by the nutritional value of the food sp ecies supplied during settlement. Algae species known to support slowe r growth rates of bivalves gave lower spatfalls,