Effect of food supply on oyster spatfall

Abstract

The amount, concentration and type of algae diet supplied to larvae of the European oyster ( Ostrea edulis L.) and the Pacific oyster ( Crassostrea gigas Thunberg) prior to and during spatfall (setting) were varied. Both oyster species gave lower spatfalls when feeding was discontinued immediately prior to settlement. In comparison with control larvae, that were fed a normal diet throughout, 47.7% (range 19–78%) of O. edulis set if the larvae had grown to 295–325 μm (mean shell length) before feeding was stopped but 8.8% (3–16%) if they had only grown to 275–295 μm. Mean spatfall of C. gigas was 10.6% (2–20%) compared with fed controls and, in contrast with O. edulis, was not related to size of larvae, in the range 262–312 μm, at the time that feeding was withheld. The proportionally greater spatfall from larger larvae was related 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 more slowly than spat which set from fed larvae. Between 95 and 100% of the larvae of both oyster species survived without food for 2 days, but only 20–30% after 6 days. Subsequent spatfall on resumption of feeding was lower than in controls that were fed throughout. This was related to loss in organic weight of surviving unfed larvae of 0.32–0.79 μg 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 (about 70% of organic weight lost) during the unfed period. The food requirements for maximum spatfall were estimated as 0.4 μg (organic weight of algae) · larva −1 · day −1 for O. edulis and 0.6 μg · larva −1 · day −1 for C. gigas. With this amount of food per larva available, spatfall at a food concentration as low as 0.035 mg (organic weight of algae) · 1 −1 was similar to that at the control concentration of 1.0 mg ·1 −1. Spatfall was also affected by the nutritional value of the food species supplied during settlement. Algae species known to support slower growth rates of bivalves gave lower spatfalls.