Influence of food density and temperature on ingestion, growth and settlement of Pacific oyster larvae, Crassostrea gigas

Ingestion, growth and metamorphosis of Pacific oyster, Crassostrea gigas, larvae were studied under controlled conditions of food density and temperature using a combination of a flow-through rearing system and a hydrobiological monitoring device. In a first experiment larvae were exposed to three different phytoplankton densities (12, 20 and 40 cells µl− 1) while in a second trial larvae were reared at five different temperatures (17, 22, 25, 27 and 32 °C). Both food concentration and temperature significantly affected the larval physiology throughout the entire development from D-veliger to young spat. Larvae survived over a wide range of both environmental parameters with high survival at the end of experiments. The feeding functional response provided the maximal ingestion rate (50 000 cells larva− 1 day− 1) which occurred at an algal density of 20 cells µl− 1 surrounding the larvae and 25 °C. At the highest temperature (32 °C), maximal growth and metamorphosis performances were reached in less than 2 weeks while the lowest temperature (17 °C) consistently inhibited ingestion and growth over the entire larval period. The estimate of the Arrhenius temperature (TA) was 11 000 K for C. gigas larvae. Larval development could be divided on the basis of feeding activity into an initial mixotrophic period with a lower and constant ingestion over the first days (from D-stage to early umbonate larva of ≈ 110 µm length) followed by an exotrophic phase characterized by a sharp increase in ingestion (umbonate to eyed of ≈ 300 µm length) and, finally, a third period for larvae ≥ 300 µm during which ingestion decreased suddenly because of metamorphosis. Optimum larval development and settlement of the oyster C. gigas occurred at 27 °C and an increasing food supply as the larvae were growing. A food density of ≥ 20 cells µl− 1 of T-ISO + CP or CG (1:1 cells number) in the culture water was required to maximise growth and metamorphosis success.