Microalgae commonly used as feed for bivalves, Pavlova lutheri (P), Isochrysis affinis galbana (T) and Chaetoceros calcitrans forma pumilum (Cp), were fed to Pacific oyster Crassostrea gigas to assess their nutritional value for larval development and metamorphosis during two experiments. Monospecific, bispecific and trispecific diets were firstly evaluated during 3 weeks from D larvae to young postlarvae. Then bispecific diets, based on different T and Cp proportions, were assessed during a similar period. Concurrently, ingestion was studied through the whole larval and postlarval development for each diet and/or diet mixture. Because lipids are assumed to be a key nutrient for bivalves, biochemical analysis was undertaken on the second set of trials focused on fatty acids and sterols. Compared to the other diet mixtures (mono and plurispecific diet) TCp induced the best larval growth performance (13.2 μm day− 1), a high larval survival (98%) but did not result in higher metamorphosis (72%). In contrast, monospecific diet P was the poorest for larvae with low growth and low survival. When varying T and Cp proportions, best larval developments were induced with 25T/75Cp and 50T/50Cp diets, though quite similar to that obtained with 75T/25Cp. In contrast, unbalanced diets (95T/5Cp and 95Cp/5T) led to low larval performances. In addition, grazing experiences showed preferential uptake of microalgae with P < PT much less-than T much less-than Cp much less-than TCp = PCp = PTCp. For mixed diets a low daily consumption (< 10 000 microalgae per larvae) was noted during the first week followed by a second phase (next 8-10 days) with a sharp increase and regular intake, reaching 90 000 microalgae per larvae per day. Finally, a marked drop (40 000 microalgae per larvae) was observed at the beginning of metamorphosis from days 20 to 21. Principal component analysis between main fatty acids (19) and sterols (7) detected in larvae and postlarvae was used to discriminate profiles according to diets and/or metamorphosis competence. The correlation circle representation showed that the 26 variables are well explained by these combined variables (78%) with a repartition along the first principal component according to diets with a gradient from 5T/95Cp to 95T/5Cp. In contrast, postlarvae and larvae were discriminated on the second principal component while no relationships were found between competent and incompetent larvae.

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 11000 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.

While literature is relatively abundant on adult shellfish, ecophysiological studies at the larval stage are scarce because of both technical difficulties and inadequate methodology. A tool dedicated to provide basic information for larval ecophysiology was accordingly developed. Two steps were followed: first a flow-through method of Crassostrea gigas larval culture was perfected during a set of experiments in which rearing systems and larval density were assessed. Then a continuous hydrobiological data recorder was adapted with modifications to comply with our experimental aim. SILO (Système d'Instrumentation des Larves en flux Ouvert) allowed the successive acquisition of hydrobiological parameters in ten 150 l larval tanks in which larvae were reared using flow-through techniques under controlled environmental conditions. An automated system enabled sequential measurement of hydrobiological parameters from one larval culture to the next. A chamber system contained several probes for measurement of temperature, salinity, pH and fluorescence. The electronic system allowed real time acquisition, storage and transfer of data. SILO was successful as a larval rearing device, reducing larval disturbance that could result from transfer and handling for measurement. It was also efficient as a tool to provide basic information for larval ecophysiology research. The effect of temperature (17, 22, 25, 27, 32°C) on growth and ingestion of microalgae was studied to test SILO. Maximum growth occurred at 27 °C (16 ± 2 µm d-1) and 32 °C (15 ± 3 µm d-1) whereas larvae reared at 22 and 25°C showed lower development (8.6 ± 3.1 and 11.7 ± 2.5 µm d-1, respectively). Moreover, metamorphosis exhibited no significant difference at 27 or 32 °C with 87.5 ± 7.1 and 85.9 ± 9.6% respectively, while at 22 or 25 °C lower metamorphosis was recorded with 55.2 ± 8.3 and 57.6 ± 9.6%, respectively. The lowest temperature (17 °C) strongly inhibited ingestion whereas the highest temperature (32 °C) stimulated maximum feeding activity over the whole larval rearing period.

Dynamic Energy Budget (DEB) theory aims to quantify the energetic framework of an individual organism as a dynamic model, from the uptake of food to its utilisation in metabolic processes (maintenance, growth, development and reproduction). The purpose of the present paper is to extend the existing DEB model for adult Pacific oyster Crassostrea gigas to the larval life stage of this species. We present the application of generic DEB theory to oyster larvae, with the formulation of the specific assumptions based on the characteristics of this stage. The model depends on seawater temperature and food density, as forcing variables, followed throughout the whole larval development. We calculated DEB parameter values for larvae by means of laboratory experiments specifically designed to collect datasets on ingestion and growth at different levels of phytoplankton density and temperature. The DEB model developed here showed good growth simulations and provided an extensive description of the energetic needs of C. gigas during its larval stage. It was demonstrated that, at 27 °C, a food density of 1400 µm 3 µl − 1 must be maintained throughout larval development to maximise growth and metamorphosis success. Timing of metamorphosis decreases exponentially with increasing temperature.