Finding new diatoms for intensive rearing of the pacific oyster (Crassostrea gigas): Energy budget as a selective tool

Intensive shellfish rearing in the polders of Bourgneuf Bay on the French Atlantic coast (46-47°N, 1-2°W) relies on the diatom Skeletonema costatum (Grev.) Cleve produced in nutrient-rich saline ground water. An episodic reduction in biomass production caused by an unidentified protozoon stressed the need to find new microalgal species as substitutes for S. costatum and diversify the microalgae used by the bivalve industry. Three species met the requirements for year-round availability in Bourgneuf Bay coastal waters and growth potential in saline ground water: Nitzschia acicularis (Kützing), Nitzschia closterium (Ehrenberg) Wm Smith and Nitzschia gandersheimiensis Krasske = Nitzschia tubicola Grunow. These microalgae were tested as food sources for adult Pacific oysters (Crassostrea gigas) by comparing short-term bivalve physiological responses with those obtained with S. costatum as reference. Suspended particulate matter concentrations in experimental diets ranged from 9.3 to 18.6 mg l^-1 and particulate organic matter concentrations from 3.3 to 5.7 mg l^-1. Significant differences were observed, with clearance rates ranging from 4.0 1 h^-1 g^-1 for N. acicularis to 7.3 1 h^-1 g^-1 for N. gandersheimiensis. The filtration rate for organic matter was significantly higher for N. gandersheimiensis than the other species, but this algae was also significantly more rejected in pseudofaeces. No differences were found among the four mean faeces production rates. Net energy balance differed significantly among the four microalgae tested, ranging from 282 J h^-1 g^-1 for S. costatum to 27 J h^-1 g^-1 for N. closterium. However, no differences were found between S. costatum, N. acicularis and N. gandersheimiensis. These results suggest that N. acicularis and N. gandersheimiensis could be tested on a larger scale involving the production of microalgae in 50-m³ outdoor tanks in association with Pacific oyster growth experiments.