Daniele Maurer

Shell growth and gross biochemical composition of oyster larvae (Crassostrea gigas) in the field

Abstract In summer 1985, Crassostrea gigas (Thunberg) larvae were isolated from the plankton of the Bay of Arcachon. Larvae of the same cohort were collected at densities varying from 3×10 6 m −3 (D larvae) to 2×10 3 m −3 (pediveligers). Shell growth rate, total dry weight and gross biochemical composition of larvae from the field were studied. Shell growth rate showed a slow decrease towards the end of the pelagic life, and conversely total dry weight increased more rapidly. Carbohydrate levels were low (0.7–1.7% of total organic matter) and remained practically unchanged throughout the pelagic life. Protein levels increased during the larval stages (61.6–81% of total organic matter) and seemed to be the most important reserves for metamorphosis. Both larval growth and biochemical composition of larvae indicated good environmental conditions for larval development in the Bay of Arcachon during July 1985. It is suggested that studies on growth and biochemical composition of Crassostrea gigas larvae from the field could be used to detect any degradation of the environment.

Growth, fatness and gross biochemical composition of the japanese oyster Crassostrea gigas in stanway cylinders in the bay of Arcachon, France

Abstract A new method of oyster cultivation, the Stanway oyster cylinder, has been investigated in the Bay of Arcachon since 1989. A comparative study was carried out between the growth of spat and 18-month-old Crassostrea gigas oysters, in cylinders and in traditional bags. The growth in height and in whole weight was lower in cylinders. In contrast, the tests showed a better quality of the meat, with higher condition index and higher carbohydrate content, and a better quality of the shell, with higher density and better shape. Therefore, because of the improvement in oyster quality, the use of the cylinder seems to be advantageous for the Arcachon oyster industry.

Weak interactions between groups and physical drivers of community dynamics in coastal phytoplankton

Phytoplanktonic communities maintain a high diversity in a seemingly homogeneous environment, competing for the same set of resources. Many theories have been proposed to explain this coexistence despite likely competition, such as contrasted responses to temporal environmental variation. However, theory has developed at a faster pace than its empirical evaluation using field data, that requires to infer biotic and abiotic drivers of community dynamics from observational time series. Here, we combine autoregressive models with a data set spanning more than 20 years of biweekly plankton counts and abiotic variables, including nutrients and physical variables. By comparing models dominated by nutrients or physical variables (hydrodynamics and climate), we first explore which abiotic factors contribute more to phytoplankton growth and decline. We find that physical drivers - such as irradiance, wind, and salinity - explain some of the variability in abundances unexplained by biotic interactions. In contrast, responses to nutrients explain less of phytoplankton variability. Concerning biotic drivers of community dynamics, multivariate autoregressive models reveal that competition between different groups (at the genus level for most) has a much weaker effect on population growth rates than competition within a group. In fact, the few biotic interactions between genera that are detected are frequently positive. Hence, our system is unlikely to be best represented as a set of competitors whose differing responses to fluctuating environments allow coexistence, as in “paradox of the plankton” models with a storage effect or a relative nonlinearity of competition. Coexistence is more likely to result from stabilizing niche differences, manifested through high intragroup density-dependence. Competition between planktonic groups and nutrients are often invoked as drivers of phytoplankton dynamics; our findings suggest instead that more attention should be given to the physical structure of the environment and natural enemies, for coastal phytoplankton at least.