Jean-Pierre Baud

Selecting the flat oyster Ostrea edulis (L.) for survival when infected with the parasite Bonamia ostreae

Abstract In the early 1970s, the appearance of the protozoan parasite Marteilia refringens restricted the production of the European flat oyster Ostrea edulis to subtidal areas. Ten years later a second protozoan parasite, Bonamia ostreae, caused a further serious decline in flat oyster farming in subtidal areas. Prophylactic measures were taken to sustain oyster farming, but the continuing presence of the two parasites led to the initiation of a programme to select for resistant oysters. Efforts were concentrated on B. ostreae and two selected strains were obtained in 1985 (S85) and 1989 (S89). The programme was based on mass selection and parasite inoculation or natural infections. This paper focuses on the survival and weights of the third generation of S85 (S85-G3), the second generation of S89 (S89-G2) and a cross between S85-G2 and S89-G1. The % survival of S85-G3 oysters was more than 4-times higher than the control group after a 20 month experiment in the wild, and significant differences in parasite prevalence were recorded. No significant differences between S89-G2 and the control group were observed for either survival or parasite prevalence. This was attributed either to a less powerful experiment or to the effect of genetic load clearing as a result of inbreeding. The % survival of the cross between the two strains was more than twice as high as the control group and significant differences were recorded for parasite prevalence. Good evidence for additivity of the resistance was provided by both the intermediate behaviour of a cross between S85-G2 and the controls and the rapid improvement that was obtained from the first generations. The selected strains showed a tendency toward higher weights and higher weight variances when compared to controls. These populations are suspected to have undergone bottlenecks, which would explain the increase in phenotypic variance. No measurable natural resistance has arisen in the wild. This could be due first to the relatively short time that has elapsed since the first infections, secondly to the early sale of resistant oysters as soon as they reach a marketable size, and thirdly to the slow kinetics of B. ostreae infection that allows both susceptible and potentially resistant oysters to participate in recruitment. Improved resistance seems to be related to the delayed mortality of selected oysters. This last result is discussed in the light of a previous study conducted in the laboratory which showed that resistant and susceptible oysters have different haemograms.

Influence of temperature on clearance and oxygen consumption rates of the flat oyster Ostrea edulis: determination of allometric coefficients

Abstract Clearance and oxygen consumption rates of Ostrea edulis were measured at different temperatures (10–30°C) in individual animals of a total weight ranging from 5 to 120 g (0.1 to 2.7 g dry tissue weight), outside the period of gametogenesis. For each temperature, the allometric relationship between the physiological response and dry tissue weight of the animal (DW) was estimated using the formula: Y = a ∗DW b . A statistical model is proposed for the variation in clearance rate or oxygen consumption rate ( Y ) with temperature ( T ) and DW: Y =[ a +( b ∗ c T )]∗DW d . The calculated value for d was 0.62 for clearance rate and 0.74 for oxygen consumption rate. The rates of both physiological functions increased as the temperature increased and reached maximum levels at 30°C (clearance rate:2.83±0.32 l h −l g −l ; oxygen consumption rate: 1.78±0.23 mg O 2 h −l g −l ) when the flat oysters were fed a Skeletonema costatum microalgal grown in underground seawater.

Use of saline ground water for intensive rearing of Ruditapes philippinarum juveniles in a nursery system

Abstract Saline ground water was used as a thermal source for intensive rearing of juveniles of Ruditapes philippinarum and as a source of nutrients for phytoplankton production. In experiments concerning growth at different seasons, controlled factors were density of juveniles, flow of water, phytoplankton concentration, temperature and frequency of feeding. Their effects on growth and mortality were classified through correspondence analysis, analysis of multiway contingency tables and analysis of variance. An optimal stratery for summer and winter rearing was then defined. The different strategies are discussed in biological terms, with reference to the literature. Observed growth rates in this intensive culture were compared with available data for artificially fed and naturally reared populations.

Comparative growth of Bonamia ostreae resistant and wild flat oyster Ostrea edulis in an intensive system : II. Second year of the experiment

This paper reports the comparative growth and mortality of two crosses involving S85 and S89, two Ostrea edulis strains selected by IFREMER for their resistance to Bonamia ostreae. A first year of monitoring had revealed that the two crosses grew significantly better than two control populations from Quiberon Bay (Brittany) and Palavas (Mediterranean Sea). In a second and final year experiment which lasted from April to November 1995, the cross between the first generation of S89 and wild oysters (G0G1) performed better for most growth characteristics (whole weight, shell weight, length and width), than both the cross between the first generation of S89 and the second generation of S85 (G1G2) and the Quiberon control group, which were not significantly different from each other. Mortality was quite high during the experiment but was lower for the G0G1 cross (61%) than for both G1G2 (67%) and the Quiberon controls (76%). A low level of B. ostreae infection was detected during the experiments indicating early infections. These results are consistent with the previous one showing a direct or indirect effect of selection to B. ostreae resistance on growth.

Artificial filament breakage of the diatom Skeletonema costatum intended for mollusc aquaculture

Abstract Optimization of intensive rearing systems, especially those involved in yields of marketable sizes of bivalve molluscs, must pass through several technical improvements. One such improvement is described here: it concerns the experimental use of a simple centrifugal pump that causes artificial breakage of the chains of the diatom Skeletonema costatum (Grev.) Cleve used to feed adults of the Manila clam Ruditapes philippinarum (Adams and Reeve). The breakage process progressively reduced the size distribution of the algal particles but eliminated all algal chains longer than 60 μm. Scanning electron microscope (SEM) observations indicated that the breakage process impacts on both algal cells themselves (60% of broken structures) and siliceous structures connecting sibling cells (40% of broken structures). Some expected implications of the breakage process are presented in connection with the intensive rearing system and nutritional requirements of Manila clam.