Philippe Geairon

Allometric relationships and effects of temperature on clearance and oxygen consumption rates of Crassostrea gigas (Thunberg).

Abstract Clearance and oxygen consumption rates of Crassostrea gigas were investigated with animals of 5–200 g total wet weight (0.1–3 g dry tissue weight), and at different temperatures (5–32 °C) after 10 days acclimation. During this period significant mortalities were observed at 32 °C, which may be close to the upper thermal limit for this species. For each temperature, allometric relationships between physiological rates and the dry weight (DW, g) of the animal were estimated. Clearance rate (CR, 1·h−1) was maximal at 19 °C; oxygen consumption rate (VO2, mgO2·h−1) increased over the range of experimental temperatures (T, °C). Two statistical models are proposed: CR = [a − (b ∗ (T − c) 2 )] ∗ DW d and VO 2 = [a + (b ∗ c T )] ∗ DW d . However, neither model is appropriate during the reproductive period.

Effects of the feeding behavior of Crassostrea gigas (Bivalve Molluscs) on biosedimentation of natural particulate matter

Some of the particulate matter found in bottom sediment comes from material rejected as pseudofaeces or produces as faeces by Crassostrea gigas. The amount of pseudofaeces rejected is proportional to the amount of seston for total seston values up to 4.6 mg 1−1. For this values up to pseudofaeces production, the amount of faeces is mainly limitated by pseudofaeces production and reaches a constant mean value of 8.9 mg 1−1 g DW−1 when seston concentration increases.The proportion of faeces versus pseudofaeces is over 1 when seston concentrations are below 10 mg 1−1, and remains less than 1 when seston concentrations are higher than 10 mg 1−1.The pseudofaeces show a lower organic content (13 ± 3%) than the material trapped through the gills (27 ± 4%), as well as a lower percentage of proteins and lipids, suggesting a selection by the labial palps. On the contrary, an enrichment with labile carbohydrates is recorded in pseudofaeces, probably due to mucus agglutination of rejected particulate matter.The feeding of Crassostrea gigas can package and transform (agglutinate, change the organic content, change the biochemical composition) suspended matter, thus influencing both the quality and quantity of organic matter sedimentation.

Comparative analysis of oxygen consumption rates between cupped oyster spat of Crassostrea gigas of French, Japanese, Spanish and Taiwanese origins

Respiration rates of various geographical strains of Crassostrea gigas were compared to assess the respiratory expenditure as a physiological indicator of catabolism (cost of maintenance). Parental oysters, sampled in France (Marennes-Oleron), Japan (Hiroshima), Taiwan (Tunkang) and Spain (Cadix), were differentiated by both their geographic origin and by mitochondrial DNA markers, allowing the distinction between the two cIosely related taxa Crassostrea gigas and Crassostrea angulata. After reproduction of these parental oysters, respiration rates of spat of each strain, reared under common controlled conditions, were individually estimated at 20 oC by using a volumetric microrespirometer. Our results demonstrated that physiological variability existed among the Crassostrea gigas strains and is likely to be related to physiological differences between geographical regions and/or genetic adaptations. The French strain showed the highest rates (0.71 IlL·h-I), while the Japanese, Taiwanese and the hybrid between Spanish and French strains consumed less, 0.53, 0.43 and 0.40 IlL·h-l, respectively. Our results confirmed the discrepancies previously suggested between Crassostrea gigas and Crassostrea angulata. Comprehensive physiological assessments should be carried out over a wide temperature range to confirm our results and to further evaluate growth potential. Appropriate decision making based on these studies will help future shellfish management in shellfish rearing areas such as the overstocked Marennes-Oleron Bay.

Respiratory time activity of the Japanese oyster Crassostrea gigas (Thunberg)

A continuous monitoring system has been developed to measure the respiratory time activity (RTA) of the oyster Crassostrea gigas (Thunberg). This activity was recorded over 24 h using this technique, at constant temperatures of 15 and 21°C in the laboratory. At 21°C, respiratory activity was measured before and after a period of starvation, during which spawning occurred. In the field, RTA was measured during two successive tidal cycles of a day. The respiratory time activity was defined as the percentage of time spent active by the oyster relative to the total experimental time. Important individual variability was noticed, percentage of time spent active ranged from 0 (RTA=0%) to 100% (RTA=100%). RTA measured on the whole experimental population, taking into account active and non-active oysters, varied from 52 to 86% under laboratory conditions and, according to the season, from 44 to 82% in the field. Considering only the active oysters (RTA>0%) which represented, according to the different experiments, from 75 to 100% of the studied population, RTA varied from 54 to 86% in the laboratory and from 59 to 94% in the field experiments. The influence of RTA on growth predictions is discussed.

Optimisation of the traditional Pacific cupped oyster (Crassostrea gigas Thunberg) culture on the French Atlantic coastline: autumnal fattening in semi-closed ponds

Abstract Oyster farming in France is a traditional activity. Each year, 60,000 tons of C. gigas are fattened before being sold. Present-day fattening techniques for improving oyster taste and colour and increasing the meat weight are not particularly reliable. To optimize the fattening process, large phytoplanktonic blooms were induced in ponds, then distributed in oyster ponds. Despite the variability of the autumn weather conditions, diatoms ( Skeletonema costatum ) have been successfully cultured in outdoor ponds. During neap tides (when no seawater replenishes the water in the oyster beds), between mid-October and mid-December 1996, the fattening index (the weight of the meat) was constant or increased by 10–15% when a supplement of algae of 380,000 cells/oyster/day was added to the ponds. Algal supplement was clearly responsible for fattening improvement. In 1995, the addition of 110,000 cells/oyster/day, a significantly lower quantity, was not sufficient to prevent weight loss. Over two periods of 3 months (in 1995 and 1996) of the experiment, a significant reduction in weight (up to 20%) was observed in oysters grown in ponds which did not receive additional algae.