Jean-Claude Cochard

Broodstock conditioning of the oyster Crassostrea gigas: origin and temperature effect

Abstract Broodstock conditioning of the pacific oyster Crassostrea gigas (Thunberg) was examined in the laboratory under controlled conditions. Three experiments were undertaken to determine (i) the effect of the origin on gametogenesis, (ii) the time necessary to obtain the first mature oocytes in standard conditioning procedures, and (iii) the impact of different conditioning temperatures (16, 19, 22 and 25 °C) on gametogenesis. Oocyte size was used to identify mature oocytes. Significant differences among populations were observed in the number of mature oocytes produced. Only the oysters from northern locations (Normandy and Brittany) reached the mature stage by the end of the standard conditioning. Concerning the effect of different temperatures during conditioning, the first mature oocytes were detected (absolute days) by the 27th day at 16 °C, 23rd day at 19 °C, 19th day at 22 °C and 22nd day at 25 °C. This information was applied to fit a logistic model to predict oocyte growth using temperature and time as main affecting parameters; D = D max /(1+ a e − kt ) with D —oocyte diameter at time t (μm), D max —the asymptotic oocyte diameter (μm), a —adjustment parameter, k —slope of the curve during growing stage and t —time (days). The model fit consistently with values achieved at 16, 19 and 22 °C. The results obtained during this study could be used to improve conditioning procedures for C. gigas .

Osmoregulatory capacity according to nutritional status, molt stage and body weight in Penaeus stylirostris

Abstract Hypo-osmoregulatory capacity (or hypo-OC, i.e., the difference between the osmolality of the haemolymph and that of sea water), haemolymph glycemia and haemolymph sodium and chloride concentrations of Penaeus stylirostris were studied in experimental tanks according to the size, the molt stage and the nutritional status of the shrimp. Osmolality and haemolymph glycemia of fed and starved P. stylirostris were also studied in earthen ponds in fluctuating sea water according to time. In experimental tanks, the absolute hypo-OC of shrimp decreased linearly with increasing wet weight for each molt stage considered. For specimens weighing 16.6±1.7 g, the absolute hypo-OC was maximum in stage C (217±17 mosm kg −1 ) and significantly lower in stages B (165±16 mosm kg −1 ), D 0 (181±18 mosm kg −1 ), D 1 (146±20 mosm kg −1 ) and D 2 (135±13 mosm kg −1 ). The hypo-OC and haemolymph glucose concentration varied significantly after the food supply. No variations in haemolymph sodium and chloride concentrations were observed. In earthen ponds, haemolymph osmolality and haemolymph glucose concentration of molt stage C shrimp increased shortly after the food supply. When shrimp were kept starved for 24 and 48 h, haemolymph osmolality remained constant and haemolymph glucose concentration varied only slightly according to time. In light of these results, the use of shrimp hypo-OC (and/or haemolymph osmolality) and haemolymph glycemia in aquaculture as potential physiological indicators of disturbance in the aquatic environment is discussed.

Pearl oysters Pinctada margaritifera grazing on natural plankton in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia).

In atoll lagoons of French Polynesia, growth and reproduction of pearl oysters are mainly driven by plankton concentration. However, the actual diet of black-lip pearl oysters Pinctada margaritifera in these lagoons is poorly known. To fill this gap, we used the flow through chamber method to measure clearance rates of P. margaritifera in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia). We found: (i) that pearl oysters cleared plankton at a rate that was positively related to plankton biovolume, (ii) that nanoflagellates were the main source of carbon for the pearl oysters, and (iii) that the quantity and origin of carbon filtrated by pearl oysters was highly dependent on the concentration and composition of plankton. These results provide essential elements for the comprehension of growth and reproduction variability of pearl oysters in atoll lagoons of French Polynesia.

Bacteriological survey of antibiotic-treated sea waters in a Pecten maximus hatchery

Abstract Quantitative and qualitative bacteriological studies were conducted on two different sea waters utilized in a scallop ( Pecten maximus ) hatchery. During the 48-h cycles separating two waters changes, the total microflora estimated by epifluorescence microscopy remained almost constant. However, viable heterotrophic bacteria increased while Vibrio -like bacteria decreased sharply. The dynamics of bacterial communities as a function of water treatment was estimated by numerical taxonomy. The antibiotic-selected resistant strains formed a bacterial community showing very little diversity.

The limits of morphometric features for the identification of black-lip pearl oyster (Pinctada margaritifera) larvae

Abstract As with most cultivated bivalves, culture of the Tahitian pearl oyster Pinctada margaritifera is particularly dependent on the natural environment, especially for spat supply. The ability to track in real time the abundance and the development of pearl oyster larvae in the plankton would help optimize spat collection in atolls of French Polynesia. However no identification criteria are available for the larvae of several bivalves species present in the lagoons and it is not yet possible to specifically monitor pearl oyster larvae. The aim of this study is to determine the most pertinent morphological identification criteria, to specifically identify the larvae of P. margaritifera and differentiate them from those of three other abundant species: Pinctada maculata, Crassostrea cuculata and Chama sp. The method of image analysis after photon microscopy was assessed. It allowed automatic measurement of numerous morphometric features that were tested alone or in combination and identification threshold for P. margaritifera larvae were determined by statistical analyses. These results led to a key that allowed correct identification for 77% of P. margaritifera larvae. The hinge diagnosis method under scanning electron microscopy, a prime method for the identification of specific criteria on bivalve larva shells, was also used on larvae of both Pinctada species. The two species could be differentiated precisely because of specific differences in the thickness of their hinge provinculum and the number of denticles it bears. However this approach is too time-consuming and technically demanding to use in real time field studies. This study showed the limitations of image analysis as an identification tool of the P. margaritifera larvae, but proper statistical analyses and especially the decision tree approach could be used to evaluate and efficiently prioritize the choice of the species identification criteria.