Gametogenesis, reproductive investment, and spawning behavior of the Pacific giant oyster Crassostrea gigas: evidence of an environment-dependent strategy

The progress of gametogenesis was studied in oysters Crassostrea gigas having the same origin (Tremblade), but cultured during 1 year in two distinctive French marine areas, the Baie des Veys and Marennes-Oléron. We assessed seasonal changes in the reproduction cycle on the basis of stereological techniques to estimate reproductive investment and measurement of gonad evolution area by quantitative histology. From a qualitative point of view, both oyster groups presented typical reproductive stages, but showed differential timing, in particular during the sequence of spawning and duration of the re-absorption stage. Oysters in Baie des Veys had a single partial spawning in August and a re-absorption stage that extended until winter. Oysters in Merennes-Oléron had a partial spawning in July and massive release of gametes during August. Spawnings in both the areas were related to maximum temperature (19°C). The quantitative analysis showed, on an annual basis, a higher reproductive investment by oysters from Baie des Veys, 86% against 53% in the other group. Larger gonads, higher gamete production, and more intensive spawning were the characteristics of oysters in Baie des Veys. Recently, the reproduction pattern and investment has been related with summer mortalities; therefore, a quantitative understanding of reproductive processes becomes necessary for C. gigas. Environmental conditions at each site may explain differences in the progress and intensity of gametogenesis. While temperature regulated the time and speed of gametogenesis, results suggest that the intensity was influenced by the quantity of available food but may need further research. However, nutrient recycling from unreleased gametes in the gonads of oysters from Baie des Vey is a factor to be considered in the results of this study.     

Lipopolysaccharides from Commensal and Opportunistic Bacteria: Characterization and Response of the Immune System of the Host Sponge Suberites domuncula

Marine sponges harbor a rich bacterioflora with which they maintain close relationships. However, the way these animals make the distinction between bacteria which are consumed to meet their metabolic needs and opportunistic and commensal bacteria which are hosted is not elucidated. Among the elements participating in this discrimination, bacterial cell wall components such as lipopolysaccharides (LPS) could play a role. In the present study, we investigated the LPS chemical structure of two bacteria associated with the sponge Suberites domuncula: a commensal Endozoicomonas sp. and an opportunistic Pseudoalteromonas sp. Electrophoretic patterns indicated different LPS structures for these bacteria. The immunomodulatory lipid A was isolated after mild acetic acid hydrolysis. The electrospray ionization ion-trap mass spectra revealed monophosphorylated molecules corresponding to tetra- and pentaacylated structures with common structural features between the two strains. Despite peculiar structural characteristics, none of these two LPS influenced the expression of the macrophage-expressed gene S. domuncula unlike the Escherichia coli ones. Further research will have to include a larger number of genes to understand how this animal can distinguish between LPS with resembling structures and discriminate between bacteria associated with it.     

Intraspecific variation in gametogenesis in two populations of the Chilean molluscan bivalve, Argopecten purpuratus (Lamarck)

An increased understanding of the general biology of Argopecten purpuratus (Lamarck) is necessary to produce successfully high quality larvae in hatcheries and to collect enough spat in situ for growth on long lines. In the present study, the reproductive cycles of two populations of A. purpuratus were examined, the Mejillones and Rinconada scallop banks, from 1989 to 1993. Calculation of the gonosomatic index and a complementary histological study of the gonad revealed differences between the two populations in their reproductive cycle as well as in the quantity of gamete emissions. The Rinconada population has a major spawning period occuring at the end of the year and lasting about three months. Minor spawnings also occur in early July to late September. In Mejillones individuals, only one major period of gametogenesis and spawning occurs. Gametogenesis begins in January and spawning occurs between February and March. This is followed by a period of quiescence lasting four to five weeks. These results suggest that the Mejillones individuals are a more appropriate source of genitors to be used for conditioning in hatcheries. Existing data on biometrics, particularly the dimensions of the adductor muscle, as well as enzyme polymorphism, suggest that the Rinconada population would be a more appropriate source of genitors to be used in future aquaculture attempts.     

Autumn conditioning of the oyster Crassostrea gigas: a new approach

Gametogenesis, emitted gametes and larval yield (D larvae) of the oyster, Crassostrea gigas (Thunberg), were simultaneously examined in laboratory under two different conditions of temperature and photoperiod. One batch (“natural” cycle, NC) followed the equivalent cycle of average conditions of these parameters measured in La Tremblade (Charente-Maritime, France) during the last 15 years. The second batch (“accelerated” cycle, AC) was maintained under a variation two times faster of these parameters. Three conditioning experiences were performed during October, November and December 1998 with oysters from both NC and AC conditions. “Accelerated individuals” produced oocytes in growing stage from October and mature oocytes were noticed from December. In contrast, oysters from NC produced growing oocytes until January. Animals in AC produced 42% (October), 56% (November) and 96% (December) of mature oocytes after conditioning. Only the “natural” lot conditioned in December showed growing (23%) and mature oocytes (8%). Higher Walne–Mann index (WMI) values were recorded for “accelerated oysters” and significant differences (P<0.05) between treatments were noticed during October and November, suggesting nutrient accumulation before conditioning in the “accelerated” condition. Gamete emission and D larval yield results were equivalent to those reported in literature during early spring for the same species. The effect of temperature and photoperiod is discussed in order to understand their relationship with gametogenesis under these particular experimental conditions. It was demonstrated that internal clocks regulating gametogenesis can be change if stimulating factors (environmental cues) also change.     

Les antibiotiques dans les elevages de larves de bivalves marinsAntibiotics in larval rearing of marine bivalves

Fifty publications (1956–1976) concerning the use of antibiotics in rearing marine bivalve larvae are analysed. Most of the existing antibiotics have been tested alone or in combination, very frequently in an arbitrary fashion and without preliminary study to determine the most suitable antibiotic. The results obtained as regards to the survival of larvae or their growth have been variable, at times contradictory and not repeatable.The problems posed by the utilisation of antibiotics in rearing bivalve larvae are discussed and a standard routine for the use of antibiotics is proposed.