Gametogenic cycle of Crassostrea gigas in contrasting Mediterranean habitats: marine (Gulf of Tunis) and continental (Bizert lagoon) culture sites

Annual gametogenic phenology of oyster, Magallana bilineata (Röding, 1798) collected from the west coast of Moheshkhali Island, Cox's Bazar, Bangladesh

The current study aimed to describe the annual gametogenic phenology of the oyster Magallana bilineata (Röding, 1798) (=Crassostrea madrasensis), which is found on the west coast of Moheshkhali Island, Cox's Bazar, Bangladesh. Samples were drawn monthly from the intertidal region during low tide, from which 20 adult individuals were selected at random for biometry and histology. The mean condition index (CI), a ratio of tissue wet weight (g) to shell length (cm), varied from 0.58 ± 0.08 to 1.32 ± 0.36. Histology revealed two spawning cycles in the habitat of M. bilineata over the 12 months of the study. Gametogenesis initiated in December and May, and ripe animals principally occurred from July to September and February to May. In the habitat, M.bilineata exhibited two spawning peak periods: April to June and August to October. The undifferentiated stage as a preparatory step for the next spawning extended from November to February for the first spawning cycle and for a brief period in June for the next spawning cycle. The initiation of spawning in March could be associated with the high-level decline of salinity and increased temperature between February and March, in association with the annual rainfall start, whereas the next spawning cycle could be associated with a gradual increase in salinity. No spawning activity was reported from December to February, when the water temperature remained below 22 °C. Further study could be undertaken on the timing of spatfall of M.bilineata in the habitat to harvest spats for commercial farming of this promising species.

Oyster Crassostrea gigas, a good model for correlating viral and chemical contamination in the marine environment

To establish a relationship between viruses and chemicals, they were analysed in oyster Crassostrea gigas from an Italian experimental station. The chemicals concentrations were: Σ6 NDL-PCBs 0.82-7.12 ng g^-1; BaP LOQ (<0.2 μg kg^-1) to 1.2 μg kg^-1; PAH4 LOQ (<0.2 μg kg^-1) to 9.8 μg kg^-1; Cd 0.073-0.365 mg kg^-1; Pb 0.010-0.487 mg kg^-1; and Hg < LOQ (0.089 mg kg^-1). The viruses identified included: noroviruses (NoVGI/GII), astrovirus (AsV), rotavirus (RV), adenovirus (AdV), and sapovirus (SaV), while hepatitis A, hepatitis E, and Aichi viruses were not detected. Significant correlations were observed for NDL-PCBs with NoVGI, NoVGII, and AdV; BaP and PAH4 with NoVGI and AsV; Cd with RV; Pb with NoVGI and AsV; PAHs with Pb; AsV with NoVGI; and AdV with NoVGII. The study indicated as C. gigas is a model for correlating pollutants and foodborne viruses, whose co-presence may represent an additional food safety risk.

Seascape genomics of eastern oyster (Crassostrea virginica) along the Atlantic coast of Canada

Interactions between environmental factors and complex life-history characteristics of marine organisms produce the genetic diversity and structure observed within species. Our main goal was to test for genetic differentiation among eastern oyster populations from the coastal region of Canadian Maritimes against expected genetic homogeneity caused by historical events, taking into account spatial and environmental (temperature, salinity, turbidity) variation. This was achieved by genotyping 486 individuals originating from 13 locations using RADSeq. A total of 11,321 filtered SNPs were used in a combination of population genomics and environmental association analyses. We revealed significant neutral genetic differentiation (mean F ST = 0.009) between sampling locations, and the occurrence of six major genetic clusters within the studied system. Redundancy analyses (RDAs) revealed that spatial and environmental variables explained 3.1% and 4.9% of the neutral genetic variation and 38.6% and 12.2% of the putatively adaptive genetic variation, respectively. These results indicate that these environmental factors play a role in the distribution of both neutral and putatively adaptive genetic diversity in the system. Moreover, polygenic selection was suggested by genotype-environment association analysis and significant correlations between additive polygenic scores and temperature and salinity. We discuss our results in the context of their conservation and management implications for the eastern oyster.