Spatial and temporal adjustments in gill and palp size in the oyster Crassostrea gigas

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Spatial and temporal variations in gill and palp size were studied during 1 year in naturally-settled populations of the Pacific oyster Crassostrea gigas, reciprocally transplanted between two contrasting sites located along a marked gradient of turbidity conditions. The variability of suspended particulate matter (SPM) and food particles, estimated by the concentration of chlorophyll-a, was measured with in situ water-quality probes. Over a full seasonal cycle, oysters exposed to high-turbidity (HT) conditions exhibited a lower gill-to-palp (G:P) ratio, compared with those exposed to low-turbidity (LT) conditions. Seasonal variations in the G:P ratio were observed at the LT site in relation to the spring phytoplanktonic bloom, but differed from those that had been observed previously. In fact, palp enlargement and gill narrowing (lower seasonal G:P ratio) suggest that oysters improved their pre-ingestive selection efficiency as a priority, rather than their filtering capacity. This result indicates that suspension-feeding bivalves do not have independent plastic responses of their foraging structures to either SPM quality or quantity, and that the direction of variations in the G:P ratio depends on the combination of these two factors. At the HT site, no seasonal pattern was observed in the G:P ratio. This can be explained by the strong hourly variations in SPM and chlorophyll-a, associated with tidal cycles, with daily variations that can be similar to those observed over a year. Reciprocal transplantations showed that oysters originating from the same site can differ in their feeding apparatus morphology when they grow in different environments and that temporal variations in the G:P ratio of oysters transplanted to a new environment converge towards that of individuals that have spent their entire life in this environment. Variations in the relative gill and palp sizes of C. gigas appear therefore to be partly the consequence of reversible phenotypic plasticity in response to spatial and temporal variations in SPM quantity and quality. The results also suggest that the limits and dynamics of gill and palp plasticity are dependent on the origin of oyster populations. Considering its role in the exploitation of different trophic conditions, and consequently in the biological performances (growth and reproduction), plasticity in the feeding apparatus morphology of C. gigas could be a determinant in the establishment of invasive populations in new ecosystems or in the management of farmed oysters.

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