Spatial and temporal variability in growth and reproduction of the Japanese cupped oyster Crassostrea gigas (Thunberg, 1793): are they adapting to northern European waters?

No abstracts are to be cited without prior reference to the author.

The Japanese cupped oyster, Crassostrea gigas, is native to Japan and was introduced in Europe for commercial purposes in the early 1960’s. In France and in the south of the Netherlands, this species is now a very important cultured species. Initially, it was thought that the cold northern waters of the Netherlands would restrain the species’ reproduction and settlement but surprisingly C. gigas is reproducing and expanding in several estuaries, and has already been seen in northern Germany. This suggests that over time eco-physiological changes have occurred, indicating either phenotypic and/or genotypic plasticity. In order to understand how this species was able to invade northern habitats, the energy flow over growth and reproduction in C. gigas was studied along a latitudinal gradient from La Rochelle, France to Eastern Scheldt and Wadden Sea in the Netherlands. Overall, the French population had a lower body condition than the two Dutch populations had. Animals from the north of the Netherlands (Wadden Sea) showed the highest gonad mass ratio, although the smallest egg size. Since egg volume directly affects development time and hence reproductive success, we will present evidence that there is a trade off between egg size and larval development time. In fact, a clear latitudinal trend in egg size was found from France to the German Wadden Sea. We will discuss the results in the light of the different lifehistory strategies of each population in terms of energy allocation over growth and reproduction and we will postulate a hypothesis explaining how the Japanese oyster has passed from “an immigrant to a permanent member'.