Ecophysiologie du ver tubicole Lanice conchilega : compétition trophique avec l'huître japonaise Crassostrea gigas en Baie des Veys (Baie de Seine Occidentale)

The trophic competItIon between the Pacific oyster Crassostrea gigas and the polychaete Lanice conchilega was studied by assessing the polychaete suspension feeding activity. Particle size distributions were compared at the input and output of the experimental design so as to estimate the polychaete retention efficiency. Although particles ranging from 4 µm to 12 µm were kept by Lanice conchilega, no upper treshold and maximum retenti on rate were reached. In contrast, Crassostrea gigas showed a retention efficiency starting at 2 µm and reached a 6 to 8 Ilm upper treshold. Based on our results, a trophic competition is likely to occur between Crassostrea gigas and Lanice conchilega. Standardized filtration rates reached 0.27 l.h-1.dmw-1 and 2.41 l.h-1.dmw-1 for a 1 g Lanice conchilega and Crassostrea gigas respectively. Lanice conchilega assimilation rate (0.37) were significantly lower than those of Crassostrea gigas (0.49). Respiration rates were estimated to 0.21 and 0.62 ml 02.h-1.dmw-1 for Lanice conchilega and Crassostrea gigas respectively. Therefore, polychaete scope for grovvth (SFG) (2.17 J.h- 1.dmw-1) was significantly lower compared to the 63.7 J.h-1.d1nw-1 Crassostrea gigas SFG. Based on these results and both species field stocks assessments, Lanice conchilega was responsible for 21 % carrying capactiy decrease and 44 % oxygen depletion. However, Lanice conchilega's SFG represented onl~' 5 % of Crassostrea gigas population's SFG. Several hypothesis regarding both populations interactions were discussed.