Croissance et composition de la coquille de Gryphaea angulata LMK. élevée sur parc de pleine eau après enrichissement phosphaté du sol

On two adjoining parcels, one used as a control plot, the other spread with 25 kg of phosphate "F" (followed by a light harrowing) on a one are area, we placed some grypheae aged 21 months. The molluscs are emerged for an average of 4h30 at each tide.1° it seems that, at the level used and for the first few months, the phosphate caused a physiological derangement in oysters which resulted in a higher mortality rate than in the control oysters.2° after an experimental period of eleven months, laboratory experiments showed a great similarity between the two populations. However:a) If the weight of the wet tissues is about the same, the weight of the dry flesh of the molluscs which stayed on the phosphated soil was lower than that of the control oysters.b) Concerning their shells, it looks like the molluscs which stayed on the phosphated soil show a tendency to create larger quantities of chalk layers. This tendency only becomes significant when the dry weight of the shell is greater than 24 g.As the grypheae answering this criterion only represent 5 to 6 % of the specimens in each batch, the differences between these two populations become negligible when considered as a whole.3° in the two studied grypheae batches, the mudblisters created by polydorids are located around the upper valve, more compact statistically speaking. This damage is less frequent in oysters which stayed on the phosphated soil, and this is probably the only advantage provided by this technique. 4° Finally, after about one year, 43% of the phosphate had disappeared.