Oocyte size, a means to evaluate the gametogenic development of the Pacific oyster, Crassostrea gigas (Thunberg)

This study reports on the results of 38,100 measurements of the oocyte diameters of the Pacific oyster, Crassostrea gigas, in three populations living along the French Atlantic shoreline. The analysis of oocytes' diameters and histological examinations of the gonads indicated that C. gigas individuals collected in 1996, 1997 and 1998 in the Bay of Brest (Anse du Rot and Points du Chateau) and Marennes-Oleron showed a clearly defined pattern of seasonal gametogenic development. Comparisons between reproductive stages, sites and years did not reveal significant differences in oocyte diameter (*P > 0.05). Considerations from qualitative and quantitative characteristics of this species allowed us to propose a reproductive scale based on oocyte diameter and consisting of four stages: (1) "Early gametogenesis" with an oocyte diameter mean of 8.47 +/- 3.6 mu m, (2) "Growing stage" with a diameter mean of 21.4 +/- 8.4 mu m, (3) "Mature stage'' with a mean of 36 +/- 4.4 mu m and (4) "Degenerating stage" with 46 +/- 7.3 mu m in mean diameter. This scale was generated from a modal distribution analysis (interactive program for fitting mixtures of distributions) and microscopic descriptions of gonad characteristics. The use of this scale for reproductive studies in C. gigas is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.