Differences in gametogenic cycle among strains of the European flat oyster Ostrea edulis and relationship between gametogenesis and bonamiosis

An experiment was carried out to compare the performance of various cohorts of oysters Ostrea edulis from each of four different geographic origins that were born and cultured in the same environment. This paper reports the variation of gonad condition of the oysters through on growing and analyses of the relationship between gametogenesis and infection by Bonamia ostreae. Nineteen cohorts of spat were produced in a hatchery from four oyster populations: Irish, Greek, and two Galician: Ortigueira and Coroso; the spat were transferred to a raft in the Ría de Arousa, an area affected by bonamiosis, for grow-out for two years. Significant differences were observed in the temporal pattern of gonad condition between oysters from different geographic origins. Irish oysters showed a short period of gonad activity and a long gonad resting phase. By contrast, Greek oysters had the longest period of gonad activity and the shortest gonad resting phase, as well as the highest percentage of male oysters compared to oysters from other origins. Galician oysters, from both origins, showed a pattern of gonad development intermediate between Irish and Greek, with a higher percentage of females during the second reproductive period. Significant differences were also observed between cohorts within Greek, Coroso and Irish origin, but not from Ortigueira, whose cohorts showed a similar temporal pattern of gonad condition. The differences between oysters from different origins would indicate adaptation to different environments; nevertheless the significant differences that were found between cohorts within origins would indicate that there is variability that eventually could allow adaptation to new environments. Considering sex categories, heavy infections by B. ostreae were significantly more frequent in oysters with a predominant female component; considering gonad stage, heavy infections were significantly more frequent in ripe and partially spawned oysters. The results suggest a hypothetical enhancement of progression of infection by B. ostreae associated with female maturation.