DIFFERENTIAL DYNAMICS OF Bonamia ostreae AND Marteilia refringens IN A SAME POPULATION OF FLAT OYSTER, Ostrea edulis, IN RADE DE BREST, BRITTANY, FRANCE

Although the flat oyster, Ostrea edulis, is naturally present in Rade de Brest, Brittany (France), density of populations remains very low (< 1 indiv.m^-2). The production of this species is locally hampered by several factors: fishing activities, predation (starfish, oyster drill and daurade) but also two constraining diseases, marteiliosis and bonamiosis. These diseases are due to the protozoan parasites Marteilia refringens and Bonamia ostreae which are reported in Rade de Brest since 1974 and 1980, respectively.

Eradication of these diseases is not possible and a better understanding of the dynamics of the parasites in the oysters is required to identify their respective impact on the oyster populations and to come out with relevant stock management, conservation or restoration measures.

In this context, we have investigated the presence of these both parasites by PCR in a cohort of oysters from spat to 3 years-old adults. Spat was sampled monthly and oysters older than 9 months were kept in bags and collected every 4 months. Oysters found infected by PCR were selected for further characterization analysis. Histology and in situ hybridization allowed estimating the level of infection and describing the distribution and development of the parasite in the tissues. In addition, PCR products were sequenced in order to evaluate the diversity of these parasite species at the oyster population level.

Our results show that both diseases evolve concurrently in the same oyster population but display different dynamics pattern. Marteiliosis seems to occur in spat when environmental conditions are suitable for the transmission of the parasite which then presents several infection cycles with moderate prevalence. B. ostreae was continuously detected in oysters older than 18 months with two peaks of prevalence preceding periods of mortality.

Each of these endemic diseases may deplete host resources, which in turn could affect the development of the other parasite during co-infection. These results highlight the interest of investigating the dynamic of multiple co-infections and within-host competition.