Nathalie Cochennec

Establishment of an experimental infection protocol for the flat oyster, Ostrea edulis, with the intrahaemocytic protozoan parasite, Bonamia ostreae: application in the selection of parasite-resistant oysters

Abstract The availability of purified Bonamia ostreae suspensions and the development of an inoculation protocol enabled experimental infections with the European flat oyster, Ostrea edulis , to be carried out. Different infectious doses were tested and the kinetics of bonamiasis studied. The 50% infectious dose (ID), determined from inoculations with 10 to 10 6 B. ostreae per individual, was estimated to be 80 000 parasites for 3-year-old oysters. This 50% ID was used to screen for parasite-resistant oysters. Preliminary results suggested that the F 1 progeny of oysters that survived previous outbreaks of bonamiasis were more resitant to B. ostreae .

First report of a Mikrocytos-like parasite in European oysters Ostrea edulis from Canada after transport and quarantine in France

As part of a disease resistance experiment, 112 apparently healthy European flat oysters Ostrea edulis L. were exported from Canada (Nova Scotia) into France to test their susceptibility to Bonamia ostreae infection. Twelve oysters died in transit and 17 others died within 2 wk of laboratory quarantine acclimation. All oysters were examined histologically, and the 17 that died during quarantine were assayed for microcells (Bonamia sp. and Mikrocytos mackini) using molecular techniques. A microcell parasite was detected in the connective tissue of 5 of the 112 oysters. Morphological appearance, tissue affinity and molecular characterization through PCR, in situ hybridization (ISH), fluorescence in situ hybridization (FISH) and sequencing revealed a protist related to M. mackini. This is the first report of a parasite of the genus Mikrocytos in a species belonging to the genus Ostrea from the Atlantic Ocean.

Shell disease in eastern oysters, Crassostrea virginica, reared in France

Progeny of eastern oyster, Crassostrea virginica, introduced into France in 1992, were reared in IFREMER facilities to test their growth performances. During the summer of 1993, sporadic mass mortalities (80-90%) occurred among C. virginica spat reared in the IFREMER laboratories in La Tremblade (Charente Maritime, France) and Bouin (Vendée, France). Affected oysters presented mantle retraction and deposition of an anomalous conchiolin layer on the inner surface of the shell. The incidence of oysters with gross signs exceeded 80%. No obvious pathogen was identified in soft tissues by histology and transmission electron microscopy (TEM). However, histological examination showed the presence of anomalous basophilic round structures, 0.5-1 microm in diameter, in gill and mantle connective tissues. These extracellular Feulgen-negative structures reacted positively with the von Kossa stain. TEM examination on mantle and gill samples in diseased spat showed that the basophilic bodies consisted of concentric deposits of an amorphous substance interpreted as containing calcium. These observations may indicate that the mineralization process in spat shells was disturbed without exact determination of the cause. Based on the similarities of the gross signs to those reported in juvenile eastern oysters in the United States, we believe that the cause of the mortalities observed in France was probably the Juvenile Oyster Disease. Moreover, we report for the first time the detection of anomalous amorphous structures in gill and mantle connective tissues associated with mortalities and deposition of an anomalous conchioloin layer on the inner shell surface in C. virginica spat.

Influence of Crassostrea gigas mixed with Ostrea edulis on the incidence of Bonamia disease

Abstract The effect of mixing Japanese oysters (Crassostrea gigas) with flat oysters (Ostrea edulis) in bags on the incidence of bonamiasis was tested with four proportions of C. gigas (0%, 25%, 50% and 75%) and two total densities (200 and 400 oysters per bag). At density 200, the survival rates of flat oysters, in 15 months, increased significantly from 20% without C. gigas to 35% with 75% C. gigas; with 400 oysters per bag, survivals were lower. This apparent effect of C. gigas might be the result of correlative variations in the number of O. edulis per bag.