Presence of Bonamia-like microcells in flat oyster, Ostrea puelchana (d'Orbigny, 1842) from San Matias Gulf, NW Patagonia, Argentina

In Argentina, especially in San Matias Gulf, oyster culture is a recent activity. Culture of native flat oyster, Ostrea puelchana (d'Orbigny, 1842), was particularly promising until an abnormal mortality occurred in 1996. Preliminary diagnosis, based on histology, revealed the presence of Bonamia-like cells in some individuals. In order to identify the pathological agent apparently involved in this mortality and to quantify its impact on natural beds, 42 batches of 30 individuals were sampled on natural beds and culture areas between years 1996 and 2001. Samples were analyzed by traditional techniques including heart imprints and histology, complemented by PCR and in situ hybridization. Histology and heart imprints revealed the presence of Bonamia-like cells in connective tissue of O. puelchana, free or within haemocytes, mainly in gills, digestive gland, stomach, intestine and gonad. PCR and in situ hybridization confirmed the belonging of the parasite to the genus Bonamia. Cultivated oysters and close natural beds presented highest frequency of parasite detection (50-60%) when mortality was maximal (October 1997). Subsequently, prevalence decreased slowly until 2000 and increased again from 6% to 10% between 2000 and 2001. Spatial and temporal propagation of the parasite was observed from culture areas to natural beds during two time intervals (1997 - 1999 and 2000 - 2001). Now, it needs to characterize this new microcell from molecular and ultrastructural point of views in order to assess it taxonomic relationships with other Bonamia species.