Microcells in flat oysters, Ostrea chilensis from Chiloe Island, Chile

The flat oyster Ostrea chilensis is susceptible to infection with two microcells, Bonamia exitiosa and Bonamia ostreae. A haemocytic parasite very similar to Bonamia species has been previously reported in two of 167 Chilean oysters sampled from Chiloe Island in 1996-1997. In order to characterise the parasite, 46 and 120 two-years-old oysters were sampled in the same area in 2001 and 2004, respectively, and analysed by histology. The presence of Bonamia-like parasite was noted in 20% and 10% of animals respectively. No significant mortality was reported despite high levels of parasitic cells in most infected oysters. Some oysters found to be infected in 2001 and 2004 were selected for molecular analysis, including partial sequencing of 18S and ITS regions of the rDNA gene cluster. On the basis of the obtained sequences, the parasite appears closely related, but distinct from Bonamia exitiosa and Bonamia ostreae. This strongly suggests the existence of a new species of Bonamia in Ostrea chilensis. However, arguments pertaining to ultrastructure of the parasite and its pathogenesis in O. chilensis need to be considered to complete the definition of this potential new species and to determine the impact of the haemocytic parasite on the Chilean production of flat oysters