The health status of two species of Tasmanian farmed shellfish, Crassostrea gigas (thunberg, 1793) and Ostrea angasi (Sowerby, 1871)

A project to assess the health of Tasmania's farmed shellfish was conducted during the period October 1990 - June 1992. A total of 5290 Pacific oysters (Crassostrea gigas) and 630 flat oysters (Ostrea angasi) were collected during the program which involved near-monthly collections of shellfish from each of four growing areas in Tasmania. Pacific oysters were free of any prescribed or potential pathogen. Flat oysters were found to be infected with a serious pathogen, Bonamia sp, and a viral inclusion of unknown significance. Histological examination of these samples revealed the presence of low numbers of commensal organisms in the tissues.of both species of oyster. Pacific oysters were infected with a viral infection of the gametes, rickettsia1 inclusions, two species of ciliates, two protozoans of unknown taxonomy, a turbellarian and two types of copepods. Flat oysters were infected with rickettsia1 inclusions, a ciliate and two types of copepod. Three species of spionid polychaetes were dissected from shellblisters affecting Pacific oysters. Changes in histological appearance of Pacific oysters including changes in the leydig tissue, the types and degree of infiltration of haemocytes and atrophy of digestive tubules show some seasonal trends and ire correlated to the gonadal stage of the oyster. Also, digestive tubule atrophy and abundance of brown cells are correlated with lower salinity.