THE ANNUAL PATTERN OF INFECTION BY BONAMIA SP IN NEW-ZEALAND FLAT OYSTERS, TIOSTREA-CHILENSIS

Detailed examination was carried out on 1745 oysters (Tiostrea chilensis syn. T. lutaria) sampled from two pairs of stations, 15 km apart, in Foveaux Strait between September 1986 and May 1989. Occurrence, intensity of infection and parasite stages were recorded in the gills/mantle, kidney, subepithelium, gonad, Leydig tissue and digestive system of each oyster, to determine the annual pattern of infection. A consistent pattern was apparent at both station pairs over the 3 years. Infections were lowest in spring (October/November) when basophilic dense forms occurred primarily in haemocytes around the gut. Rapid proliferation occurred in early summer (December/January) when infected haemocytes entered the gonad to phagocytose, mainly male, reproductive products. Gonad infections with larger dense forms persisted through the mainly female cycle, with heavy gonad infections and highest prevalence of infection in autumn (April). In early winter (May/June) infection rates declined, but a second peak in prevalence occurred in winter (August) with a subsequent decline to low prevalence and light infections in spring. Egress from the host was effected primarily through the gonad (December-August), kidney (April-August), digestive diverticulae and gill (January-August), and presumably on the death of the host. Parasite tissue distribution was related to crowding except in the male gonad where it coincided with haemocyte phagocytosis of gonad products.