THE ECOLOGY OF BONAMIA AND DECLINE OF BIVALVE MOLLUSKS

Bonamia is a protozoan parasite of the haemocytes of oysters (Tiostrea chilensis), in which it has an annual developmental cycle between Nov ember and August each year. The parasite transmits directly, oyster to oyster, and therefore disease spread is related to host stock density . The Foveaux Strait oyster population experiences large mortalities e very 20-30 years, and these may be attributable to Bonamia. The parasi te appears to become less pathogenic at the end of, and probably betwe en, mass mortalities, and some oysters appear more tolerant of infecti on than others. On the basis of these observations, and considering ot her protist pathogen:oyster models, the apparently reduced pathogenici ty of Bonamia is discussed in terms of parasite kinetics. The populati on dynamics and selection of parasite tolerant host stocks, and kineti cs of parasite transmission, may explain the cyclic nature of large-sc ale mortalities in Foveaux Strait, without change in parasite pathogen icity.