CLEARING OF THE KIDNEY-DISEASE BACTERIUM RENIBACTERIUM-SALMONINARUM FROM SEAWATER BY THE BLUE MUSSEL MYTILUS-EDULIS, AND THE STATUS OF THE MUSSEL AS A RESERVOIR OF THE BACTERIUM

The blue mussel Mytilus edulis commonly fouls netpens on salmonid farm s along the British Columbia coast, Canada, and salmon farmers have ex pressed concerns that this bivalve may serve as a source of infectious agents for penned salmonids. The chief concern was that the mussel mi ght concentrate the bacterial kidney disease agent Renibacterium salmo ninarum (Rs) from the water column (into which it is shed with the fec es of infected salmon) and then serve as a continuous source of infect ions with this important salmonid pathogen. Our results showed that th e mussel is capable of rapidly clearing seawater of suspended Rs cells . In the process, most or all of the ingested Rs cells were killed by the mussel. The bactericidal eff ect most likely occurred in the musse l's digestive tract, where Rs cells would have been exposed to the let hal substances present in the digestive gland tissues of the mussel. A small portion of viable Rs cells found in the sedimented mussel's fec es likely represented Rs cells removed from the water column by the fe ces during settling. Mussels that were exposed to Rs appeared to free themselves of Rs quite rapidly after they were removed from the Rs-con taining water. Thus, although mussels may cause problems for salmon fa rmers as a result of netpen fouling, it is unlikely that they serve as long-term reservoirs of Rs. In fact, mussels should have the benefici al effect of reducing Rs levels in the netpen environment and thus pro bably play a role in reducing the frequency of horizontal Rs infection s.