Pathogenicity and infectivity of a rickettsia-like organism in the sea scallop, Placopecten magellanicus

Abstract. A rickettsia-like organism from the gills and plicate membrane of the sea scallop, Placopecten magellanicus , was prevalent in near-shore Rhode Island scallop populations such as Block Island Sound and Narragansett Bay, but relatively scarce in Georges Bank samples. Fifty-three per cent of scallops transplanted in cages to Narragansett Bay became infected with the rickettsiae in 2.5-3.5 months. Scallops that were transferred to aquaria with Narragansett Bay water were 100% infected with rickettsiae in 5 months or less. There is some evidence that heavy infection of P. magellanicus inhibits swimming response and that rickettsial infection may contribute indirectly to mortalities by reducing scallop mobility and avoidance of predators. The organism was successfully cultured in gill organ culture and by direct injection of infected tissue homogenate, but could not be grown on artificial media or in yolk sac of embryonated hens' eggs. Other bivalve species could not be infected with the rickettsia of P. magellanicus through the water column or by direct injection.