W2 virus infection of the crustacean Carcinus mediterraneus: a reovirus disease.

Most of the viruses described in marine invertebrates have been related to known virus families only on the basis of ultrastructural properties. Recently a viral agent was isolated and studied in the Mediterranean shore crab Carcinus mediterraneus. This agent, which was 65 to 70 nm in diameter, developed in the cytoplasm of connective tissue cells of C. mediterraneus and produced unusual viral structures, 'rosettes', consisting of an empty sphere bounded by arrangements of viral particles. The capsid consisted of two protein shells. After purification, full virions exhibited a density of 1.34 g/ml in CsCl. The nucleic acid composition of virions was estimated at about 22% and was shown to be a dsRNA with at least nine segments in four different size classes. The capsid contained six polypeptides with Mr of 120 x 10(3), 94 x 10(3), 76 x 10(3), 44 x 10(3), 32 x 10(3) and 24 x 10(3), as determined by SDS-PAGE. From its biological, ultrastructural and physicochemical properties, we propose that this virus should be classified as a new member of the family Reoviridae.