marnaviridae is a family of positive-stranded rna viruses in the order "picornavirales". the first marnavirus that was isolated, and which is the type species for the family, infects a microphyte: the toxic bloom-forming raphidophyte, "heterosigma akashiwo". algae therefore seem to serve as natural hosts. there is only one genus ("marnavirus") and one species in this family, the type species "heterosigma akashiwo rna virus" (harnav).