Phylogenetic and morphological characterization of the green alga infesting the horse mussel Modiolus modiolus from Vityaz Bay (Peter the Great Bay, Sea of Japan).

In this work, the ultrastructural features and taxonomic position of the green microalga infesting the horse mussel Modiolus modiolus from the north-western Pacific (Vityaz Bay, Peter the Great Bay, Sea of Japan) are reported. Mussels were collected monthly from May to September of 2009. In different months, the prevalence of mussels with green tissues was 16.6-62.5% (mean 43%). The most affected organs were the mantle, digestive gland and gonad. Histological analysis revealed severe infiltration of the connective tissue by hemocytes containing the alga cells. Electron microscopy showed that the alga was morphologically similar to the green algae from the genus Coccomyxa (Chlorophyta: Chlorococcales). Two new primers were designed to generate partial small subunit (SSU) rRNA sequences of the green alga from M. modiolus. Phylogenetic analysis based on the comparison of the SSU rRNA sequences of the trebouxiophyceans confirmed an affiliation of the green alga with the genus Coccomyxa. The sequence (1296 bases) of the green alga from M. modiolus was most closely related to the sequence CPCC 508 (AM981206) (identity 100%), obtained from an acid-tolerant, free-living chlorophyte microalga Coccomyxa sp. and to the sequences EU127470 (identity 99.3%) and EU127471 (identity 99.7%) of the green alga, presumably the true Coccomyxa parasitica, infecting the blue mussel Mytilus edulis from the Flensburg Fjord (North Atlantic).