First record of the green microalgae Coccomyxa sp. in blue mussel Mytilus edulis (L.) from the Lower St. Lawrence Estuary (Québec, Canada)

- Mytilus edulis infested with microalgae Coccomyxa sp. were found in the St. Lawrence Estuary.
- M. edulis was the only bivalve with visible signs of infestation with the alga.
- Coccomyxa sp. was identified by rRNA sequencing and HPLC pigment analysis.
- Concentration of algal cells in mussels hemolymph reaches up to 3.8 × 107 mL−1.
- The source of Coccomyxa infestation in the Lower St. Lawrence Estuary is unknown.

During autumn 2012 and spring 2013, blue mussels Mytilus edulis (L.) with strongly deformed (L-shaped) posterior shell margins and green spots in soft tissue (microalgae) were collected from intertidal zone along the south shore of the Lower St. Lawrence Estuary near Rimouski (Québec, Canada). Identification of algal cells infesting mussels as Coccomyxa sp. was confirmed by rRNA sequencing and HPLC pigment analysis. Flow cytometric analysis revealed the presence of algal cells in the hemolymph and extrapallial fluid in mussels with deformed and non-deformed shells; concentrations of algal cells were ranged from about 200 mL−1 in mussels with actually non-deformed shells to concentrations reaching up to 3.8 × 107 mL−1 in mussels with heavily deformed ones. Chemical analyses of soft tissues led us to conclude that butyltin compounds and trace metals cannot be considered among factors responsible for the shell deformity observed. Using scanning electron microscopy, the biogenic nature of the erosion on the external shell surface and aragonitic lenses of prisms in the curvature zone of deformed shells (in sections) were recorded. The sequence of the green algae from M. edulis of the Lower St. Lawrence Estuary was closely related to Coccomyxa sp. infecting M. edulis from the Flensburg Fjord (North Sea) and Modiolus modiolus (L.) from the Vityaz Bay (Sea of Japan).