New three-way symbiosis: an eukaryotic alga, a blue mussel, and an endolithic cyanobacteria

In several parts of the world, mytilid mussels, Mytilus spp., are infected with pathogenic, single-celled, photosynthetic algae belonging to the genus Coccomyxa . The posterior shell edge of heavily infected mussels becomes considerably thickened with an extra shell material. Also, the external shell surface is usually eroded as a result of the microboring activity of endolithic cyanobacteria. We compared the number of bioeroded shells, the bioerosion degree, and the number of badly eroded shells, in uninfected and Coccomyxa -infected Mytilus spp. from the Lower St. Lawrence Estuary, Québec, Canada. The thickness of prismatic and nacreous layers was measured. The epibionts (pink calcareous algae, crustose brown algae, and barnacles) which encrusted surface of studied shells, were counted. Epibionts did not occur frequently and their possible relationship with the partners of a three-way symbiosis, Coccomyxa sp. – Mytilus spp. – endolithic cyanobacteria, has been neglected. We suggest that the mussel provides the alga Coccomyxa a protected space and metabolic carbon for photosynthesis. The alga stimulates shell thickening, and this protects mussel against ocean acidification and predators. The endolithic cyanobacteria remove black-colored periostracum providing the mussel and alga with an increased ability to survive during sunny days when exposed at low tide. The eroded shells become more translucent which encourages alga photosynthesis. However, shell degradation caused by endolithic cyanobacteria is a possible reason for the death of the Coccomyxa -infected mussels at the studied sites.