Comparison of two calcein administration techniques for marking the shell of the blue mussels (Mytilus spp.) infected with pathogenic microalgae Coccomyxa

The presence of the L-shaped shell deformity (LSSD) is a reliable biomineral marker for non-lethal detection of wild blue mussels (Mytilus spp.) infected with the unicellular photosynthetic microalgae Coccomyxa sp. However, the LSSD formation rate is unknown. Most literature regarding bivalve shell sclerochronology advocates the fluorochrome calcein as an ideal growth marker. Administration technique of calcein-seawater solution for mussels, injection into mantle cavity or immersion, influences mark quality and staining. The best technique cannot be predicted in advance. Three circumstances may impact the dyeing success for Coccomyxa-infected mussels: mantle colonization with algae, increase in mucus production and biophysiological control in alga-mussel association. This report examines how injection (in field condition) and immersion (for 2 h - field condition; 20 h - laboratory condition) affect shell marking in adult (60–70 mm shell length) Coccomyxa-infected Mytilus spp. Uninfected wild and farmed mussels are used in parallel experiments for comparative purposes. After the staining procedures (calcein concentration 150 mg L ‐1), mussels were caged for 55 days in the Lower St. Lawrence Estuary (Québec, Canada). Results demonstrate that uninfected mussels showed fluorescent marks regardless of the administration techniques, whereas in Coccomyxa-infected mussels marks were visible only after immersion for 20 h. This may suggest that dyeing success could be managed by unknown aspects of biophysiological control in alga-mussel symbiosis. A comparison of images made in 2019 and 2023 indicates no change in calcein marks brightness 4 years after the end of shell staining experiments.