Reproduction-related effects of green alga Coccomyxa sp. infestation in the horse mussel Modiolus modiolus

► Coccomyxa sp. delays gametogenesis in the horse mussel Modiolus modiolus. ► Acinus area in the gonads of infected mussels of both sexes is significantly lower. ► The results suggest a reduced reproductive output for the infected mussels. ► First study on reproduction-related effects of Coccomyxa sp. infection in mussels.