'Phototaxis' in the absence of light? Locomotory patterns in unionid mussels

Locomotion in benthic invertebrates can strongly affect habitat selection and ecosystem nutrient cycling. In the case of freshwater mussels, the drivers of locomotion are largely unresolved. Our aim was to assess the influence of light presence and intensity on the locomotory behaviour of freshwater mussels in controlled laboratory experiments. The species investigated in our study were Anodonta anatina and Unio pictorum, two widely distributed mussels in European lentic and lotic inland waters. At low algal concentrations, known to be associated with more frequent locomotory activities, we found that both species moved primarily in the absence of light (72.7% of all movements across experiments). However, the movements of both species were directed towards the light source, resembling a net-positive 'phototactic' response but in the absence of light. The distance to the light source, which was negatively correlated to light intensity, had a positive effect on the distance covered in locomotory activities by A. anatina but not by U. pictorum. Intraspecific variation in shell size had no impact on movement distance, indicating that the energetic costs of movement were not a limiting factor. We suggest that the observed movement towards brighter locations helps to enhance food quantity and quality, whilst movement in darkness mitigates predation risks.