The effects of artificial light at night (ALAN) on the gaping activity and feeding of mussels

Artificial light at night (ALAN) is one of the most widespread forms of environmental pollution. Studies on terrestrial organisms have shown that the effects of ALAN can be pervasive, and importantly, can depend on the colour (i.e., wavelength) of light. ALAN also affects marine environments as it is present in more than 22% of the worlds coastlines and can reach depths of up to 100m. However, the impact of different colour ALAN on coastal marine organisms is under-investigated. In this study, we tested the effects of different ALAN colours on Mytilus edulis, a widespread coastal bivalve known for its high phytoplankton clearance capacity amongst other valuable ecosystem services. Using a lab-based valvometry system, we recorded the impact of red, green, and white ALAN on gaping activity patterns and phytoplankton clearance capacity of individual mussels and compared these to dark night control. Mussels exhibited a semi-diel activity pattern in both proportion of time open and gaping frequency. Although ALAN did not have significant effects on the proportion of time open it did significantly affect the gaping frequency. This effect was colour-specific with red and white ALAN resulting in lower activity compared to the dark night control but there were no effects on the phytoplankton consumption. Under green light, however, mussels showed a higher gaping frequency and reduced phytoplankton consumption with increasing time spent open compared to the other ALAN treatments and the dark control. Our findings suggest that ALAN does have colour-specific effects on mussels and indicate the importance of further investigating the physiological mechanisms behind these patterns, and their potential ecological consequences.