Light-related behaviour of the zebra mussel (Dreissena polymorpha, Bivalvia)

We studied the impact of light of various intensities (0.1-100 lux) and wavelengths upon zebra mussel behaviour in the laboratory. Mussels selected dark attachment sites rather than illuminated ones. The response of large individuals (>10 mm of shell length) was stronger than that of small ones (<10 mm). Mussels avoided even very dim illumination (as low as 0.1 or 1.2 lx, in the case of large and small individuals, respectively). All mussels avoided light of various wavelengths (red, green, blue) equally. Horizontal locomotion of mussels was reduced in 100-lx light. Upward movement, exhibited normally by small mussels in darkness, was reduced by light of intensity ≥12 lx. Finally, the percentage of time spent by large mussels with open valves was shorter in dim light (3 and 6 lx) than in darkness. Furthermore, illuminated individuals closed and opened their valves more often than darkened ones. Both these reactions disappeared in strong light (100 lx). In the field, the light-related behaviour of mussels, observed here, might lead to selecting deeper and sheltered sites, which are better protected against predators and dislodgement.