Effect of attachment status and aggregation on the behaviour of the zebra mussel Dreissena polymorpha

Abstract

The zebra mussel Dreissena polymorpha lives attached to a hard substratum, often in dense aggregations. It can thus be expected that both its attachment status and conspecifics may greatly affect its responses to various environmental stimuli. We studied whether the attachment status influenced mussel relocation frequency and responses to light and gravity in laboratory conditions. We also tested the impact of conspecifics upon mussel attachment strength and relocation frequency. Small (<10 mm) and large (>10 mm) mussels attached for 1 or 6 days relocated from their initial positions less frequently than unattached individuals. Large mussels crawled less often than small individuals, but neither attachment status nor size modified their responses to light: illuminated (100 lx) mussels always moved towards dark areas and changed their initial sites less often than those kept in darkness. On a slope inclined at 8°, unattached small mussels preferred upward movement in darkness whereas large mussels moved similarly in both directions and were less mobile than small individuals. The behaviour of mussels attached for 6 days was reversed: negative geotaxis disappeared in small mussels, while large mussels preferred to crawl upwards and moved more often than small individuals. Aggregated mussels of both size groups were more strongly attached to the substratum and stayed at their initial sites more often than singletons. Our study shows that attachment status and conspecifics can modify responses of zebra mussels to environmental stimuli and should be taken into account in behavioural studies on this species.