Impact of abiotic factors on aggregation behaviour of the zebra mussel Dreissena polymorpha

Abstract

Many bivalves exhibit contagious distribution, which reduces their exposure to environmental dangers including predation, dislodgement or desiccation, but at the cost of increasing intraspecific competition and waste accumulation. We studied the impact of depth (1 or 5 m), substratum (loose or solid), light (0 or 1000 lux), temperature (10-30 °C, every 5 °C) and water flow (0 or 8.7 cm s-1) on aggregation-forming in the gregarious byssate zebra mussel Dreissena polymorpha in the field (the depth factor) and laboratory (the other factors). Mussels were more clustered at a shallower site, at intermediate temperatures (15-25 °C) and in running water. There was no significant effect of light on mussel aggregation. Greater initial distances between mussels reduced clumping to some extent, but not within 15-30 mm, suggesting that mussels can move actively towards conspecifics over such short distances. Mussels rarely formed druzes (groups of mussels attached to conspecifics) except for those tested on sand, which is unsuitable for attachment. In general, mussels grouped in the presence of potential dangers (shallow depth, running water, unsuitable substratum) and when unfavourable effects of clumping were lower (intermediate temperatures and water flow). However, they did not attach to each other unless other substrata were unavailable, suggesting that overcrowding was avoided when possible. Thus, mussel aggregation behaviour seems to be a compromise between the advantages and shortcomings of solitary and gregarious life, and may change depending on local environmental conditions.