Impacts of microcystins on the feeding behaviour and energy balance of zebra mussels, Dreissena polymorpha: A bioenergetics approach

Microcystins are produced by bloom-forming cyanobacteria and pose significant health and ecological problems. To investigate the impacts of these biotoxins on the physiology of the zebra mussels, Dreissena polymorpha, a series of short-term feeding experiments were conducted in the laboratory. We used five microalgal diets consisting of single-cell suspensions of the green algae, Chlorella vulgaris, the diatom, Asterionella formosa, the cryptophyte, Cryptomonas sp. and two strains of the toxic cyanobacterium, Microcystis aeruginosa (strains CCAP 1450/06 and CCAP 1450/10). A sixth diet was a mixture of the diatom and the CCAP 1450/10 cyanobacterial strain. The low-toxicity strain CCAP 1450/06 contained 7.4 μg l−1 of the MC-LR variant while the very toxic strain CCAP 1450/10 contained 23.8 μg l−1 of MC-LR and 82.9 μg l−1 of MC-LF. A flow-through system was designed to measure the following feeding parameters: clearance, filtration, ingestion and absorption rates. Ultimately the scope for growth (SFG) was determined as a net energy balance. We observed that mussels cleared the cyanobacterial species containing MC-LF (mean ± 95% confidence interval) at a significant lower rate (498 ± 82 ml h−1 g−1 for the single cell suspension and 663 ± 100 ml h−1 g−1 for the mixture diet) than all of the non-toxic species and the cyanobacterium containing MC-LR (all above 1 l h−1 g−1). The same pattern was observed with all the feeding parameters, particularly absorption rates. Furthermore, MC-LF caused an acute irritant response manifested by the production of ‘pseudodiarrhoea’, unusually fluid pseudofaeces, rich in mucus and MC-LF-producing Microcystis cells, ejected through the pedal gape of the mussels. This overall response therefore demonstrates selective rejection of MC-LF-producing cyanobacteria by zebra mussels, enhancing the presence of the very toxic MC-LF-producing M. aeruginosa in mixed cyanobacterial blooms and in the benthos.Finally, we observed that the SFG (mean ± 95% confidence interval) of mussels feeding on M. aeruginosa containing MC-LF was significantly lower (34.0 ± 18.8 J h−1 g−1 for the single cell suspension and 83.1 ± 53.0 J h−1 g−1 for the mixture diet) than for mussels ingesting non-toxic diets, except for C. vulgaris (all above 200 J h−1 g−1). This reveals a sublethal, stressful effect of microcystins (particularly MC-LF) on the feeding behaviour and energy balance of the zebra mussel.