Reduction of cyanobacterial toxins through coprophagy in Mytilus edulis

An experiment was conducted to follow the fate of the cyanobacterial toxin, nodularin, produced by Nodularia spumigena through ingestion by Mytilus edulis and re-ingestion of faecal material (coprophagy). Mussels were fed with cultures of N. spumigena, and the faeces that were produced were fed to other mussels not previously exposed to N. spumigena. Concentrations of nodularin were measured in the food (N. spumigena), the mussels and in the faeces in order to make a toxin budget. High concentrations of nodularin were found in the mussels and their faeces after 48 h incubation with N. spumigena. When the toxic faeces were fed to new mussels, the toxin content of faeces was reduced from 95 μg nod g−1 dry weight (DW) to 1 μg nod g−1 DW through the process of coprophagy. Hence, when toxic faeces were fed to mussels, the nodularin concentration of the resulting faecal material was reduced by 99%. Pseudofaeces were produced when the mussels were grazing on N. spumigena, but not when grazing on faeces. The pseudofaeces contained high concentrations of nodularin and apparently intact N. spumigena cells. However, these cells were growth-inhibited and their potential contribution to seeding a bloom is probably limited. Our data indicate that a large fraction of ingested nodularin in M. edulis is egested with the faeces, and that the concentration of nodularin in the faeces is reduced when faeces are re-ingested.