Unionacean molluscs (naiads)

While walking along a riverbank or skirting the edge of a large pond or lake there is a good chance that we will encounter the shell valves of the swan mussel, Anodonta cygnea (Figure 16.1), most probably left high and dry by changing water levels. On finding these impressive shells, fish parasites could not be further from our minds, but, surprising as it may seem, these large mussels spend a brief period of their early lives as parasites of fishes. Moreover, this parasitic phase is obligatory and these mussels depend entirely on freshwater fishes for their survival.