First observation of a spawning mantle display in a European unionid mussel.

Freshwater bivalve mussels in the order Unionida are highly endangered ecosystem engineers with a parasitic lifecycle necessitating a fish host to metamorphose from larval glochidia to juvenile mussel. While many species are broadcast spawners and release a large number of glochidia into the water column, many other species have a variety of highly evolved lure mechanisms and mantle displays to attract hosts to ensure a more targeted infestation. Almost all lure mussels are found exclusively in North America, with only one European species ( Unio crassus ) occasionally displaying a host attraction behaviour referred to as larval spurting . Here, I present evidence that the depressed river mussel ( Pseudanodonta complanata ) exhibits mantle displays to attract fish to gravid mussels for a targeted infestation, the first description of mantle displays in Europe.