Hitching a ride and going with the flow – dispersal of unionid mussels (Bivalvia: Unionidae)

Unionid mussels are a highly imperiled group. Little is known about their dispersal, which is a complex process that depends on the transport of larvae (glochidia) and juveniles in the water column, and attached on a host fish. We investigated dispersal abilities of four species that differ in several ecological traits, including their host attraction strategy. Field experiments showed that the generalist (Actinonaias ligamentina), without a host attraction strategy, can broadcast its glochidia considerable distances. A combination of lab experiments and modeling indicated that this is also the case for the generalists’ juveniles. Conversely, dispersal of glochidia and juveniles of the most specialized species (Epioblasma triquetra) that captures its host fish between its valves is significantly reduced. Examination of host fish movements based on a mark-recapture study and a literature review further supported the finding that the generalist (A. ligamentina) has the largest dispersal distances (hundreds of meters to kilometers), while the specialist (E. triquetra) has the shortest ((tens of) meters). The other two species (Lampsilis fasciola and Ptychobranchus fasciolaris) fill positions between these extremes. Our findings show that dispersal distances can differ considerably among mussel species, but are predictable based on their ecology, which is a crucial first step to understand connectivity of populations and to develop sound conservation strategies.