The ability to get everywhere: dispersal modes of free-living, aquatic nematodes

Nematodes colonize almost all aquatic habitats worldwide. Despite their small size, restricted locomotion and lack of pelagic larvae, they can reach even isolated habitats within a short time. In this review, we examine the underlying dispersal modes, considering their active movement in substrates and water, their drift by water and wind, rafting, zoochory as well as human-mediated vectors. These modes are limited by morphology and habitat structure, ecological factors and especially by hydrodynamics. Active dispersal is effective over short distances, but with increasing water-flow velocity, passive dispersal modes, which enable long-range transfer, become important. In fact, the transport of nematodes over thousands of kilometers via ship water tanks and by hitchhiking on sea turtles has been documented. Overland dispersal vectors include wind and birds whereas rafting enables an aggregated distribution because food is available, and reproduction is possible onboard the rafts. The diversity of possible dispersal modes is high and offers a reasonably chance for gravid females or groups of nematodes to be transferred even to remote environments. Their immigration is continuous, and supported by their rapid, parthenogenetic reproduction, nematodes are effective pioneers with the ability to (re)colonize new or disturbed habitats or rebalance already existing communities.