How to cross the sea: testing the dispersal mechanisms of the cosmopolitan earthworm Pontodrilus litoralis

Dispersal capability often decides the geographical distribution and long-term success of a species. In this investigation, Pontodrilus litoralis, a widely distributed species along shores throughout mid- and low latitudes of the world, was investigated. We tested three hypotheses explaining its dispersal: helped by humans, transported by birds and carried by currents. Although the earthworms seemed to be associated with artificially planted wind-breaking woods and mangroves along the west coast of Taiwan, they were also found on isolated beaches in the Pescadores Islands without such plantings. They are approximately 2 mm wide, making them too small for use as fishing bait. These two mechanisms invoking human help were not supported. In a laboratory experiment, we moved the earthworms to the plumage of various body parts of pigeons, and they dropped off or died within a short time, a result incompatible with the bird hypothesis. The earthworms stayed alive and grew when immersed in freshwater or seawater for at least a month. They also survived on floating wood in an in situ experiment lasting approximately two months. Thus, the current hypothesis was the only one we were unable to falsify; driftwood and perhaps wooden vessels could provide both food and transport on long journeys. Wood boats exist for a short time on an evolutionary time scale, but it may be long enough to disperse the earthworm around the world. The phase-out of wood boats, thus, may start the divergence of P. litoralis populations around the world.