Active and species-specific dispersal behaviour in a marine nematode cryptic species complex

Dispersal is a 3-step process, consisting of a departure, transience and settlement step. In marine nematodes, dispersal is expected to be passive because nematodes lack an active planktonic phase. The transoceanic distribution of several species suggests effective large-scale dispersal. Nevertheless, it remains unclear whether nematodes can actively influence their dispersal and whether species-specific differences exist. Such differences could contribute to the commonly observed co-existence of ecologically similar species. Three separate experiments were conducted on 4 species of the cryptic species complex of Litoditis marina (Pm I, Pm II, Pm III and Pm IV) to test the effect of species identity and behaviour in the dispersal steps. A flume experiment demonstrated that nematode emergence increased with increasing flow speed and that they could also actively emerge from decomposing algae (departure step). Movement analysis of nematodes in seawater highlighted behaviour which contributes to the transience step. The settlement experiment demonstrated that L. marina can choose on which substratum to settle in still water. Species-specific differences were found in the departure and transience steps. In the departure step, Pm IV exhibited more active behaviour than Pm I. In the transience step, Pm II and Pm IV showed the highest body bend frequency. These species-specific differences correspond with geographical distribution patterns, which are smaller in the species with weaker dispersal behaviour, and lend support to the idea that dispersal can facilitate coexistence of closely related species through a trade-off between competition and dispersal.