Colonization of macroalgal deposits by estuarine nematodes through air and potential for rafting inside algal structures

Our study investigated nematode dispersal through air and potential for rafting by a various set up of field experiments conducted in the Western-Scheldt estuary. In experiment I, we assessed differences in relative abundance of nematodes colonizing algae in boxes on high poles through air by wind transport ("open boxes") and by exclusion of invertebrate vectors ("gauzed boxes"). In experiment II, we investigating the importance of nematode colonization through air ("air only") versus through air and tidal currents ("air+sea", lower boxes). In experiment III, we investigated the difference in relative abundances of nematodes inside and outside macroalgal structures of Fucus sp., namely floating bladders and receptacula; using qPCR for nematode Litiditis marina species on a cryptic level if present.

Dispersal is an important life-history trait. In marine meiofauna, and particularly in nematodes, dispersal is generally considered to be mainly passive, i.e. through transport with water currents after erosion from the sediment or from algal substrata. Because nematodes have no larval dispersal stage and have a poor swimming ability, their dispersal capacity is expected to be limited. Nevertheless, many marine nematode genera and even species have near-cosmopolitan distributions, and at much smaller spatial scales, can rapidly colonise new habitat patches. Here we demonstrate that certain marine nematodes, like the morphospecies Litoditis marina, can hide inside macroalgal structures such as receptacula (especially PM I L. marina cryptic species) and, to a lesser extent, floating bladders, which may allow them to raft over large distances with drifting macroalgae. We also demonstrate for the first time that these nematodes can colonize new habitat patches, such as newly deposited macroalgal wrack in the intertidal, not only through water but also through air. Our experimental set-up demonstrates that this aerial transport is probably the result of hitchhiking on vectors such as insects, which visit and move between the patches of deposited algae. Transport by wind, which has been observed for terrestrial nematodes and freshwater zooplankton, could not be demonstrated. These results can be important for our understanding of both large-scale geographic distribution patterns and of the small-scale colonization dynamics of habitat patches by marine nematodes.