Free-living marine nematodes actively choose habitat when descending from the water column

Benthic organisms living in shallow high-energy areas are regularly suspended into the water column, where they dwell for longer or shorter periods before they re-enter the benthos. Marine free-living nematodes, normally the most abundant metazoans in soft sediments, lack a pelagic larval stage and are exceedingly poor swimmers. Although empirical data are lacking, nematodes are regularly considered to settle as passive particles through the water column. We carried out an experiment to assess whether nematodes are able to choose settling spots when descending from the water column. In the laboratory, nematodes were extracted from sediment collected from a shallow sandy bay and allowed to settle in a water column of 1 m to the bottom, which was seeded with 4 small containers containing: nothing, sediment without organic matter, sediment with added pelagic algae and sediment with added benthic algae (BA). The nematodes showed a clear preference for the BA containers, being about 7 times as abundant there than in the other treatments (p < 0.001). The nematode composition was different among treatments with 3 abundant species more or less confined to BA. The size distribution of the nematodes shows that the smallest nematodes are also the most efficient in choosing habitat. This is in concordance with the prediction that swimming ability is coupled to nematode size and water viscosity. The results obtained show that the conception of nematodes as passive particles in the water column is not entirely valid.