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

Abstract

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.

Colonization patterns of Nematoda on decomposing algae in the estuarine environment: Community assembly and genetic structure of the dominant species Pellioditis marina

“…Nevertheless, passive dispersal of meiofauna can be substantial. Nematodes, for instance, are frequently observed in the water column, especially in areas with high tidal activity (Ullberg and Olafsson 2003), and appear to be among the most abundant rafting organisms (Thiel and Gutow 2005). Moreover, several meiofaunal taxa have relatively low abundances in the sediment but abound in epiphytic habitats or on macroalgal and other wrack deposits.…”

The clam Macoma balthica prevents in situ growth of microalgal mats: implications for meiofaunal assemblages

“…The fact that the genera that contributed most to dissimilarities between treatments i.e. Cobbia, Theristus and Chromadorita were also found to actively choose the same sort of microalgal mats in a settling experiment (Ullberg & Ólafsson 2003), indicates that their increased abundance is likely to be more a result of their attraction to these algal habitats than enhancement of their population in the absence of M. balthica once settled in the jar. On the other hand, harpacticoids, particularly the phytal and epibenthic ones, are in general very mobile and are often found swimming in the water column.…”

“…Attraction of meiobenthic taxa and particularly nematodes and harpacticoids to microalgal patches is often referred to in the literature and has been suggested to contribute to explain patchiness and distribution of these animals both in the sediment and the water column (Santos et al 1995, Ullberg & Ólafsson 2003. Once in the sediment, animals which are more sediment-bound, such as nematodes or certain harpacticoid species, are likely to be more affected by the presence and varying densities of the clams than those showing more nomadic/ mobile and epibenthic lifestyles.…”

“…We found that this was the case, at least under laboratory conditions, and therefore we were particularly interested in the following questions: (1) do nematode assemblages develop differently depending on the density of the clam? Our earlier studies indicate that this is likely to be so, especially in areas of in situ growth of microalgal mats, as some nematode species are able to choose such habitats when settling from the water column (Ullberg & Ólafsson 2003). (2) Can we expect more mobile taxa such as copepods to develop differently from the nematode assemblages?…”

Saccate thallus of the red alga Halosaccion glandiforme harbor diverse invertebrate fauna

“…Marine roundworms inhabiting high-energy rocky shores are exposed regularly to changing tidal conditions. Being poor swimmers, they often remain suspended in the water column before re-entering the benthos as passive particles when tides recede; yet, nematodes are efficient in choosing algal habitats (Ullberg and Ó lafsson 2003). Nematodes in our algal samples may have entered the saccate thallus via passive or active mechanisms.…”