Free-living nematodes in the freshwater food web: a review

Abstract

Free-living nematodes are well-recognized as an abundant and ubiquitous component of benthic communities in inland waters. Compelling evidence from soil and marine ecosystems has highlighted the importance of nematodes as trophic intermediaries between microbial production and higher trophic levels. However, the paucity of empirical evidence of their role in freshwater ecosystems has hampered their inclusion in our understanding of freshwater food web functioning. This literature survey provides an overview of research efforts in the field of freshwater nematode ecology and of the complex trophic interactions between free-living nematodes and microbes, other meiofauna, macro-invertebrates, and fishes. Based on an analysis of the relevant literature and an appreciation of the potential of emerging approaches for the evaluation of nematode trophic ecology, we point out research gaps and recommend relevant directions for further research. The latter include (i) interactions of nematodes with protozoans and fungi; (ii) nonconsumptive effects of nematodes on microbial activity and the effects of nematodes on associated key ecosystem processes (decomposition, primary production); and (iii) the feeding selectivity and intraspecific feeding variability of nematodes and their potential impacts on the structure of benthic communities.

Keywords: algae; bacteria; ecology; fish; food web; free-living; freshwater; fungi; interaction; macrofauna; meiofauna; method; organic matter; predation; protozoa; selectivity.

Fig. 1

Trophic interactions involving nematodes in freshwater ecosystems based on their frequencies of occurrence in literature. Arrow size is relative to the number of trophic interactions directly or indirectly evidenced in the relevant literature (93 studies; listed in Table 1). Arrow trajectory shows the trajectory of energy. Scale bar for metazoans is 1 mm. Meiofauna include permanent meiofaunal taxa, such as rotifers, tardigrades, water mites, harpacticoid copepods, and oligochaetes; as well as temporary meiofauna, such as larvae of chironomids. Macrofauna include gastropods, flatworms, and shrimps; Vertebrates include fishes and amphibian larvae. Macrophytes also include bryophytes.

Fig. 2

(A) Yearly publication rate of studies considering broad aspects of free-living freshwater nematodes versus studies highlighting their trophic interactions, from 1991 to 2013. (B) Global distribution of study/collection sites where evidence of trophic interactions involving free-living freshwater nematodes has been obtained (collection/study sites listed in Table 1). Some sites have been investigated repeatedly (N > 3 different studies): Lake Erken (E), Lake Constance (C), Garonne River (G), and Movile Cave (M).

Fig. 3

(A) Scheme of detritus-pellet formation by a Chromadoridae nematode. After adherence of the nematode’s tail to the substrate via caudal secretions, the surrounding particles are collected by oscillation movements and further agglutinated into a pellet (after Meschkat 1934; modified in Riemann and Helmke 2002). (B) Anterior part of a living Chromadorina bioculata retrieved from epilithic biofilms of the Garonne River (France). The white arrow shows the constriction of the nematode cuticle due to the attachment of secretions. A diatom cell and detrital particle are trapped in secretions. Bacterial density is high in the pellet and around the trapped diatom. Scale bar is 50 µm.