Key role of free-living nematodes in the marine ecosystem.

Nematodes are the most abundant metazoans on Earth. Estimates of global nematode species diversity vary widely, but currently seem to agree that there are about one million species, a number that is comparable to what is reported for other very diverse animal phyla. Nematodes are the dominant taxon in marine sediments in terms of both abundance and biomass. However, the great importance of this group is not only related to these characteristics, but also to its close relationship with the other organisms playing a key role in trophic webs. Theeffects of nematodes on sediment stability are similarly crucial. The relative stability of populations, the short turnover rate, and the generallyhigh tolerance to ecosystem alterations make Nematoda particularly suitable as bioindicators, a benefit that is enhanced because collection methods and sample treatment for these species are relatively simple. For this reason this phylum has been used as an indicator for assessing the ecological quality of marine ecosystems according to the Water Framework Directive (WFD, Directive 2000/60/EC). In addition, the high taxonomic diversity of nematodes is also related to their large trophic spectrum and the variety of their buccal structures. The latter, and other traits such as body and tail shape and life strategy, are easily recognizable, meaning that nematodes can be distinguished on both a morphological and functional basis. Functional diversity is an important component of biodiversity, but if compared to taxonomic diversity, it is still poorly applied. Nevertheless it could be very promising when it comes to improving our knowledge of ecosystems and relationship between the diversity patterns of nematodes and the abiotic environment. Recent studies have suggested that biodiversity loss might impair the functioning and sustainability of ecosystems. Accordingly, monitoring the quality of the environment appears to be essential for devising effective protection strategies and appropriate forms of management. In this respect, the benthic environment is the best domain and nematodes the ideal candidates. The aim of this paper, therefore, is to review past and present advances in the use of nematodes as bioindicators for the monitoring of marine ecosystems, and to highlight future perspectives.