4 Meiofauna as a Tool for Marine Ecosystem Biomonitoring Maria Balsamo, Federica Semprucci, Fabrizio Frontalini and Rodolfo Coccioni Department of Earth, Life and Environmental Sciences (DiSTeVA), University of Urbino, Italy 1. Introduction Meiofauna are the metazoan component of the benthos, and also include large protozoans (e.g. foraminifera). They are defined by their body size (44-1000 μm) and are the most diversified element of the marine biota: as many as 24 of the 35 animal phyla have meiobenthic representatives which live in meiofauna, whether for all their life or just temporarily. It is the most abundant benthic group in the marine realm, and is thought to be closely connected to other faunal compartments of the benthic system. The function of meiofauna in marine benthic systems seems to be much more complex than previously supposed, and requires investigation to clarify their ecological importance in the benthic domain (see Balsamo et al., 2010 for review). The aims of this paper are: to review advances in the use of meiofauna as a bio-indicator for the monitoring of marine ecosystems; and to highlight future perspectives of this approach. In particular, the use of the two most abundant and diverse meiofaunal groups (Foraminifera and Nematoda) will be considered. 2. Meiofauna as an environmental bio-indicator in marine ecosystems The use of meiofauna as a biological indicator is a more recent development than the utilization of macrofauna in the assessment and monitoring of aquatic ecosystems (Coull & Chandler, 1992). The advantages of the former are numerous and strongly emphasized by Kennedy & Jacoby (1999), while some of the arguments traditionally advanced against their use underline difficulties in identification, the high rate of sampling frequency and the microscopic size of these organisms. However, new technologies and tools, such as standardized methodologies, electronic identification keys, molecular approaches and the creation of new indices, currently allow for and promote the use of meiofauna in ecological studies (see Giere, 2009, for review). 3. Two representative meiofaunal groups: Foraminifera and Nematoda Among the meiobenthic protozoans, Foraminifera (class Foraminifera, phylum Granuloreticulata) are the most abundant and diverse of the shelled microorganisms in the www.intechopen.com