Functional group definitions in aquatic ecology vary depending on the type of ecosystem (e.g., marine compared to fresh water ecosystems, stream compared to lake ecosystems). Since the benthic environment functions as the major storage and recycling compartment for virtually all material that flows in the aquatic system, biological processes that take place there are interesting models for identification of the different invertebrate functions. The accurate species function includes the effects of an organism on the abiotic as well as the biotic properties of the habitats. Therefore, a functional group may be defined as a group of species that share common biogeochemical and interspecific attributes. The main difficulty of applying this definition in aquatic ecosystems comes from the high diversity of organism functions (compared to terrestrial systems) that potentially exist at different levels: i) interspecific diversity supported by the large morphological and behavioural diversity of organisms, reinforced by a low degree of species redundancy; ii) intraspecific functional diversity due to changes in life history strategy during the life cycle, and enhanced by optimal foraging theory. The major question remains to find accurate functional group sizes and classifications that permit the distinction of the different biological activities involved in ecosystem key processes without missing other biological functions. After reviewing some of the problems in current functional classifications of benthic invertebrates, we propose a classification system based on the mechanical activities that characterize each species rather than consideration of the multiple consequences of these activities. This sorting strategy will result in a sub-classification of classical feeding groups into more precise functional groups. Such groups as bioturbation groups or functional feeding groups may be composed of representative taxa in both marine and freshwater environments
