Nematodes are unsegmented worms with elongated, cylindrical bodies; they are present worldwide in fresh, brackish, and salt waters, and in soil. A freshwater nematode has been defined as “any nematode species inhabiting either fresh water or non-brackish swampy soil below the water table; hence a species that will not drown in fresh water; a species fitted to utilize oxygen dissolved in fresh water.”1 Nematodes are a food source for other invertebrates, small fish, and fungi, and play a fundamental role in cycling carbon and nitrogen through the benthic ecosystem. Predaceous nematodes from the Aporcelaimidae, Diplogastridae, Dorylaimidae, and Mononchidae families abound in fresh water, devouring other nematodes, oligochaetes, and other small invertebrates. Their role in ingesting algae and diatoms is less clear; however, algae in the gut can occasionally cause dorylaimids to turn bright amber, yellow, or green.
In recent years, examinations of nematode communities in soil or aquatic sediments have been increasingly used to study many aspects of environmental health.2 The maturity index (MI) originally was developed to measure the ecological succession status of a terrestrial nematode community3–5; the concept was later extended to marine, estuarine, and then freshwater nematodes.6,7 The role of nematodes in wastewater treatment systems is also increasingly being investigated.8 Bactivorous freshwater nematodes can ingest human enteric pathogens, which then can survive chlorination inside nematode bodies. These nematodes often appear in large numbers in secondary wastewater effluents and have been used as bioindicators of water quality.9