Nematode worms are small (mostly less than 3mm long) slender, unsegmented, often smooth and shiny (at low magnification), and often with one sharply-pointed end. They are generally white or semi-transparrent. The Phylum Nematoda contains many thousands of species, and we don’t know how many live in fresh waters.
Some nematodes are free-living and may be common in streambed habitats, while others such as the mermithids are parasitic, living inside larger invertebrates.
Mermithid nematodes parasitise stream invertebrates including caddisfly and midge larvae. Free-living species may feed on detritus, algae, fungi, or small invertebrates.
Nematode worms are found in many habitat types, including streams and ponds with poor water quality. They have low tolerance values of 3 (hard bottom sites) and 3.1 (soft bottom sites).