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Nematode worms (Nematoda)

Taxonomic group

Phylum: Nematoda

Diagnostic features

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.

Typical habitats

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.

Indicator value

  • Hard bottom: 3
  • Soft bottom: 3.1

The tolerance values (ranging from 0 to 10) give an indication of which are the sensitive taxa (values of 8 or more) and which are the tolerant taxa (values of 3 or less). For more information see: Indicator species

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).