Nematoda

Free-living nematodes are bilaterally symmetric, cylindrical, and non-segmented worms1,2. The external cuticle can bear striations, longitudinal ridges or cuticular grooves. The internal anatomy of a free-living nematode is very complex and its structures are used for identification1,2. There are more than 104 free-living species described2.

• Benthic freshwater nematodes are less than a centimetre long, and usually range between 0.1 to 3 mm.
• Nematodes have female and male individuals but also parthenogenetic or hermaphroditic groups do occur. Females are larger in size and the sexual organs vary in shape and arrangement with family or species level.
• Nematodes have the ability to colonise any kind of freshwater habitat because certain species can tolerate also low oxygen concentrations (2 to 10% oxygen saturation). In the streambed sediments of an alpine gravel stream, 45 nematode species were found reaching densities of more than 300 ind/l, while some sandy streams can harbour more than 100 nematode species2-4.
• Nematoda are represented by species that consume dead plant and animal matter or detritus. Herbivorous species are specialised for biting and chewing living plant tissues, while some nematode genera are predatory (Mononchus, Dorylaimus). Prey items consist of protozoans, small metazoans including oligochaetes, rotifers, gastrotrichs and tardigrades3.
• The internal anatomy plays an important taxonomical role for identification and some characters are based on the shape of the anterior end, shape of oesophagus and the amphids (sense organs). Overall, taxonomically relevant features are based on numeric assessments and many taxonomic characters are difficult to observe. Thus, considerable experience is needed to arrive correctly at the species level1,2.