Oligochaeta

Oligochaetes are Annelida, bilaterally symmetrical segmented worms with four bundles of chaetae on every segment, except the first one1. In streams, the size varies from minute worms less than 1 mm (Class Aphanoneura: Family Aeolosomatidae) and the Familiy Naididae (Class Oligochaeta) to app. 400 mm (Class Oligochaeta: Family Haplotaxidae). There are records of larger worms in other type of habitats.

• Oligochaetes are coelomates, with a mouth, located under the head. They have a simple, straight digestive tract with a pharynx, oesophagus, and an intestine, terminating in an anal opening1. They exhibit asexual or sexual reproduction depending on the family.
• In streams and rivers, the most overlooked meiofaunal-sized worms are the genus Aphanoneura (Family Aeolosomatidae) and species of the families Naididae and Enchytraedidae, all smaller than 1 mm. The bodies of these oligochaetes are delicate and often disappear when using fixatives such as alcohol or formaline. Well studied are the families Tubificidae that are found in soft sediments, rich in organic matter, and Lumbriculidae because of their larger size.
• In some streams, meiofaunal-sized oligochaetes (≤ 500 μm) are amongst the most productive taxa within the meiofauna and are about 75% as productive as all the macrofaunal sized larval insects together2
• Species identification of almost all oligochaetes is based on the arrangement and shape of hair chaetae (Aelosomatidae, Naididae, Enchytraedidae) which have to be examined from dorsal and ventral bundles across various regions of the body. Additionally, within the familiy Enchytraedidae, the location of the gonads is used widely taxonomically, which means the examination of mature specimens.