Annelida: Oligochaeta

Description

Oligochaetes are segmented, bilaterally symmetrical worms with tapering ends. There is an anterior prostomium, followed by the first segment, the peristomium, which carries the ventral mouth. The posterior extremity is the pygidium, with terminal anus. Other segments are similar in form excepting a group of up to 6 segments in the genital region which form a thickened opaque clitellum in sexually mature specimens. Chaetae are typically present in two pairs of bundles per segment. Chaetae include both hair-like capillaries and simple or distally-toothed hooks. Compound chaetae are absent.

Comments

The Oligochaeta includes the terestrial earthworms and a much greater diversity of smaller freshwater, estuarine and marine species. Most marine and freshwater oligochaetes belong to one of 3 families: Naididae, Enchytraeidae or Tubificidae.

Some oligochaetes are easily confused with polychaetes, but oligochaete hooks are distinctive and are never hooded as in many polychaetes including the capitellid polychaetes which they most closely resemble. Oligochaetes are common in benthic samples, and are often dominant in organically enriched sediments if a sieve of 0.3 mm or less is used.

Identification of oligochaetes requires dissection, and, often, use of histology and serial sections to infer anatomy. This is not to be undertaken lightly, and there are no recent identification tools for the Australian fauna. Brinkhurst (1982), though it addresses the British fauna, includes notes on study methods and simply and concisely describes families and genera, many of which are likely to occur in Australian waters.

References

Brinkhurst, RO. 1982. British and other marine and estuarine oligochaetes. Cambridge University Press, Cambridge.

Brinkhurst, RO and Cook, DG (eds) 1980. Aquatic Oligochaete Biology. Plenum Press, New York. ix and 1-529 pp.

Brinkhurst, RO and Nemec, AFL. 1987. A comparison of phenetic and phylogenetic methods applied to the systematics of Oligochaeta. Hydrobiologia 155, 65-74.

Erseus, C. 1993. Taxonomy of Capilloventer (Capilloventridae), a little-known group of aquatic Oligochaeta, with descriptions of two new species. 27, 1029-1040.

Ersèus, C. 1997. The Oligochaeta. In Blake, JA, Hilbig, B and Scott, PH (eds), Taxonomic Atlas of the Benthic Fauna of the Santa Maria Basin and the Western Santa Barbara Channel. The Annelida Part 2 Oligochaeta and Polychaeta: Phyllodocida (Phyllodocidae to Paralacydoniidae). Santa Barbara Museum of Natural History, Santa Barbara, California, pp. 5-36.

Jamieson, BGM. 1988. On the phylogeny and higher classification of the Oligochaeta. Cladistics 4, 367-410.

Sims, RW. 1969. Outline of an application of computer techniques to the problem of the classification of the Megascolecoid earthworms. Pedobiologia 9, 35-41.