Oligochaeta
Introduction
About 3100 species worldwide of primary freshwater annelids,
but most of the species are secondarily terrestrial, some are secondarily marine.
Most freshwater secies are much smaller than the familar earthwoms.
Recognition
Annelids. More or less similar to the familiar earthworm.
Oligochaets lack the parapodia of the polychaets, they lack the suckers of the
Brachiobdella, Hirudinea, and Acanthobdellida, they normally have 4 bundles of setae per
segment, but lack the ventral hair setae and the ciliated prostomium of the
the Aphanoneura.
Life Cycle
Hermaphroditic with mutual copulation resulting in cross-fertilization.
A few are parthenogenetic.
Breeding may be seasonal or occur throughout much of the year, and it may occur
only once or multiple times.
They use the clitellum to secrete a cocoon around the eggs.
The trochophore larva is passed in the cocoon and miniature adults emerge upon hatching.
Many of the smaller species (Naididae) reproduce mostly by asexual budding.
Life cycles may be univoltine, multivoltine, semivoltine or variable.
The life cycle is non-emergent.
Trophic Roles
Usually collectors, feeding on the dead organic material and bacteria in the sediment.
A few are commensal to parasitic or even more predatory.
Communities
Moderately complex in fine sediments of both lentic and lotic water.
The species are separated by food size, biotic factors (competitors and predators),
and by water and sediment properties.
Many of the smaller species occur more on solid substrates, a few live on other animals.
Some build tubes. Some occur in deep water.
Some species increase in enriched waters with reduced oxygen, and some species
tolerate anaerobic conditions for part of the year.
Many species belong to the meiofauna.
Some are hyporheic, some interstitial and some restricted to groundwaters.
Some normally terrestrial species may occur in shallow, cold, well oxygenated water.
Economic Importance
Oligochaetes are important primary consumers in many systems, where they
are important in the early links of some food chains.
The importance of these soft-bodied worms in diets is often underestimated.
Some are used as commercial fish food.
They are often largely responsible for the bioturbation of lake sediments and where
they can affect nutrient dynamics of the system.
Some are good environmental indicators, especially the Tubificidae.
Some are secondary hosts for important parasites,
including salmonid whirling disease.
Zoogeography
The freshwater species are often more widespread than terrestrial species.
Many species are cosmopolitan, most are widespread.
Man has moved some species between the continents.
The cocoon stage and encysted worms may be transported overland, often aided
by larger animals. Many areas are not yet well collected.
Collecting
The larger species may be taken in routine benthic samples, but they are often
underrepresented and not well preserved in such.
The best oligochaete samples are just specially processed sediment samples.
The oligochaete sample can be quantitative only if the sediment sample is also quantitative.
Larger specimens may be concentrated by field seiving.
If the seiving is brief, and the sample kept in motion the worms will ball-up and
not pass through a seive that they could easily burrow through.
Most field samples should at least have the couse debries and heavy sand and gravel
removed in the field.
Preservation
The best samples have the oligochaetes mostly killed by poisoning thier water with
ethanol before fixing with Kahle's fluid, Bouin's fluid or 10% formalin.
This takes only a few minutes as the delicate worms are killed much more quickly
than insects.
The worms may deteriorate if too much time passes before fixing.
Oligochaetes take rose bengal more strongly than nearly anything else.
This can aid in sorting, but it may interfer with identification.
In many cases, unstained odigochaete samples more easily sorted against a black background.
Identification requires slide mounts.
The best mounts require alcohol series dehydration and clearing in xylene before
mounting in resin.
Minimizing the length of the xylene bath keeps the worms from getting brittle.
Taxonomy
Fairy well studied because of the limited size of the group and wide distribution
of many species.
The higher classification of many groups remains unsettled.
Many keys are highly artificial to avoid the difficult internal anatomy characters that
define the subfamilies, tribes and genera.
Primarily a specialist's group at the species level.
Key Referencs
- Brinkhurst, R.O. 1986. Guide to the freshwater aquatic microdrile oligochaetes
of North America. Can. Spec. Publ. Fish. Aquat. Sci. 84: 259 p.
- Klemm, D.J., ed. 1985. A guide to the freshwater Annelida (Polychaeta, naidid and
tubificid Oligochaeta, and Hirudinea) of North America. Kendall/Hunt, Dubuque, Iowa 198 p.
- Brinkhurst, R.O. and B.G.M. Jamieson. 1971. Aquatic Oligochaeta of the world.
Oliver & Boyd, Edinburgh, 860 pp.
Some changes after Pennak
- Aeolosomatidae (Aeolosoma) was removed from the Oligochaeta and placed in the new class Aphanoneura, which arose between the basal Polychaeta and the remaining 4 annelid classes, which are phlogenetically unresolved.
Synopsis Of Major Neartic Families
- Megadriles- 2 simple pointed chaetae per bundle, larger, thick bodied worms with
thick clitellum several segments behind the gonopores, earthworms, but some aquatic.
- Family Lumbricidae- earthworms, the clitellum begins on segment 18 or more posterior,
the intertine has an anterior gizzard.
- Family Sparganophilidae- one genus, Sparganophilus,
sometimes treated as a subfamily of Glossoscolecidae.
The clitellum begins on segment 25 or more anterior,
the intestine lacks an anterior gizzard,
long, slender, unpigmented worms living in mud, often associated with Sparganium.
- Microdriles- Chaetae various, thin-bodied with thin clitellum (1 cell layer) over the
gonopores or further anterior.
- Family Haplotaxidae- ventral chaetae single, elongate thin worms, one genus, Haplotaxis.
- Family Lumbriculidae- 2 chaetae per bundle,
- Family Enchytraeidae- 2 or more chaetae per bundle, all simple pointed and
often straight, cutical thickened, poorly known.
- Family Opistocystidae- 3 or more chaetae per bundle, tail end with 3 processes,
head end with a proboscis, a single North American species in the southeast, under 3 mm.
- Family Naididae- 3 or more chaetae per bundle, transparent worms mostly under 10 mm,
often in chains, 20 genera and 68 species.
- Family Tubificidae- 3 or more chaetae per bundle, redish worms over 10 mm,
19 genera with 56 species.