Aquatic Invertebrates of Montana

Oligochaeta

Introduction

About 3100 species worldwide of primary freshwater annelids, but most of the species are secondarily terrestrial, some are secondarily marine. Most freshwater species are much smaller than the familar earthwoms. The list of species in Montana can be found here.

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.