Oligochaeta

(Earthworms)

Phylum Annelida

Subphylum Clitellata

Class Oligochaeta

Number of families 17

Thumbnail description
Terrestrial worms that typically dwell in soil and that are characterized by a "tube within a tube" construction, with an outer muscular body wall surrounding a digestive tract that begins with the mouth in the first segment and ends with the anus in the last segment

Evolution and systematics

Earthworms belong to a well-defined clade, the Clitellata, which includes leeches, branchiobdellids, many aquatic and small terrestrial worms with a single cell-layered clitellum, and the earthworms, most of which have a multi-layered clitellum. However, earthworms as a group lack a defining characteristic unique to earthworms. This is because they include the Moniligastridae, a south and east Asian earthworm family, which have a single-layered clitellum and prosoporous (male genital openings in front of the female genital openings). All other earthworms have a multi-layered clitellum and male genital openings behind the female pores (opisthoporous) and are called the Crassiclitellata. As soft-bodied invertebrates, earthworms lack a fossil record, other than burrow traces that may or may not have been created by earthworms. Their phylogenetic relationships have been a matter of controversy since the early twentieth century. Based on analysis of DNA sequence data, Jamieson et al. (2002) concluded that the large family Megascolecidae (in the broad sense, including the Acanthodrilidae and Octochaetidae, of some authors) is the sistergroup of the Ocnerodrilidae, and that these in turn are together the sister-group of a clade composed of several families: Sparganophilidae, Komarekionidae, Almidae, Lutodrilidae, Hormogastridae, Lumbricidae, and Microchaetidae. The remaining two numerically important families, Glossoscolecidae and Eudrilidae, form a third major clade of Crassiclitellata, but relationships to the other two were not clear. Several small families, plus the Moniligastridae, were not included in the analysis. These families complete the family list of the Crassiclitellata: Ailoscolecidae, Alluroididae, Biwadrilidae, Diporochaetidae, and Kynotidae. Overall, there are 17 families, one order, and more than 4,000 species.

Physical characteristics

Earthworms have a "tube within a tube" construction, an outer muscular body wall surrounding a digestive tract that begins with the mouth in the first segment and ends with the anus in the last segment. Body wall musculature consists of an outer circular layer and an inner longitudinal layer, which respectively extend and shorten the body. Between the body wall and the gut is the body cavity, within which various other organs are arranged, generally segmentally. Segments are repeated units of the body, externally manifested as rings and internally separated by septa. In earthworms, each segment except the first bears setae, small chitinous bristles used for traction in the burrow.

A typical earthworm gut consists of the mouth, a muscular pharynx for taking in food, a gizzard for reducing food particles to smaller sizes, an esophagus, and an intestine. In the family Lumbricidae, the gizzard is located after the esophagus, just prior to the expansion of the intestine. Intestinal gizzards have evolved independently in other families and genera. The esophageal wall may secrete digestive enzymes, and in some earthworms, parts of the esophagus are modified as glands for the secretion of calcium carbonate into the gut contents. The intestine may be differentiated into digestive and absorptive regions, and often has a dorsal in-folding of the intestinal wall, called the typhlosole.

Small excretory organs, the nephridia, are arranged segmentally, from two per segment to many small nephridia per segment. Urine is excreted through nephropores to the out-side, or is collected via systems of tubules and excreted into the intestine. In some families, nephridia of the anterior segments have been modified as glands for digestive secretions.

Earthworms are hermaphrodites. Reproductive organs are located in the anterior segments. The female reproductive system consists of paired ovaries in the 13th segment, ovarian funnels leading from the ovaries to an external female genital pore on the 14th segment, and depending on the family, there may be sperm receptacles called spermathecae. If present, these will generally be in some of segments 5–10. Spermathecae receive sperm from the mate during copulation. Alternatively, sperm may be deposited in packets called spermatophores, which will be found clinging to the exterior of the worm. The clitellum provides an outer casing for the ova and also secretes food used by the developing embryo.

Male organs consist of testes in one or both of segments 10 and 11, testicular funnels leading to sperm ducts through which sperm passes to the male genital openings, seminal vesicles in segments adjacent to the testicular segments (one or more of segments nine, 11, 12), and in some families, prostate glands that secrete fluids associated with the male genital pores. In other families, there are often glands associated with setae modified for use in copulation.

Distribution

Earthworms are globally distributed, but do not occur in deserts or regions where there is permafrost or permanent snow and ice. They may also be absent from the taiga biome and other cold climate vegetation types where soils are strongly acid (pH below 4). It has been shown that during the last 20,000 years, many glaciated areas have lacked the presence of earthworms, but in these and other places where they do not occur naturally, some species have been introduced by human activity. The Megascolecidae have the widest natural distribution, being present on all continents, except Europe. The Glossoscolecidae are confined to tropical South America, Central America, and a few Caribbean islands, while the Eudrilidae are found only in sub-Saharan Africa. The Lumbricidae are mainly in Europe, with a few species native to North America. Australian indigenous species are exclusively megascolecids. A few species have attained global temperate or tropical distributions with human assistance.

Habitat

The typical earthworm habitat is soil, but there are species living in freshwater mud, saltwater shorelines, and in suspended soils of tropical forests. The soil habitat can be divided into litter layer, topsoil, and deeper soil horizons, with different earthworms utilizing each.

Behavior

The three ecological categories of earthworms have very different behavior patterns. The anecic feeding behavior has been described. Their primary escape tactic is to rapidly withdraw into the burrow. Epigeic species crawl or burrow through organic matter deposits and feed on it. They have well-developed escape behavior that includes rapid motions, even the ability to jump and thrash about randomly, and to drop tail segments for the predator. Endogeics have little escape behavior, may just writhe or coil in the hand, and may exude some body cavity fluids. In some instances, these fluids may be noxious. Further details of earthworm behavior are poorly known, because they inhabit an opaque medium and are shy of light.

Feeding ecology and diet

Earthworms feed on dead and decomposing organic material such as fallen leaves, decaying roots, and soil organic matter. Epigeic worms are those feeding at or near the surface, or within accumulations of organic matter on or above the soil surface (e.g., logs, epiphyte root mats in trees, etc.). These will consume relatively freshly dead plant matter, as do anecic worms. Anecic earthworms maintain a deep burrow from which they emerge to ingest plant matter from the soil surface; the best known is the European night crawler, Lumbricus terrestris. Endogeic worms operate deeper in the soil and utilize organic matter that has already been somewhat or extensively modified from its original condition. Body size, coloration, and gut morphology are consistently different among these three categories. Epigeics are typically small, darkly colored, and have little secondary development of gut surface area. Anecics are large, colored only in the head, and have gut morphology similar to epigeics. Endogeic worms may be small or very large, but are usually un-pigmented, and show the greatest degree of gut surface area development.

Reproductive biology

Most earthworms are simultaneous hermaphrodites and exchange sperm during copulation. Sperm transfer may be external, in which the seminal fluid flows from male genital openings to the spermathecae, or there may be penis-like organs to insert the seminal fluid directly into the spermathecal openings. Sperm transfer by spermatophores is also known to occur. After copulation, fertilization takes place in the egg case. The case, or cocoon, is formed by the clitelum and passes over the female pores to receive one or more ova. It is then worked forward over the spermathecal pores, from which sperm are expelled into the case, and fertilization results. The cocoon is deposited in the soil or other substrate. The developing embryo feeds on clitellar and/or prostatic secretions, passes through larval stages, and emerges as a miniature earthworm. Growth and maturation may take months or years, depending on the species. In temperate zones, mating and cocoon deposition generally take place in the spring, with a secondary period possible in the autumn. In tropical areas, the peak of activity occurs during rainy seasons. However, the details of mating seasons in tropical earthworms are poorly known.

Some species of earthworms are clonal and reproduce by parthenogenesis. In this case, a diploid ovum is produced that is a genetic copy of the parent. No fertilization is necessary, so a single individual can reproduce unaided. This is important among the many species that have attained wide artificial distributions. In other instances, hermaphroditic species have been observed self-fertilizing. It is not known how common this is, or under what circumstances an individual may choose this course.

Conservation status

The 2002 IUCN Red List includes six species of earthworms; four are categorized as Vulnerable, one as Lower Risk/Near Threatened, and one (Phallodrilus macmasterae) as Critically Endangered. Only one is clearly protected, the Gippsland giant worm of Australia (Megascolides australis), which the IUCN classifies as Vulnerable. It has a very narrow range. Driloleirus macelfreshi, a giant worm from western Oregon in the United States, is suspected to be extinct, although the IUCN classifies it as Vulnerable. It is quite probable that many species are extinct because of habitat destruction, particularly in mountainous regions where the topography and earthworms' low dispersal rates contribute to high species diversity and small species ranges.

Significance to humans

Several species of earthworms (Eisenia fetida, E. andrei, Eudrilus eugeniae, and Perionyx excavatus) are used for production of vermicompost; some of these are used for fish bait as well. The use of earthworms as fish bait seems to be almost universal, and people use whatever worms they can find for this purpose. There are a few species commercially harvested and sold for bait: Lumbricus terrestris (Canada, northern United States), Diplocardia riparia (south-central United States), and D. missippiensis (Florida). As transformers of soil structure and organic matter, earthworms are significant to the maintenance and improvement of soils and plants growing in them, and thereby to humans who benefit from those plants.

Species accounts

List of Species

African worm

Eudrilus eugeniae

order

Haplotaxida

family

Eudrilidae

taxonomy

Eudrilus eugeniae Kinberg, 1867, St. Helena Island.

other common names

None known.

physical characteristics

Dark mauve or pink throughout; 3.1–4.7 in (8–12 cm) length; clitellum covering segments 14–18, prominent widely placed female pores on segment 14, male pores on segments 17–18; eight setae per segment in four pairs.

distribution

Sub-Saharan West Africa origin, now pantropical in rich organic microhabitats; cultured in earthworm farms globally.

habitat

Naturally occurring in savanna and forests; does well in vermicomposting situations.

behavior

Little is known apart from observations of seasonality of activity.

feeding ecology and diet

Cultured in pure organic media; in nature, produces piles of fecal pellets (castings) during rainy season.

reproductive biology

An outcrossing hermaphrodite; high enough reproductive rate to make it economically useful.

conservation status

Not listed by the IUCN. Status in native range not known; probably stable. Now invasive in many tropical areas.

significance to humans

Cultured for fish bait, pet food, and the compost it produces. Large numbers are grown annually in the United States, mainly in southern states.

Common field worm

Aporrectodea caliginosa

order

Haplotaxida

family

Lumbricidae

taxonomy

Aporrectodea caliginosa Savigny, 1826, France.

other common names

None known.

physical characteristics

Slightly dusky pigmentation in the head segments, approximately 1.9–3.5 in (5–9 cm) length, with a clitellum (thickened band) covering segments 27–34, with small paired markings ventral side of segments 30, 32–34.

distribution

Western Europe, with peregrine distribution in New Zealand, northeast Asia, United States, and Canada and temperate South America, South Africa, and Australia. (Specific distribution map not available.)

habitat

Frequently found in human-influenced habitats, including arable land, pastures, and forests.

behavior

In warmer temperate areas, it undergoes a temperature-induced summer dormancy, so that its peak activity periods are in the spring and autumn. Activity also requires moist soil conditions.

feeding ecology and diet

Flexible; may be found active to depths of 7.8 in (20 cm) or near the surface under litter or other decomposing organic material; seldom in purely organic substrates.

reproductive biology

It is known to mate; most likely an outcrossing hermaphrodite. Reproduction is in the spring, with a secondary peak in the autumn.

conservation status

Not listed by the IUCN. Abundant where it occurs, and may be considered an invasive exotic throughout much of its artificially acquired range.

significance to humans

Capable of persisting under agricultural tillage conditions, it may be of some importance in creating soil properties favorable to plant growth.

No common name

Amynthas corticis

order

Haplotaxida

family

Megascolecidae

taxonomy

Amynthas corticis Kinberg, 1867, Oahu, Hawaii, United States.

other common names

None known.

physical characteristics

Red to reddish brown dorsal pigmentation, lighter at segmental equators; 1.9–5.9 in (5–15 cm) length; clitellum on segments 14–16, single female pore ventral on segment 14, small paired male pores ventral side of segment 18, spermathecal pores at furrows 5/6/7/8/9, small oval discs ventrally in segments 6–9; setae in continuous rings of 10–50 per segment.

distribution

Subtropical China, with global warm temperate to cooler tropical peregrine distribution. (Specific distribution map not available.)

habitat

Frequently found in human-influenced habitats, including arable land, pastures, and forests.

behavior

Capable of violent thrashing movements and jumping, may autotomize tail segments.

feeding ecology and diet

Flexible; may be found active to depths of 7.8 in (20 cm) or near the surface under litter or other decomposing organic material.

reproductive biology

All known populations are probably asexual parthenogenetic morphs, a factor in its very wide artificial distribution.

conservation status

Not listed by the IUCN. An invasive exotic throughout its artificially acquired range. The original homeland is not known, but may be in China.

significance to humans

Some attempts have been made to culture it for bait, and it has been blamed for damage in greenhouses. Like other peregrine species, it is able to tolerate disturbed soil conditions, so may be of some value if populations do not become too large.

River worm

Diplocardia riparia

order

Haplotaxida

family

Megascolecidae (Acanthodrilinae)

taxonomy

Diplocardia riparia Smith, 1895, Illinois, United States.

other common names

None known.

physical characteristics

Dark brown anterior dorsal pigmentation; 4.7–7.8 in (12–20 cm) length; clitellum covering segments 13–18, with small paired longitudinal grooves ventrally on segments 18–20.

distribution

Central United States, in Iowa, Illinois, Missouri, Kansas, and Nebraska.

habitat

Riparian forests and fine-textured alluvial soils of river banks; said to be most common under silver maple stands.

behavior

Nothing is known.

feeding ecology and diet

Probably anecic or endogeic, feeding on organic debris buried in riverbank sediments.

reproductive biology

Because specimens show evidence of sperm production, it is an outcrossing hermaphrodite.

conservation status

Not listed by the IUCN. Locally abundant in banks of low-gradient streams in the central United States, but invasive species brought in as fish bait may threaten its populations in some areas.

significance to humans

Collected from natural populations and sold for fish bait. It is better able to withstand summer heat than other common bait species.

Gippsland giant worm

Megascolides australis

order

Haplotaxida

family

Megascolecidae

taxonomy

Megascolides australis McCoy, 1878, Australia.

other common names

Aboriginal: Karmai.

physical characteristics

Dark purple anterior pigmentation; approximately 31.5–39.3 in (80–100 cm) length; clitellum covering segments 27–34, with small paired markings ventral side of segments 30, 32–34.

distribution

Limited to the Bass River Valley, Victoria, Australia.

habitat

In clay soils along watercourses and other moderately wet places.

behavior

The entire life is spent underground, including feeding, mating, and waste deposition.

feeding ecology and diet

Forms deep burrows reaching to the water table; feeds on organic matter in soil.

reproductive biology

Egg cases are deposited underground, and embryos develop for 12–14 months, emerging at 7.8 in (20 cm) length.

conservation status

Listed as Vulnerable by the IUCN. Has a very narrow range and is threatened by land and water use practices in the region.

significance to humans

It is of no economic importance, other than as a wonder of nature with some tourism drawing power. There is a Giant Worm Museum in Bass and a worm festival in the town of Koramburra.

No common name

Pontoscolex corethrurus

order

Haplotaxida

family

Glossoscolecidae

taxonomy

Pontoscolex corethrurus Müller, 1856, Brazil.

other common names

English: Brushy-tail.

physical characteristics

Un-pigmented, light pink in head, posterior section color depends on ingested soil; approximately 1.9–3.9 in (5–10 cm) length; clitellum covering segments 13–22; setae of tail enlarged, located in alternate positions between segments, either irregular or in 16 rows, but only eight per segment.

distribution

Originally from northeast South America, with pantropical peregrine distribution in disturbed habitats. (Specific distribution may not available.)

habitat

Frequently found in human-influenced habitats, including arable land, pastures, and second-growth forests; from low elevations to tropical montane cloud forests.

behavior

When exploring, it extends a tentacle-like proboscis. Goes dormant when soils dry out.

feeding ecology and diet

Endogeic, flexible; may be found active to depths of 7.8 in (20 cm) or near the surface under litter or other decomposing organic material; able to utilize low-quality organic matter.

reproductive biology

Possibly parthenogenetic, can reproduce as long as soil conditions permit.

conservation status

Not listed by the IUCN. Abundant where it occurs, and may be considered an invasive exotic throughout much of its artificially acquired range.

significance to humans

As one of the few tropical earthworms capable of persisting under agricultural tillage conditions, it may be of some importance in creating soil properties favorable to plant growth, but it has been implicated in soil structure breakdown when population density becomes high.

Resources

Books

Edwards, C. A., and P. J. Bohlen. Biology and Ecology of Earthworms. 3rd edition. New York: Chapman and Hall, 1996.

Lee, K. E. Earthworms, Their Ecology and Relationships with Soils and Land Use. Sydney, Australia: Academic Press, 1985.

Stephenson, J. The Oligochaeta. Oxford, U.K.: Clarendon Press, 1930.

Periodicals

Gates, G. E. "Burmese Earthworms. An Introduction to the Systematics and Biology of Megadrile Oligochaetes with Special Reference to Southeast Asia." Transactions of the American Philosophical Society new series 62, no. 7 (1972): 1–326.

Michaelsen, W. "Oligochaeta." Tierreich 10 (1900): 1–575.

Other

Jamieson, B. G. M. Native earthworms of Australia (Megascolecidae, Megascolecinae). PDF Document on CDROM. Enfield, NH: Science Publishers, Inc., 2000.

Samuel Wooster James, PhD