Oligochaeta

(invertebrate zoology) A class of the phylum Annelida including worms that exhibit both external and internal segmentation, and setae which are not borne on parapodia.

(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

Resources

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.

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.

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

[Article by: Samuel Wooster James, PhD]

A class of the phylum Annelida including worms such as the earthworms. There are 21 families with over 3000 species. These animals exhibit both external and internal segmentation. They usually possess setae which are not borne on parapodia. Oligochaetes are hermaphroditic. The gonads are few in number and situated in the anterior part of the body, the male gonads being anterior to the female gonads. The gametes are discharged through special ducts, the oviducts and sperm ducts. A clitellum is present at maturity. There is no larval stage during development.

The oligochaetes are primarily fresh-water and burrowing terrestrial animals. A few are marine and several species occur in the intertidal zone.

Oligochaetes are cylindrical, elongated animals with the anterior mouth usually overhung by a fleshy lobe, the prostomium, and the anus terminal. The body plan is that of a tube within a tube. Externally, the segments are marked by furrows. The setae or bristles are borne on most segments. Other external features are the pores of the reproductive systems opening on certain segments, the openings of the nephridia, and in many earthworms dorsal pores which open externally from the coelom. Some aquatic species have extensions of the posterior part of the body which function as gills.

The oligochaetes have been used in studies of physiology, regeneration, and metabolic gradients. Some aquatic forms are important in studies of stream pollution as indicators of organic contamination. Earthworms are important in turning over the soil and reducing vegetable material into humus. It is likely that fertile soil furnishes a suitable habitat for earthworms, rather than being a result of their activity. See also Annelida.

For the plant genus from the sunflower family (Asteraceae), see Oligochaeta (plant).

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This article is in need of attention from an expert on the subject. WikiProject Animal or the Animal Portal may be able to help recruit one. (August 2008)

Oligochaeta
Earthworm (Lumbricus terrestris)
Scientific classification
Kingdom: Animalia Phylum: Annelida Class: Clitellata Subclass: Oligochaeta
Orders
Haplotaxida Lumbriculida Moniligastrida

Oligochaeta (singular Oligochaete, pronounced /ˈɒlɨɡɵkiːt/) is a subclass in the biological phylum Annelida and includes various earthworms. Specifically, it contains the terrestrial megadrile earthworms (some of which are semi- or fully aquatic), and freshwater or semi-terrestrial microdrile forms including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms (Lumbriculidae) and several interstitial marine worms.

With around 10,000 known species the Oligochaeta make up about one half of the phylum.

These worms usually have few setae (chaetae) or "bristles", and lack parapodia, unlike polychaeta.

Contents 1 Common characteristics 1.1 Internal anatomy 2 Life cycle 3 Habitat 4 Families 5 References 6 Bibliography

Common characteristics

Oligochaetes are well-segmented worms and most have a spacious body cavity (coelom) that is used as a hydroskeleton. They range in length from less than 0.5 millimetres (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft), in the case of species such as the giant Gippsland earthworm.

The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone no sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or setae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes.

Each segment has four bundles of setae, with two on the underside, and the others on the sides. The bundles can contain anything from one to twenty-five setae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward.

A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form the clitellum, which is important in reproduction.^[1]

Internal anatomy

Most Oligochaetes are detritus feeders, although some genera are predaceous, such as Agriodrilus and Phagodrilus. The digestive tract is essentially a tube running the length of the body, but has a powerful muscular pharynx immediately behind the mouth cavity. In many species, the pharynx simply helps the worm suck in food, but in many aquatic species, it can be turned inside out and placed over food like a suction cup before being pulled back in.

The remainder of the digestive tract may include a crop for storage of food, and a gizzard for grinding it up, although these are not present in all species. The oesophagus includes "calciferous glands" that maintain calcium balance by excreting indigestible calcium carbonate into the gut. A number of yellowish "chloragogen cells" surround the intestine and the dorsal blood vessel, forming a tissue that functions in a similar fashion to the vertebrate liver. Some of these cells also float freely in the body cavity, where they are referred to as "eleocytes".^[1]

Most oligochaetes have no gills or similar structures, and simply breathe through their moist skin. The few exceptions generally have simple, filamentous gills. Excretion is through small ducts known as metanephridia. Terrestrial oligochaetes secrete urea, but the aquatic forms typically secrete ammonia, which dissolves rapidly into the water.^[1]

The vascular system consists of two main vessels connected by lateral vessels in each segment. Blood is carried forward in the dorsal vessel (in the upper part of the body) and back through the ventral vessel (underneath), before passing into a sinus surrounding the intestine. Some of the smaller vessels are muscular, effectively forming hearts; from one to five pairs of such hearts is typical. The blood of oligochaetes contains haemoglobin in all but the smallest of species, which have no need of respiratory pigments.^[1]

The nervous system consists of two ventral nerve cords, which are usually fused into a single structure, and three to four pairs of smaller nerves per body segment. Only a few aquatic oligochaetes have eyes, and even then they are only simply ocelli. Nonetheless, their skin has several individual photoreceptors, allowing the worm to sense the presence of light, and burrow away from it. Oligochaetes can taste their surroundings using chemoreceptors located in tubercles across their body, and their skin is also supplied with numerous free nerve endings that presumably contribute to their sense of touch.^[1]

Life cycle

Earthworms are hermaphrodites, which means that each animal has both male and female reproductive organs. They have external fertilization (except for some members of the African family Eudrilidae), but copulate and store sperm in a receptacle called a spermatheca. Like leeches, they have a clitellum which secretes a "cocoon" or capsule into which both eggs and sperm are deposited and acts as an incubator for the embryonic worms. The cocoon is deposited in the soil. On hatching, the young worms resemble small adults and grow continually until they reach maturity. They lack a trochophore larval stage.

Habitat

Native earthworm species are often eradicated from natural areas as people clear native vegetation and introduced species become more dominant in these disturbed habitats. Introduced earthworms are most common in disturbed environments such as suburban gardens and farmland paddocks.

Families

The following list of Oligochaeta families follows ICZN convention so that family-group name (ending in -idae) is followed by authorship and date.

Oligochaete worm

• Randiellidae Erséus & Strehlow, 1986
• Tubificidae Vejdovsky, 1884 (including Naidinae Ehrenberg, 1831)
• Narapidae Righi, 1983
• Opistocystidae, Cernosvitov 1936
• Dorydrilidae Cook, 1971
• Parvidrilidae Erséus, 1999
• Phreodrilidae Beddard, 1891
• Propappidae Coates, 1986
• Haplotaxidae Michaelsen, 1900
• Tiguassuidae Brinkhurst, 1988
• Lumbriculidae Vejdovsky, 1884
• Enchytraeidae Vejdovsky, 1879
• Moniligastridae Claus, 1880
• Alluroididae Michaelsen, 1900
• Syngenodrilidae Smith and Green, 1919
• Glossoscolecidae Michaelsen, 1900
• Tumakidae Righi, 1995
• Ailoscolecidae Bouché, 1969 (including Komarekionidae Gates, 1974)
• Sparganophilidae Michaelsen, 1918
• Microchaetidae Michaelsen, 1900
• Lumbricidae Claus, 1876 (including Diporodrilinae Bouché, 1970; Eiseniinae Omodeo, 1956; Spermophorodrilinae Omodeo & Rota, 1989; Postandrilinae Qiu & Bouché, 1998; Allolobophorinae Kvavadze, 2000 and Helodrilinae Kvavadze, 2000)
• Kynotidae - Brinkhurst & Jamieson, 1971
• Hormogastridae Michaelsen, 1900 (including Vignysinae Bouché, 1970 and Xaninae Diaz Cosin et al., 1989)
• Lutodrilidae McMahan, 1978
• Criodrilidae Vejdovsky, 1884 (including Biwadrilidae Brinkhurst & Jamieson, 1971)
• Almidae Duboscq, 1902
• Ocnerodrilidae Beddard, 1891 (including Malabariinae Gates, 1966)
• Acanthodrilidae Claus, 1880 (including Diplocardiinae Michaelsen, 1900)
• Octochaetidae Michaelsen, 1900 (including Benhamiinae Michaelsen, 1895/7)
• Exxidae Blakemore, 2000
• Megascolecidae Rosa, 1891 (including Pontodrilinae Vejdovsky, 1884; Plutellinae Vejdovsky, 1884 and Argilophilinae Fender & McKey-Fender, 1990)
• Eudrilidae Claus, 1880.

References

1. ^ ^{a b c d e} Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 528-547. ISBN 0-03-056747-5. 

Bibliography

• Blakemore, R.J. (2005). Whither Octochaetidae? – its family status reviewed. In: Advances in Earthworm Taxonomy II. Eds. A.A. & V.V. Pop. Proceedings IOTM2, Cluj University Press. Romania. Pp. 63–84. http://www.oligochaeta.org/ITOM2/IOTM2.htm.
• Blakemore, R.J. (2006). Revised Key to Earthworm Families (Ch. 9). In: A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World – 2nd Edition (2006). Eds.: N. Kaneko & M.T. Ito. COE Soil Ecology Research Group, Yokohama National University, Japan. CD-ROM Publication. Website: http://bio-eco.eis.ynu.ac.jp/eng/database/earthworm/.
• Erséus, C. and Källersjö, M. (2003). 18S rDNA phylogeny of basal groups of Clitellata (Annelida). Zoologica Scripta 33(2): 187-196.
• Jamieson, B.G.M., Tillier, S., Tillier, A., Justine, J.-L., Ling, E., James, S., McDonald, K. and Hugall, A.F. (2002). Phylogeny of the Megascolecidae and Crassiclitellata (Annelida, Oligochaeta): combined versus partitioned analysis using nuclear (28S) and mitochondrial(12S, 16S) rDNA. Zoosystema 24(4): 707-734.
• Michaelsen, W. (1900). Das Tierreich 10: Vermes, Oligochaeta. Friedländer & Sohn, Berlin. Pp. xxix+575, figs. 1-13. Online here: http://mail2web.com/cgi-bin/redir.asp?lid=0&newsite=http://www.archive.org/details/oligochaeta10mich.
• Plisko, J. D. (2006). The Oligochaeta type material housed at the Natal Museum, South Africa. African Invertebrates 47: 57-61. [1]
• Siddall, M.E., Apakupakul, K, Burreson, E. M., Coates, K. A., Erséus, C, Gelder, S. R., Källersjö, M, & Trapido-Rosenthal, H. (2001). Validating Livanow's Hypothesis: Molecular Data Agree that Leeches, Branchiobdellidans and Acanthobdella peledina form a Monophyletic Group of Oligochaetes. Molecular Phylogenetics and Evolution, 21: 346-351. http://research.amnh.org/~siddall/pub/livanow.pdf.
• Stephenson, J. (1930). The Oligochaeta. Clarendon Press, Oxford. Pp. 978.

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