Photo: A giant earthworm (Haplotaxida) burrows back into the moist leaf litter in the subtropical rainforest floor. (Photo by Wayne Lawler/Photo Researchers, Inc. Reproduced by permission.)
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 phylo-genetic 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 Acan-thodrilidae and Octochaetidae, of some authors) is the sister-group of the Ocnerodrilidae, and that these in turn are together the sister-group of a clade composed of several families: Sparganophilidae, Komarekionidae, Almidae, Lutodrili-dae, Hormogastridae, Lumbricidae, and Microchaetidae. The remaining two numerically important families, Glossoscoleci-dae and Eudrilidae, form a third major clade of Crassiclitel-lata, 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, Di-porochaetidae, and Kynotidae. Overall, there are 17 families, one order, and more than 4,000 species.
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 seg-mentally, from two per segment to many small nephridia per segment. Urine is excreted through nephropores to the outside, 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 13 th 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 sper-matophores, 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.
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 Lum-bricidae 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.
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.
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.
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, Luminous 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.
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.
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.
Several species of earthworms (Eisenia fetida, E. andrei, Eu-drilus 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.
1. River worm (Diplocardia riparia); 2. African worm (Eudrilus eugeniae); 3. Gippsland giant worm (Megascolides australis); 4. Pontoscolex corethrurus; 5. Common field worm (Aporrectodea caliginosa); 6. Amynthas corticis. (Illustration by Bruce Worden)
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