Earthworm

Earthworm is the common name for the largest members of Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female. Theoretical cladistic studies have placed them instead in the suborder Lumbricina of the order Haplotaxida, but this may again soon change. Folk names for the earthworm include "dew-worm", "Rainworm", "night crawler" and "angleworm" (due to its use as fishing bait).

Earthworms are also called megadriles (or big worms), as opposed to the microdriles (or small worms) in the families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a distinct clitellum (which is much more obvious than the single-layered one of the microdriles) and a vascular system with true capillaries.

Anatomy
The basic body plan of an earthworm is a tube, the digestive system, within a tube, the muscular slimy, moist outer body. The body is annular, formed of segments that are most specialized in the anterior. Earthworms have a simple closed circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of "hearts" (aortic arches) which vary in number in the different taxa. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall, where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms. Most earthworms are decomposers feeding on undecayed leaf and other plant matter, others are more geophagous.

Reproduction
Earthworms are hermaphrodites: They typically have two pairs of testes, surrounded by 2 pairs of testes sacs. There are 2 or 4 pairs of seminal vesicles which produce, store and release the sperm via the male pores, and ovaries and ovipores in segment 13 that release eggs via female pores on segment 14. However, most also have one or more pairs of spermathecae (depending on the species) that are internal sacs which receive and store sperm from the other worm in copulation. Some species use external spermatophores for transfer instead.

Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The clitellum becomes very reddish to pinkish in color. The cocoon, or egg case, is secreted by the clitellum band which is near the front of the worm, but behind the spermathecae. Some time after copulation, long after the worms have separated, the clitellum secretes the cocoon which forms a ring around the worm. The worm then backs out of the ring, and as it does so, injects its own eggs and the other worm's sperm into it. As the worm slips out, the ends of the cocoon seal to form a vaguely lemon-shaped incubator (cocoon) in which the embryonic worms develop. They emerge as small, but fully formed earthworms, except for a lack of the sex structures, which develop later in about 60 to 90 days. They attain full size in about one year, sometimes sooner. Scientists predict that the average lifespan under field conditions is 4–8 years, still most garden varieties live only one to two years. Several common earthworm species are mostly parthenogenetic, that is, with asexual reproduction resulting in clones.

Digestion
There is a digestion system in an earthworm. The process of nutrition begins in the mouth, where food is sucked in by a muscular pharynx. From there, food goes down the esophagus through peristalsis (visceral muscle contractions.) After this, the food is stored in the crop, which retains food and has the ability to expand. From there, food goes into the gizzard where sand and muscular contractions churn the food and increase the surface area. From there, food enters the intestine which has the ability to absorb food, then food exits through the anus. A rectum is unnecessary because an earthworm is in a moist environment and thus does not require water reclamation.

Regeneration
Earthworms have the ability to regenerate lost segments, but this ability varies between species and depends on the extent of the damage. Stephenson (1930) devoted a chapter of his monograph to this topic, while G.E. Gates spent 20 years studying regeneration in a variety of species, but “because little interest was shown”, Gates (1972) only published a few of his findings that, nevertheless, show it is theoretically possible to grow two whole worms from a bisected specimen in certain species. Gates’s reports included: An unidentified Tasmanian earthworm shown growing a second head is reported here:.
 * Eisenia fetida (Savigny, 1826) with head regeneration, in an anterior direction, possible at each intersegmental level back to and including 23/24, while tails were regenerated at any levels behind 20/21.
 * Lumbricus terrestris Linnaeus, 1758 replacing anterior segments from as far back as 13/14 and 16/17 but tail regeneration was never found.
 * Perionyx excavatus Perrier, 1872 readily regenerated lost parts of the body, in an anterior direction from as far back as 17/18, and in a posterior direction as far forward as 20/21.
 * Lampito mauritii Kinberg, 1867 with regeneration in anterior direction at all levels back to 25/26 and tail regeneration from 30/31; head regeneration was sometimes believed to be caused by internal amputation resulting from Sarcophaga sp. larval infestation.
 * Criodrilus lacuum Hoffmeister, 1845 also has prodigious regenerative capacity with ‘head’ regeneration from as far back as 40/41.
 * Lumbriculus veriega Able to split into two segments and survive.

Rainstorms and "stranding" behaviour
Earthworms can sometimes be found on the surface of the ground following heavy rain storms, as a storm may flood the soil with excessive water. However, if the surface where they find themselves is unexpectedly paved, rocky, or compacted (hardened), they may become stranded, potentially suffering injury or death from causes such as heat, exposure, dehydration, or predation. Note, there are some earthworm species that can survive for several days in water if it is sufficiently oxygenated. Earthworms may also come to the surface during rain in order to mate, and therefore, an alternative hypothesis concerning "stranding" behavior is that as some species (notably Lumbricus terrestris) come to the surface to mate they may become stranded. However, this behavior is limited to only a few species and L. terrestris is rarely, if ever, one of those found stranded on impermeable surfaces, this hypothesis does not seem likely to be true.

Another hypothesis is that the worms may be using the moist conditions on the surface so they can breathe and to travel more quickly than they can underground, thus moving to and colonizing new areas more quickly. Since the relative humidity of the surface and air is higher during and after rain, they do not become dehydrated quite as rapidly. However, if true, this is a very risky behavior near dawn, in high summer, or in the daytime, since earthworms die quickly when exposed to direct sunlight with its high heat, light and strong UV content, and are more vulnerable to predators such as birds.

A further hypothesis is that, because there are many other organisms beside the earthworm in the ground as well, and these organisms all tend to increase respiration as water content of the soil increases, carbon dioxide gas may dissolve into the rainwater forming a higher than usual acid content carbonic acid in the soil area. As the soil becomes too acidic for the worms, they seek a more neutral environment on the surface.

Locomotion and importance to soil
Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. In all the body segments except the first, last and clitellum, there is a ring of S-shaped setae embedded in the epidermal pit of each segment (perichaetine). The whole burrowing process is aided by the secretion of lubricating mucus. Worms can make gurgling noises underground when disturbed as a result of the worm moving through its lubricated tunnels. They also work as biological "pistons" forcing air through the tunnels as they move. Thus earthworm activity aerates and mixes the soil, and is constructive to mineralization and nutrient uptake by vegetation. Certain species of earthworm come to the surface and graze on the higher concentrations of organic matter present there, mixing it with the mineral soil. Because a high level of organic matter mixing is associated with soil fertility, an abundance of earthworms is beneficial to the organic gardener. In fact as long ago as 1881 Charles Darwin wrote: It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.

Benefits
The major benefits of earthworm activities to soil fertility can be summarized as: The earthworm's existence cannot be taken for granted. Dr. W. E. Shewell Cooper observed "tremendous numerical differences between adjacent gardens" (Soil, Humus And Health), and worm populations are affected by a host of environmental factors, many of which can be influenced by good management practices on the part of the gardener or farmer.
 * Biological. In many soils, earthworms play a major role in converting large pieces of organic matter (e.g. dead leaves) into rich humus, and thus improving soil fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the dried dirt, such as leaf fall or manure, either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 9" of soil in which the worm is living.
 * Chemical. As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough—including stones up to 1/20 of an inch (1.25mm) across—into its gizzard wherein minute fragments of grit grind everything into a fine paste which is then digested in the intestine. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, minerals and plant nutrients are made available in an accessible form. Investigations in the US show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, in itself an indicator of why it pays the gardener or farmer to keep worm populations high.
 * Physical. By its burrowing actions, the earthworm is of great value in keeping the soil structure open, creating a multitude of channels which allow the processes of both aeration and drainage to occur. Permaculture co-founder Bill Mollison points out that by sliding in their tunnels, earthworms "act as an innumerable army of pistons pumping air in and out of the soils on a 24 hour cycle (more rapidly at night)". Thus the earthworm not only creates passages for air and water to traverse, but is itself a vital component in the living biosystem that is healthy soil. Earthworms continue to move through the soil due to the excretion of mucus into the soil that acts as a lubricant for easier movement of the worm. (See Bioturbation.)

Darwin estimated that arable land contains up to 53,000 worms per acre (13/m²), but more recent research from Rothamsted Experimental Station has produced figures suggesting that even poor soil may support 250,000/acre (62/m²), whilst rich fertile farmland may have up to 1,750,000/acre (432/m²), meaning that the weight of earthworms beneath the farmer's soil could be greater than that of his livestock upon its surface.

Earthworms as invasive species
From a total of around 6,000 species, only about 120 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.

Australia
Australia has 650 known species of native earthworm that survive in both rich and in nutrient-poor conditions where they may be sensitive to changes in the environment. Introduced species are commonly found in agricultural environments along with persistent natives. Most of the 75 or so exotics have been accidentally introduced into Australia. The total species numbers are predicted to exceed 2,000.

North America
A total of approximately 182 earthworm taxa in 12 families are reported from America north of Mexico, i.e., USA & Canada, of which 60 (ca. 33%) are exotic/introduced. Only two genera of Lumbricid earthworms are indigenous to North America while introduced genera have spread to areas where earthworms did not formerly exist, especially in the north where forest development relies on a large amount of undecayed leaf matter. When worms decompose that leaf layer, the ecology may shift making the habitat unsurvivable for certain species of trees, ferns and wildflowers. Another possible ecologic impact of greater earthworm numbers: larger earthworms (e.g. the night crawler, Lumbricus terrestris, and the Alabama jumper, Amynthas agrestis) can be eaten by adult salamanders, and when the salamanders do consume the earthworms they are more successful at reproduction. However, those earthworms are too large for juvenile salamanders to consume, which leads to a net loss in salamander population.

Currently there is no economically feasible method for controlling invasive earthworms in forests. Earthworms normally spread slowly, but can be quickly introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings.

United Kingdom
A recent threat to earthworm populations in the UK is the New Zealand Flatworm (Artiposthia triangulata), which feeds upon the earthworm, but in the UK, has no natural predator itself. At present sightings of the New Zealand flatworm have been mainly localised, but this is no reason for complacency as it has spread extensively since its introduction in 1960 through contaminated soil and plant pots. Any sightings of the flatworm should be reported to the Scottish Crop Research Institute, which is monitoring its spread.

Special habitats
While, as the name earthworm suggests, the main habitat of earthworms is in soil, the situation is more complicated than that. The brandling worm Eisenia fetida lives in decaying plant matter and manure. Arctiostrotus vancouverensis from Vancouver Island and the Olympic Peninsula is generally found in decaying conifer logs. Aporrectodea limicola and Sparganophilus and several others are found in mud in streams. Some species are arboreal. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own.

Ecology
Earthworms are classified into three main ecophysiological categories: (1) leaf litter/compost dwelling worms (epigeic) e.g. Eisenia fetida; (2) topsoil or subsoil dwelling worms (endogeics); and (3) worms that construct permanent deep burrows through which they visit the surface to obtain plant material for food, such as leaves (anecic), e.g. Lumbricus terrestris. Earthworm populations depend on both physical and chemical properties of the soil, such as soil temperature, moisture, pH, salts, aeration and texture, as well as available food, and the ability of the species to reproduce and disperse. One of the most important environmental factors is pH, but earthworms vary in their preferences. Most earthworms favor neutral to slightly acidic soil. However, Lumbricus terrestris are still present in a pH of 5.4 and Dendrobaena octaedra at a pH of 4.3 and some Megascolecidae are present in extremely acid humic soils. Soil pH may also influence the numbers of worms that go into diapause. The more acidic the soil, the sooner worms go into diapause, and remain in diapause the longest time at a pH of 6.4.

Earthworms form the base of many food chains. They are preyed upon by many species of birds (e.g. starlings, thrushes, gulls, crows, European Robins and American Robins), snakes, mammals (e.g. bears, foxes, hedgehogs, moles) and invertebrates (e.g. ground beetles and other beetles, snails, slugs). Earthworms have many internal parasites including Protozoa, Platyhelminthes, Nematodes; they can be found in the worms' blood, seminal vesicles, coelom, intestine, or in the cocoons.

The application of chemical fertilizers, sprays and dusts can have a disastrous effect on earthworm populations. Nitrogenous fertilizers tend to create acid conditions, which are fatal to the worms, and often dead specimens are to be found on the surface following the application of substances like DDT, lime sulphur and lead arsenate. In Australia, changes in farming practices such as the application of superphosphates on pastures and a switch from pastoral farming to arable farming had a devastating effect on populations of the Giant Gippsland earthworm leading to their classification as a protected species.

Therefore, the most reliable way to maintain or increase the levels of worm population in the soil is to avoid the application of artificial chemicals. Adding organic matter, preferably as a surface mulch, on a regular basis will provide them with their food and nutrient requirements, and also creates the optimum conditions of heat (cooler in summer and warmer in winter) and moisture to stimulate their activity.

Economic impact
Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose and compost food waste. These are usually Eisenia fetida (or its close relative Eisenia andrei) or the Brandling worm, also known as the Tiger worm or Red Wiggler, and are distinct from soil-dwelling earthworms.

Earthworms are sold all over the world. The earthworm market is sizable. According to Doug Collicut, "In 1980, 370 million worms were exported from Canada, with a Canadian export value of $13 million and an American retail value of $54 million."

Earthworms are also sold as food for human consumption. Noke is a culinary term used by the Māori of New Zealand, to refer to earthworms which are considered delicacies.

Taxonomy and distribution
The families, with distribution of the main ones:
 * Acanthodrilidae: Africa, midland and southeastern North America, Central and South America, Australia and Oceania.
 * Ailoscolidae
 * Alluroididae
 * Almidae (disputed): Africa, South America.
 * Criodrilidae
 * Eudrilidae: Tropical Africa.
 * Exxidae: Central America/Caribbean.
 * Glossoscolecidae: Central and northern South America.
 * Hormogastridae: Europe.
 * Lumbricidae: Temperate Northern Hemisphere from Vancouver Island, Canada to Japan, mostly Eurasia.
 * Lutodrilidae
 * Megascolecidae: South East Asia, Australasia and Oceania, northwestern North America.
 * Microchaetidae
 * Ocnerodrilidae: Central and South America, Africa.
 * Octochaetidae: Central/South America, western Africa, India, New Zealand, Australia.
 * Sparganophilidae: North America.