Earthworm

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Earthworm
Amynthas sp., a common Asian earthworm often Cosmopolitan and introduced around the World
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Oligochaeta
Subclass: Haplotaxida
Order: Megadrilacea
Suborder: Lumbricina + Moniligastrida
NODC v. 8.0, 1996[1]
Families

An earthworm is a tube-shaped, segmented animal that is commonly found living in soil, feeding on live and dead organic matter. Its digestive system runs through the length of its body. It conducts respiration through its skin; has a double transport system composed of coelomic fluid that moves within the fluid filled coelom and a simple, closed blood circulatory system. It has a central and a peripheral nervous system. The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve chord running back along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut and their actions move the digesting food toward the worm's anus.[2] Earthworms are hermaphrodites—each individual carries both male and female sex organs. As an invertebrate it lacks a skeleton, but it maintains its structure with fluid-filled coelom chambers that function as a hydrostatic skeleton.

"Earthworm" is the common name for the largest members of Oligochaeta (which is either a class or a 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).

Larger terrestrial earthworms are also called megadriles (or big worms), as opposed to the microdriles (or small worms) in the semi-aquatic families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a distinct clitellum (which is more extensive than that of microdriles) and a vascular system with true capillaries.

Contents

Anatomy

Form and function

Depending on the species, an adult earthworm can be anywhere from 10 mm long and 1 mm wide up to 3 m long and over 25 mm wide, but the typical Lumbricus terrestris grows to about 360 mm long.[3]

From front to back, the basic shape of the earthworm is a cylindrical tube, divided into a series of segments that compartmentalize the body. Grooves called "furrows" are generally[4] externally visible on the body demarcating the segments; dorsal pores and nephropores exude a fluid that moistens and protects the body surface. Except for the mouth and anal segments, each segment carries bristle-like hairs called lateral setae[5] used to anchor parts of the body during movement;[6] species may have four pairs of setae on each segment or more than eight sometimes forming a complete circle of setae per segment.[5] Special ventral setae are used to anchor mating earthworms by their penetration into the bodies of their mate.

Generally, within a species, the number of segments found is consistent across specimens, and individuals are born with the number of segments they will have throughout their lives. The first body segment (segment number 1) features both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals the entrance when the worm is at rest, but is also used to feel and chemically sense the worm's surroundings. Some species of earthworm can even use the prehensile prostomium to grab and drag items like grasses and leaves into their burrow.

Adult earthworms develop a belt-like glandular swelling, called the clitellum, which covers several segments toward the front part of the animal. This is part of the reproductive system, and it harbors the egg capsule. The posterior is most commonly cylindrical like the rest of the body, but depending on the species may also be quadrangular, octagonal, trapezoidal, or flattened and the last segment is called the periproct. The earthworm's anus, short vertical slit, is found on this segment.[5]

Cross section of the body of an earthworm (Oligochaeta) showing the disposition of the more important organs : the body wall (w) consists of dermis, circular and longitudinal muscles; the body cavity is divided by membranes (c) into a series of chambers, in each of which opens the mouth of a coiled nephridium (n); the axis of the cavity is occupied by the intestine (i); above and below it is a longer blood vessel (v), and below it is also the central nerve cord (nc)

The exterior of an individual segment is a thin cuticle over skin, commonly pigmented red to brown, which has specialized cells that secrete mucous over the cuticle to keep the body moist and ease movement through soil. Under the skin is a layer of nerve tissue, and two layers of muscles—a thin outer layer of circular muscle, and a much thicker inner layer of longitudinal muscle.[7] Interior of the muscle layers is a fluid-filled chamber called a coelom[8] that by its pressurization provides provides their boneless bodies, structure. A structure called a nephridium removes metabolic waste and expels it through pores on the sides; two or more nephridia are found in most segments.[9] At the center of a body is the digestive tract, which runs straight through from mouth to anus without coiling, flanked above and below by blood vessels and the ventral nerve cord. The segments are separated from each other by dividing walls called septa [10] that are perforated, which allow the coelomic fluid to pass between segments.[11]

Many earthworms can eject coelomic fluid through pores in the back in response to stress; Australian Didymogaster sylvaticus (known as the "blue squirter earthworm") can squirt fluid as high as 30 cm.[11]

Digestive system

The gut of the earthworm is a straight tube which extends from mouth to anus. It is differentiated into a buccal cavity (generally running through the first 1 or 2 segments of the earthworm), pharynx (running generally about 4 segments in length), esophagus, crop, gizzard (usually) and intestine.[12]

Food enters the mouth. The pharynx acts as a suction pump; its muscular walls draw in food. In the pharynx, the pharyngeal glands secrete mucus. Food moves into the esophagus, where calcium from the blood and ingested from previous meals, is pumped in to maintain proper blood calcium levels in the blood and food pH. From there it passes into the crop and gizzard. In the gizzard, strong muscular contractions grind the food up with the help of mineral particles ingested along with the food. Once through the gizzard, food continues through the intestine for digestion. The intestine secretes pepsin to digest proteins, amylase to digest polysaccharides, cellulase to digest cellulose, and lipase to digest fats.[2] Instead of being coiled like a mammal intestine, an earthworm's intestine increases surface area to increase nutrient absorption by having many folds running along its length. The intestine has its own pair of muscle layers like the body, but in reverse order—an inner circular layer inside an outer longitudinal layer.[13]

Circulatory system

Earthworms have a dual circulatory system composed of the coelom and a closed circulatory system both of which carry the food, waste and respiratory gasses. The closed circulatory system, with five main blood vessels: the dorsal (top) vessel, which runs above the digestive tract; the ventral (bottom) vessel, which runs below the digestive tract; the subneural vessel, which runs below the ventral nerve chord; and two lateroneural vessels on either side of the nerve chord.[14] The dorsal vessel acts moves the blood forward while the other four longitudinal vessels carry the blood to the rear. In segments six through eleven, a pair of aortic arches rings those coelom and act as hearts, pumping the blood to the ventral vessel that acts as the aorta. The blood consists of ameboid cells and hemoglobin dissolved in the plasma. The second circulatory system derives from the cells of the digestive system that line the coelom. As the digestive cells become full, they released non-living cells of fat into the fluid filled coelom where they float freely but can pass through the walls separating each segment and so move food to other parts and to assist in wound healing.[15]

Excretory system

The excretory system contains a pair of nephridia in every segment, except for the first three and last one.[16] There are three types of nephridia: integumentary, septal and pharyngeal. The integumentary nephridia lie attached to the inner side of the body wall in all segments except the first two. The septal nephridia are attached to both sides of the septa behind the 15th segment. The pharyngeal nephridia are attached to 4th, 5th and 6th segment.[16] The waste in the coelom fluid from a forward segment is drawn in by the beating of cilia of the nephrostome, carried through the septum (wall) which forms a series of loops entwined by blood capillaries that also transfer waste into the tubule of the nephrostome. The excretory wastes are then finally discharged through a pore on the worms side.[17]

Respiration

Earthworms have no special respiratory organs. Gases are exchanged through the moist skin and capillaries where the oxygen is picked up by the hemoglobin dissolved in the blood plasma and carbon dioxide is released. Water as well as salts can also be moved through the skin by active transport.

Reproduction

Earthworm reproduction
Earthworm cocoons from L. terrestris
An earthworm cocoon from L. rubellus

Mating occurs on the surface, most often at night. Earthworms are hermaphrodites; that is they have both male and female sexual organs. The sexual organs are located in segments 9 to 15. Earthworms have one or two pairs of testes contained within sacs. There are two or four pairs of seminal vesicles which produce, store and release the sperm via the male pores. Ovaries and oviducts in segment 13 release eggs via female pores on segment 14, while sperm is expelled from segment 15. One or more pairs of spermathecae are present in segments 9 and 10 (depending on the species) which are internal sacs that receive and store sperm from the other worm during copulation. As a result, segment 15 of one worm exudes sperm into segment 9 and 10 with its storage vesicles of its mate. Some species use external spermatophores for sperm transfer.

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. Some time after copulation, long after the worms have separated, the clitellum (behind the spermathecae) secretes the cocoon which forms a ring around the worm. The worm then backs out of the ring, and as it does so, it 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, but lack their sex structures, which develop in about 60 to 90 days. They attain full size in about one year. Scientists predict that the average lifespan under field conditions is four to eight years, still most garden varieties live only one to two years. Several common earthworm species are mostly parthenogenetic.

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. Despite denial of this phenomenon by some current experts,[who?] Gates’s reports included:

  • 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, i.e., two worms may grow from one.[18]
  • 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.[19]

An unidentified Tasmanian earthworm shown growing a second head has been reported.[20]

Locomotion and importance to soil

Close up of an earthworm in garden 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 their movement through their 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 of nutrients and its 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 generally considered beneficial by 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. [21]

Benefits

The major benefits of earthworm activities to soil fertility can be summarized as:

  • Biological. In many soils, earthworms play a major role in the conversion of large pieces of organic matter into rich humus, thus improving soil fertility. This is achieved by the worm's actions of pulling below organic matter deposited on the surface, such as leaf fall or manure, either for food or to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it and mingle it with the earth. Worm casts (see below) can contain 40% more humus than the top 9" (23 cm) of soil in which the worm is living.
Faeces in form of casts
  • Chemical. In addition to dead organic matter, the earthworm also ingests any other soil particles that are small enough—including sand grains up to 1/20 of an inch (1.25mm)—into its gizzard wherein those 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, deposited on the surface or deeper in the soil, minerals and plant nutrients are changed to an accessible form for plants to use. Investigations in the United States show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potassium than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plentiful humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, and is in itself an indicator of why it pays the gardener or farmer to keep worm populations high.
  • Physical. The earthworm's burrowing creates a multitude of channels through the soil and is of great value in maintaining the soil structure, enabling processes of aeration and drainage. 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)".[22] Thus the earthworm not only creates passages for air and water to traverse the soil, but it modifies the vital organic component that makes a soil healthy.(See Bioturbation.)

The earthworm's existence cannot be taken for granted. Dr. W. E. Shewell Cooper observed "tremendous numerical differences between adjacent gardens", 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.[23]

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 a farmer's soil could be greater than that of the livestock upon its surface.

Earthworms as an invasive species

Main articles: Earthworms as an invasive species and Invasive earthworms of North America

From a total of around 6,000 species, only about 150 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.[24]

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, some aquatic and some euryhaline (salt-water tolerant) and littoral (living on the sea-shore, e.g. Pontodrilus litoralis). 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) anecic (meaning "reaching up") worms that construct permanent deep burrows through which they visit the surface to obtain plant material for food, such as leaves, e.g. Lumbricus terrestris.[25]

Permanent vertical burrow

Earthworm populations depend on both physical and chemical properties of the soil, such as 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 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.

Ocypus olens trying to prey on Lumbricus sp.

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, pigs, 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 their cocoons.

The application of chemical fertilizers, sprays and dusts are believed to have a disastrous effect on earthworm populations[citation needed]. Nitrogenous fertilizers tend to create acidic conditions, which are fatal to the worms, and dead specimens are often 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.

The most reliable way to maintain or increase the levels of worm population in the soil is to avoid the application of artificial chemicals. The addition of organic matter, preferably as a surface mulch, on a regular basis will provide them with their food and nutrient requirements, and will create the optimum conditions of temperature and moisture that will stimulate their activity.

Economic impact

Earthworms being raised at the La Chonita Hacienda in Mexico

Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose and food waste. These are usually Eisenia fetida (or its close relative Eisenia andrei) or the Brandling worm, commonly known as the Tiger worm or Red Wiggler. They are distinct from soil-dwelling earthworms. In the tropics, the 'African Nightcrawler' Eudrilus eugeniae and the 'Indian blue' Perionyx excavatus are used.

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 for their chiefs.

Taxonomy and distribution

Within the world of taxonomy the stable 'Classical System' of Michaelsen (1900) and Stephenson (1930) was gradually eroded by the controversy over how to classify earthworms such that Fender and McKey-Fender (1990) went so far as to say "The family-level classification of the megascolecid earthworms is in chaos."[26] Over the years, many scientists developed their own classification systems for earthworms, which led to confusion, and these systems have been and still continue to be revised and updated. The classification system used here, developed by Blakemore (2000), is a modern reversion to the Classical System that is historically proven and widely accepted.[27]

Categorization of a megadrile earthworm into one of its taxonomic families under suborders Lumbricina and Moniligastrida is based on such features as the makeup of the clitellum, the location and disposition of the sex features (pores, prostatic glands, etc.), number of gizzards and body shape.[27] There are currently over 6,000 named species of terrestrial earthworms, as provided in a species name database,[28] but the number of synonyms is unknown.

The families, with their known distributions or origins:[27]

See also

References

  1. ^ "ITIS Report for Lumbricina, Taxonomic Serial No.: 69069". ITIS. Retrieved May 14, 2012. 
  2. ^ a b Cleveland P. Hickman Jr., Larry S. Roberts, Frances M Hickman (1984, 7th ed.). Integrated Principles of Zoology. Times Mirror/Mosby College Publishing. p. 344. ISBN 0-8016-2173-9. 
  3. ^ Blakemore 2012, p. xl.
  4. ^ Edwards & Bohlen 1996, p. 11.
  5. ^ a b c Sims & Gerard 1985, pp. 3-6.
  6. ^ Edwards & Bholen 1996, p. 3.
  7. ^ Edwards & Bohlen 1996, p. 8-9.
  8. ^ Edwards & Bohlen 1996, p. 1.
  9. ^ Edwards & Bohlen 1996, p. 6.
  10. ^ Sims & Gerard 1985, p. 8.
  11. ^ a b Edwards & Bohlen 1996, p. 12.
  12. ^ Edwards & Bohlen 1996, p. 13.
  13. ^ Edwards & Bohlen 1996, pp. 13-15.
  14. ^ Sims & Gerard 1985, p. 10.
  15. ^ Cleveland P. Hickman Jr., Larry S. Roberts, Frances M Hickman (1984, 7th ed.). Integrated Principles of Zoology. Times Mirror/Mosby College Publishing. pp. 344–345. ISBN 0-8016-2173-9. 
  16. ^ a b Farabee, H.J. "Excretory System". Retrieved 29 July 2012. 
  17. ^ Cleveland P. Hickman Jr., Larry S. Roberts, Frances M Hickman (1984, 7th ed.). Integrated Principles of Zoology. Times Mirror/Mosby College Publishing. pp. 345–346. ISBN 0-8016-2173-9. 
  18. ^ Biolbull.org
  19. ^ Links.jstor.org
  20. ^ QVmag.tas.gov.au
  21. ^ Darwin, Charles, The formation of vegetable mould through the action of worms, with observations on their habits. Found at Project Gutenberg Etext Formation of Vegetable Mould, by Darwin
  22. ^ Mollison, Bill, Permaculture- A Designer's Manual, Tagari Press, 1988
  23. ^ Cooper, Shewell; Soil, Humus And Health ISBN 978-0-583-12796-7
  24. ^ Cosmopolitan Earthworms
  25. ^ Earthworms: Renewers of Agroecosystems (SA Fall, 1990 (v3n1))
  26. ^ Fender & McKey-Fender (1990). Soil Biology Guide. Wiley-Interscience. ISBN 0471045519. 
  27. ^ a b c Blakemore, Robert J. (March, 2006). "Revised Key to Worldwide Earthworm Families from Blakemore plus Reviews of Criodrilidae (including Biwadrilidae) and Octochaetidae". A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World. annelida.net. Retrieved May 15, 2012. 
  28. ^ Earthworm species name database

Referenced works

  • Blakemore, Robert J. (2012). Cosmopolitan Earthworms -- an Eco-Taxonomic Guide to the Peregrine Species of the World. (5th Ed). Yokohama, Japan: VermEcology So(i)lutions.  Unknown parameter |credentials1= ignored (help)
  • Sims, Reginald William; Gerard, B (1985). Earthworms: Keys and Notes for the Identification and Study of the Species. London: Published for The Linnean Society of London and the Estuarine and Brackish-Water Sciences Association by E. J. Brill/Dr. W. Backhuys.  Unknown parameter |credentials1= ignored (help)
  • Edwards, Clive Arthur; Bohlen, Patrick J. (1996). Biology and Ecology of Earthworms, 3rd Ed. Springer.  Unknown parameter |credentials1= ignored (help); Unknown parameter |credentials2= ignored (help)

Further reading

  • Edwards, Clive A., Bohlen, P.J. (Eds.) Biology and Ecology of Earthworms. Springer, 2005. 3rd edition.
  • Edwards, Clive A. (Ed.) Earthworm Ecology. Boca Raton: CRC Press, 2004. Second revised edition. ISBN 0-8493-1819-X
  • Lee, Keneth E. Earthworms: Their Ecology and Relationships with Soils and Land Use. Academic Press. Sydney, 1985. ISBN 0-12-440860-5
  • Stewart, Amy. The Earth Moved: On the Remarkable Achievements of Earthworms. Chapel Hill, N.C.: Algonquin Books, 2004. ISBN 1-56512-337-9

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