All about earthworms

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“…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.” — Charles Darwin (1881)

The lowly earthworm, they’re given such short shrift. With nary a thought from us, they quietly work magic day and night, right under our very feet. We should be celebrating earthworms, as the ancient Egyptians did, because they’re the unheralded champs of soil restoration. Aristotle called them “the intestines of the earth.”

Earthworms clean up debris and recycle it as fertilizer. Their tunnels aerate the soil and erosion is almost eliminated. Their labors benefit the food we eat, the flowers we love, the trees that shade us and the wildlife who live in our yards. And, the worms themselves are food for many animals. Around the world, agricultural areas are going “no-till.” In these instances, earthworms are valued as the main force for churning crop residue into the soil and keeping it fertile.

They’re strange-looking critters, aren’t they? But they’re perfectly built for the work they do, and they’ve been doing it successfully for a long time — maybe 1.2 billion years. In 2002, Australian researchers found a fossilized trail in sandstone they believe was made by a worm-like animal that long ago. This would not only be the oldest worm evidence, but the oldest multi-celled animal.

Earthworms are classified in the phylum Annelida, (from the Latin anellus for “little ring), which also includes leeches. Most annelids live in most environments. Earthworms belong to the order Haplotaxida and the suborder Lumbricina. Today about 3,000 species of earthworms are known to exist and they’re found in on every continent, except Antarctica. The U.S. has about 100 species of native earthworms and another 15 or so non-native species.

Physical description
Earthworms vary wildly in size. Most are only a few inches long, with some less than an inch. At the other extreme is the giant Gippsland Earthworm, an endangered (and harmless) Australian species who lives in the subsoil and grows to 6 feet or more. The largest U.S. earthworms are night crawlers, who can measure up to 10 inches.

(Iceclanl / Wiki; cc by-sa 3.0)

(Iceclanl / Wiki; cc by-sa 3.0)

Earthworms seem to look pretty much alike, don’t they? To experts, though, there are many differences. There’s body length, of course, but they vary also in such characteristics as coloration, number of body segments and type and location of body hairs.

Earthworms range in color from reddish to gray to brown. Their skin (called a cuticle) is soft and very permeable. It’s also slimy. Slime is a mucus produced by a band of glandular tissue called the clitellum (kly-TELL-um), and it’s crucial: Worms breathe directly through tiny pores in their skin and this moisture allows oxygen to pass directly through and into their bloodstream. It also allows carbon dioxide to pass out. If the skin dries, the worm can’t breathe. The slimy skin also serves another role as a lubricant, to help ease the worm through soil. Mucus from the clitellum also forms cocoons to hold worm embryos. The clitellum is shown in the photo above. 

Earthworm body system

Earthworm body system. (KDS444 : Wiki; cc bysa 3.0)

Earthworms are simple animals with a body that’s basically a muscular cylinder of ring-like segments called annuli (ANN-u-lie), tapering off at both ends. Some species have as many as 150 segments. Earthworms have no skeleton, they’re soft through-and-through. Their body contains a simple circulatory system with two main blood vessels, five bands of tissue that pump blood, a digestive tract, reproductive organs and an anus. They have two heart-like organs and between them are glands that manage calcium in their diet. Earthworms have a nervous system which reacts to heat, cold, touch and vibrations. They don’t have eyes, ears or a nose, but they do have light-sensing cells and will move away from light (ultraviolet light rays can kill them).

The head end of a night crawler earthworm (Allolobophora chlorotica). (Gilles San Martin / Flickr; cc by-sa 2.0)

The head end and tail end look so much alike, can we tell which is which? Yes. The head end, called the prostomium, is at the end that’s closest to the clitellum. It may be surprising, but their head, so lacking in other features, does have a mouth. A toothless mouth, but with strong muscles for eating. And, eat they do — up to a half or more of their body weight per day.

They take in food with strong “lips” and a sucking action that pulls it into their mouth. The food is moistened (if it isn’t already wet) and swallowed, where it moves down the throat to a storage pouch called a crop. Then, on to the gizzard it goes, where powerful muscles grind and mix the food with a little help from fine sand particles that have also been consumed. Pulverized food, in the form of a thick paste, moves from there into the intestines. Most of the worm’s body is intestines. Here digestive juices break food down further and nutrients pass into the bloodstream. What’s left over, mostly revitalized soil, exits the body through the anus.

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Earthworm castings are rich in nutrients for plants. They make an excellent fertilizer. (Red58bill / Wiki; CC BY 3.0)

Worm poo is called castings. Castings are composed of dirt mixed with organic matter. They’re five times richer in nitrogen, seven times richer in phosphates, and 11 times richer in potassium than the surrounding soil, so they make a great fertilizer. It’s so good, in fact, that many people maintain a worm bin. Into the bin goes leftover food which is consumed and eliminated as castings by the worms. The resulting compost is used in potting soil, as a mulch or made into a “tea” to sprinkle as a fertilizer. There are many “vermicomposting” sites on the internet that explain how to do this.

Earthworms move by expanding and relaxing muscles while gripping surfaces with tiny bristles, called setae (SEE-tee). Here’s how it works: A set of muscles around each segment contracts, which has the effect of squeezing the worm into a long, thin shape. It then anchors the front of its body using setae. Following that, other muscles which run the length of the body contract, making the worm short and fat; this pulls the rear end forward. Setae at the rear then attach to the soil and the front releases and moves forward. The waves of relaxing and contracting muscles give worms their characteristic wriggling motion as they move. Worms generally move forward (one way to tell their head from their rear), but they’re capable of moving backward, too.

You’ve likely heard that cutting a worm in two will produce two worms. This isn’t true. The front half might live and grow into a full-size worm if it has the clitellum and 10 body sections behind it. If so, new growth will be a little smaller in diameter. The back half always dies.

Behavior and habitat

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Earthworm burrow illustrating grass at the top and a long tunnel with rainwater flowing down it. (© Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer. Rev. ed. Ankeny, IA: Soil and Water Conservation Society.)

To survive, earthworms need food, oxygen, moisture, a specific temperature range and to stay out of the sun. They’re cold-blooded (don’t generate their own body heat) and live closer to the surface in warm weather and deeper in winter. They’ll die if they freeze, so the colder it gets, the deeper they go to hibernate. Generally, worms prefer temperatures of between 50 and 70 degrees, but some species can tolerate a higher or lower range. If conditions become too dry or hot for normal activity, they estivate (or aestivate), which is the warm weather version of hibernation. Estivating earthworms can be easily identified, as they tie themselves up in a knot.

Earthworms move through the earth in burrows, or tunnels, they create. Some worms burrow horizontally, some vertically. Either way, burrowing aerates the soil, allowing oxygen and water to penetrate to plant roots and soil microorganisms.

Worms eat soil and decomposing plant materials as they burrow. Good soil may hold up to a million worms per acre eating as much as 10 tons of leaves and other plant matter per year. Their castings are rich in nitrogen, calcium, magnesium and phosphorus, all important nutrients for healthy soil.

Earthworms don’t all perform the same kind of work or live at the same depth. There are three major groups:

  • Surface-soil species – These worms live just under the soil surface or within organic matter lying on top. Researchers have found that they’re most numerous in the crowns of grass clumps, the junction where stems and roots meet. They’re shaded there and hidden from birds as they feed on decaying leaves and roots. The grass also offers some protection from high temperatures. Typically small, these worms are adapted to more extremes of temperature and moisture than other kinds of earthworms. Some species live in the highly rich environment of compost piles and can’t survive in soil containing less organic matter.
  • Upper-soil species – These earthworms live close to the surface and feed on soil and whatever organic matter it contains. Their burrows are temporary and become filled with worm droppings, adding nutrients to subsoil.
  • Deep-burrowing species – Typically called night crawlers, these worms form permanent, vertical burrows as deep as 8 feet into the soil. They create “middens” at the entrance to the burrows.

Middens are piles of worm poo and plant residue. It isn’t exactly clear to experts just why worms build middens. But it’s probably to hide the burrow entrance while providing a handy pile of food for them to feed on from below, safe from sun and birds. Middens dissolve over time, and as they do, they fertilize the soil at the top and drain into the burrow, fertilizing that, as well. In soil that isn’t tilled, burrows are permanent and used for several years. The grass around a midden is sometimes greener and taller.

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Estivating earthworm (Jtellerelsberg / Wiki; CC BY-SA 3.0)

Worms can be seen on sidewalks and driveways after a rain. Just why is another worm mystery. What is known for sure is that they don’t do it to avoid drowning. Edwin Berry, an entomologist, has studied worms for ten years and finds they can live for weeks in water. Some experts believe rain or high humidity gives worms a chance to disperse to new feeding areas, giving them a chance to move around “outdoors” without drying out. If you see one on pavement, do him a favor by picking him up and placing him on soil in a shady place — light paralyzes them after about an hour, leaving them helpless to move to safety.

Reproduction

Earthworms mating (Jack Hynes / Wiki)

Earthworms are hermaphrodites (her-MAFF-row-dites), having both testes and ovaries. The male sex organs are in certain segments and the female organs are in other sections. Still, it takes two worms to tango. So, how do they find each other? Mostly by happenstance. Although they can’t see or hear, they can feel ground vibrations or the movement of leaves or grass when another worm is passing by. With that, the worms start feeling around until they find each other. Exactly how fertilization takes place is complicated, but basically the worms align their bodies so that their heads are pointed in different directions, with the sex organs aligned for the transfer of sperm from one worm into the sperm receptacle of the other. Setae are used by the worms to hold their slippery bodies together. Following their pairing, they separate and both worms are ready to reproduce.

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Earthworm cocoons. (© Soil and Water Conservation Society (SWCS). 2000. Soil Biology Primer. Rev. ed. Ankeny, IA: Soil and Water Conservation Society.)

First, however, they must fertilize the eggs they are carrying. To do that, the clitellum secretes a ring of sticky mucus that slides forward, first over the segment containing eggs, which stick to it, and then to segments containing the sperm. There the sperm comes into contact with the eggs and fertilizes them. The clitellum continues its way forward, carrying fertilized eggs, until it slips right off the front end of the worm. It seals itself at both ends, forming a cocoon, and is left lying on or in the soil.

About the size of a grain of rice and shaped like a lemon, the cocoon holds between one and 20 fertilized eggs, depending on the species. The embryonic stage lasts from three weeks to five months, depending on the species. Embryos can survive underground until temperature and moisture conditions are just right for hatching. In unsuitable situations they may overwinter and hatch in spring. Embryos develop in a way similar to that of a bird in an egg, by consuming nutritive matter within the cocoon. When it’s all used up, the babies are ready to hatch and come out one end of the cocoon.

Earthworms produce between three and 80 cocoons a year, depending on the species. Worms that live deeper in the soil produce fewer cocoons. Earthworms become adults in 10 to 55 weeks, depending on the species. A worm without a noticeable clitellum is not yet a sexually mature adult. Lifespan ranges from four to eight years, depending on the species.

Food Sources
Worms living close to the surface eat plant matter — decaying roots, leaves, fruits, vegetables. Those living deeper mostly eat dirt, which contains fungi, bacteria, nematodes (a class of worms, usually microscopic in size), and other tiny organisms. So, either way, worm excrement contains nutrients beneficial to plants. Worms in compost bins feed also on whatever vegetative food is thrown into the bin — bananas, tomatoes, corn, potatoes, etc.

Predators
Snakes, birds, toads, rodents, moles, foxes, certain beetles, slugs, humans. Earthworms can be killed by a major fluctuation in soil pH (they prefer neutral acidity), soil lacking oxygen (such as clay soil) or moisture, insecticides, vermicides and other chemicals, and by tilling.

Night crawler at a glance
Appearance: Reddish. Slightly slimy. Head is at the end closest to the swollen segment.
Size: Body 8 to 10 in. long, about 150 segments.
Lifespan: Matures in 1 yr., lives up to 6 yrs.
Range/habitat: Common in gardens, lawns, grasslands.
Behavior: Nocturnal. Harmless. Comes to surface to mate and for food. In winter, hibernates below frost line.
Foods: Uses its mouth to pull decaying plant matter into burrow.
Cover/nesting: Vertical burrows, up to 8 ft. deep.
Reproduction: Any time of year, up to 38 cocoons.
Predators: Snakes, birds, reptiles, rodents, moles, foxes, certain insects, slugs, flatworms, humans.
Scientific classification
Kingdom: Animalia
Phylum: Annelida
Class: Clitellata
Subclass: Oligochaeta
Order: Haplotaxida
Family: Lumbricidae
Genus: Lumbricus
Species: terrestris

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