How Earthworms Work

Charles Darwin spent 39 years studying the humble earthworm and concluded that they are very valuable to the Earth's processes.
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Back in 1881, the biologist and creator of the theory of natural selection, Charles Darwin, published a book on a subject very near and dear to his heart. "The Formation of Vegetable Mould, Through the Action of Worms, With Observations on their Habits" was the culmination of 39 years of research into what is often considered among the humblest of creatures, the earthworm.

"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," he concluded [source: Darwin].

Darwin was on the money; at least by sheer numbers alone, earthworms have a tremendous impact on local ecosystems (though not all of it is good, researchers have recently learned). There are more than 3,000 species of earthworm on the planet, and they range in average sizes from.4 inches (1 centimeter) to 9 feet (2.7 meters) in length, although outsized monsters are routinely reported around the world [source: Earthworm Society of Britain]. In a single hectare (2.47 acres) of land, you may find 500,000 to 2 million individual worms. You may further find that the total wet biomass of all these worms equals 10 times the total weight of all the animals living aboveground on that same plot of land [source: Green-Pik].

Yet as ubiquitous as earthworms are, there is much we only recently have come to learn about them. In some northern regions of North America, there are no earthworm species around just a few hundred years ago. That's because during the last ice age, most earthworm species native to North America died out due to the change in climate. The earthworms you'll find today in abundance in the temperate woodlands and forests of northern North America are relatively recent immigrants from Europe and Asia, who hitched rides in the soil attached to plants imported to North America in the 19th and 20th centuries. In this clime, they've flourished and spread even farther, due to humans who purchase them as fish bait and often release them into the wild or inadvertently give them long distance rides attached to the tread of car tires [source: Kinver].

If you thought the only cool thing about earthworms was their ability to regenerate, or perhaps that they play pinochle on your snout when you die, read on and have your socks knocked off.

Classifying Earthworms

Worms are generally classified by the position in the soil they inhabit.

Earthworms typically fall into one of three categories based on where in the soil they live. If you took a spade and dug down a couple feet into soil that's covered with a top layer of dead leaves in just about any temperate woodland in North America or Europe, you'd likely encounter all three classifications of earthworms.

Around the point where the leaves sit just above ground and begin to mix with the topsoil, or litter horizon, you'll find the epigeic class of earthworms (epi means "above" and geic derives from "Gaia" or "Earth"). These worms play a big role in decomposing leaves and other organic matter that falls to the woodland floor simply by eating it. Redworms, also called manure worms, are among the comparatively small epigeic earthworms, and they're commonly sold for use in compost piles.

Beneath the soil surface, in the dark, rich topsoil layer, you'll find the endogeic class of earthworms. Endogeic earthworms generally move parallel to the surface, leaving horizontal burrows and eating organic material found only beneath the ground, like dead plant roots. As they spend their entire lives underground and out of the sun, they lack pigment and are usually pink, gray or white. Scientists know the least about the behavior and life cycle of endogeic worms.

The deep-dwelling anecic class is the most familiar types of earthworms. Their burrows are strong enough that they tend to be permanent and can reach several feet beneath the surface. They too feed on leaf litter, but anecic worms pull whole leaves down into their burrows where they feed at their leisure. In a single night, Lumbricus terrestris -- the common European earthworm, known in the U.S. as the night crawler -- can travel as far as 62 feet (19 meters) along the soil surface in search of food [source: Werner].

Earthworms are segmented, which means they belong to the phylum annelid, meaning "ringed worm" [source: Raskoff]. The 100 to 150 ringed segments that help to define earthworms are also its means of locomotion. Each ring can move as a muscle and independently from the rest of the segments, and the worms move through a process of expansion and contraction. At the front, or anterior, end of the worm are retractable bristles called setae. When extruded, the setae anchor the front end of the worm in place, which allows the worm to contract the segments in the rear, dragging them forward (they can also move backward). By extending its front, anchoring itself and then contracting the rear, the worm is able to move through the soil.

Earthworm Anatomy

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Earthworm Reproduction

Earthworm reproduction or just unfortunately tangled up?
Earthworm reproduction or just unfortunately tangled up?

One of the most interesting aspects of earthworms is their sexuality. Earthworms are simultaneous hermaphrodites, meaning worms have both male and female reproductive organs. During sexual intercourse among earthworms, both sets of sex organs are used by both worms. If all goes well, the eggs of both of the mates become fertilized. You can imagine this is a highly efficient way of ensuring the survival of the species. Vermicomposters, people who raise worms and other organisms to compost, report that their earthworm populations typically double every 60 to 90 days [source: Werner].

To copulate, two worms line up against one another facing opposite directions. In this position, both worms excrete so much mucous, that what is called a slime tube forms around their bodies. Each worm ejaculates sperm from its sex organs into this slime tube and it is then deposited in the other worm's sperm receptacle. The act of mating is completed, but the process of reproduction still continues as each worm goes its separate way [source: Conrad].

You know the wide band near the front of any earthworm? That band is called the clitellum and it's responsible for producing another tube of mucus. This band is passed forward toward the mouth end of the worm. As it travels forward, the mucus passes over the sacs containing the worm's own eggs, which stick to the slime. Attached to the slime tube, the eggs then pass over the seminal receptacle, where the other worm's sperm is kept. The eggs and sperm come in contact in the slime tube and if all goes well, the eggs are then fertilized.

The band of slime is wriggled off the head of the worm and forms a cocoon in the shape of a lemon for the anywhere from four to 20 worm eggs that the common European earthworm typically lays. In about two to three weeks, the newborn worms will hatch and emerge from the cocoon into the soil. This cycle of reproduction can happen every week to 10 days, another reason earthworm populations can grow so quickly [source: Barrett].

Earthworms can also reproduce themselves if need be. They can regenerate new segments if they lose a few. Most earthworms are better at regenerating tails than heads, but some can. They don't reproduce asexually, however; only half (and likely the head half) of an earthworm split in two will regenerate into a full worm once again [source: Tomlin].

The Life and Death of the Typical Earthworm






The Helpful Earthworm







The Harmful Earthworm?

Earthworms can reduce the understory in a woodland like this one. Without it, larger animals that feed on those plants can't thrive.

Sure, they decompose organic matter, break it into usable nutrients for their local ecosystems and recycle compost. But as helpful as they are, researchers have recently come to see the earthworm in another way: as destructor.

The very same traits that make them beneficial can also make earthworms harmful. Study after study finds that earthworms are voracious eaters: One found they can break down about 90 percent of the surface leaf litter in an apple orchard in a single winter; another estimated that earthworms can consume about 9,240 pounds (4,200 kilograms) of organic litter for every 2.47 acres in 11 weeks [sources: Werner, Werner and Bugg]. But that same leaf litter that earthworms eat so efficiently is also the habitat for spiders, lizards, beetles and other arthropods, frogs, snails and innumerable other species. Put simply, earthworms eat these other animals and plants out of house and home.

The litter horizon also serves as protection for seeds that grow to form the understory plant community of a forest -- all the smaller plants and saplings that make up the lower canopy of vegetation near ground level. Studies have found earthworm activity can reduce both the total coverage and the diversity of plant species among the understory canopy by between 25 and 75 percent [source: University of Minnesota]. This effect ripples up the food chain to affect deer and other vertebrates that depend on that vegetation for food.

There is also evidence that earthworms also have a counterproductive effect on carbon sequestration. One of the major roles of soil is to act as a sink for storing carbon and prevent a disproportionate release into the atmosphere. That organic material earthworms eat have carbon locked up inside, in addition to nitrogen; worms unlock this carbon by breaking down the organic matter, and can contribute to as much as an additional 28 percent of carbon released from the soil by researchers from Colgate University's estimate [source: Kinver].

In North America, all of this is in line with the fact that, despite the many benefits of their presence, most earthworms are non-native, invasive species. The loss of earthworm species during the last ice age left northern woodlands to successfully adapt to conditions without them. With the reintroduction of earthworms to these ecosystems in the last few hundred years, the effects have been the same as with the introduction of any other invasive species: What was once the "natural" order is changing, as ecosystems evolve once again to adapt, this time to the presence, not the absence, of earthworms.

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More Great Links


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