Earthworms are not all the same, according to Reidun Pommeresche. She studies earthworms and soil at the Norwegian centre for organic agriculture NORSØK.
Some species stay mostly in the same place, such as in the soil beneath a bush or tree in your garden. Others are more nomadic.
And not all soil is the same to an earthworm. Some prefer topsoil rich in plant remains and livestock manure, while others thrive best in compost.
The common earthworm is the largest
The common earthworm is the largest species. Its scientific name is Lumbricus terrestris, although it is also known as the lob worm and nightcrawler. It grows to about 30 centimetres long and can become as thick as a little finger, according to Miljølære (link in Norwegian).
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“The common earthworm lines the walls of its burrows with excrement and mucus. These earthworm tunnels become excellent drainage pipes and air channels in the soil,” says Pommeresche.
The common earthworm's tunnels can be as deep as three metres.
You may be lucky enough to spot a common earthworm when it comes up to the surface. It does this quite often to find food. But it's probably easier to spot the signs it leaves behind.
“It prefers to keep its tail inside its burrow. Then it gathers plant debris and other material into a large ring around the entrance,” she says.
If you pay close attention, you can see this for yourself.
It usually emerges at night
In autumn, the ground beneath a tree is often covered by an even layer of fallen leaves. By spring, those leaves may instead be gathered into distinct little piles.
“If you remove one of those piles, you'll find an earthworm burrow underneath,” says Pommeresche.
Because it puts so much effort into maintaining its burrows, the common earthworm does not like soil that is ploughed or disturbed too frequently. As a result, it is most commonly found in places with stable vegetation.
The common earthworm is nocturnal. It is sensitive to sunlight and heat and prefers damp conditions.
“That's why they usually come up at night, and sometimes when it's raining,” the researcher says.
When an earthworm emerges from the soil, it may be for reasons quite different from what most people assume.
This is where the earthworm lives.(Photo: Reidun Pommeresche)
Meet to exchange genes
"When it rains, they mainly come up because they're looking for a new place to be. Some are also looking for a mate to reproduce with," says Pommeresche.
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"It's best for earthworms to be on the surface when it's raining. They dry out quickly and are vulnerable to UV radiation. The sound of raindrops indicates that it's safe to come up."
Some birds have even learned to tap their beaks on the ground or make dripping sounds with their feet to imitate the sound of rain and lure earthworms into becoming a protein-rich meal, Pommeresche explains.
"Earthworms have no long-distance vision and navigate the ground using other senses. That's why we sometimes find them on asphalt and gravel roads when the sun suddenly comes out again, because they had nowhere to burrow down. If they manage to reach a grassy area or the edge of a ditch, they'll burrow back into the soil," the researcher says.
Speaking of reproduction: Earthworms are hermaphrodites, but they usually still mate in order to exchange sperm. This makes their offspring more robust by avoiding inbreeding. After mating, both earthworms can lay eggs, which are called cocoons.
These cocoons develop into new earthworms.(Photo: Reidun Pommeresche)
They don't drown
Pommeresche debunks the myth that earthworms come to the surface because they would otherwise drown when it rains.
"Earthworms can actually survive for several weeks in water, as long as it contains oxygen," she says.
Earthworms don't have lungs. Instead, they absorb oxygen through their skin. This also means they need their skin to stay moist in order to absorb oxygen.
"Earthworms are excellent at turning over and mixing soil when it's moist. That's one reason they thrive in many parts of Norway," says Pommeresche.
230 earthworms per square metre is completely normal, according to researcher Reidun Pommeresche.(Photo: NORSØK)
They don't like frost
Earthworms are found throughout Norway, from south to north and from east to west. But researchers still don't know exactly how high into the mountains they can survive, according to Pommeresche.
What is known is that they need temperatures above freezing to function. If the frost penetrates too deeply into the ground, as it does in the high mountains, earthworms cannot thrive.
During winter, they move deeper into the soil to escape the frost.
The milder climate means there are more species in southern Norway than farther north, especially in lowland areas. In total, Norway has at least 19 species, according to the Great Norwegian Encyclopedia.
They choose different habitats
Different earthworm species occupy different ecological niches. Some are found in nutrient-poor environments with acidic soil, such as the red wiggler (Bimastos rubidus). The square-tailed worm (Eiseniella tetraedra) lives along the edges of streams and is especially well adapted to wet environments.
In compost heaps you'll find the manure worm (Eisenia fetida/andrei). It prefers soil that is extremely rich in nutrients, says Pommeresche. Norway has two of these earthworm species, both of which gather wherever there's a large amount of organic material piled together.
"I've found these only very rarely in fields and meadows. If you have ground cover in your garden and a thick layer of lawn grass, they can also thrive there. But in cultivated soil, such as where grains or vegetables are grown, there's often too little food for them to thrive," says Pommeresche.
The manure earthworm is also known as the tiger worm because of the stripes that appear when it stretches its body.(Photo: Garna Zarina / Shutterstock / NTB)
Grey worms are the most common
With a bit of practice, you can tell the most common earthworm species apart. Manure worms, for example, have a dark, slightly burgundy-red colour. When they stretch out, pale bands appear between each segment of the body, which is why they are also called tiger worms.
Grey worms (Aporrectodea caliginosa)and pink worms (Aporrectodea rosea) can be found in gardens and all types of cultivated soil. You rarely see them on the surface.
They continually burrow through fresh soil, creating new tunnels that they seldom return to. What they eat comes out the other end as fine, nutrient-rich soil. These species are the best soil mixers we have.
Some of the most common earthworm species in Norway.(Photo: Reidun Pommeresche. English translations added by Science Norway)
Pommeresche has studied just how industrious they are.
"If you have 230 worms in one square metre of soil, together they can consume 20 kilograms of soil in a single year," says Pommeresche.
230 worms per square metre is a normal amount, according to the researcher. But there can be several times as many if the soil is nutrient-rich and favourable for earthworms.
She has found as many as 900 worms in meadows with grass and clover at depths between 0 and 20 centimetres.
It's the grey worms that are capable of consuming such large amounts of soil. The common earthworm works more slowly because it invests much more effort in building its more permanent burrows.
Earthworms produce impressive amounts of excrements relative to their size.(Photo: Reidun Pommeresche)
A piece of string with advanced abilities
Earthworms perform an important job in the soil. They make it looser, fertilise it, and redistribute nutrients. Many people also think of earthworms as fishing bait. What does earthworm expert Pommeresche think about that? Is it a little inhumane?
Pommeresche prefers not to think too much about it.
"It just looks like a little piece of string. It doesn't have eyes or other cute features that make people become attached to it," she says.
Even so, it has many senses and abilities that we consider quite advanced, says the researcher.
"The earthworm is the first of the invertebrates to have a closed circulatory system. It has five hearts that pump blood around its body. And even though it cannot see, it has countless sensory cells. It can smell its surroundings and locate organic material in the dark underground," she says.
The manure earthworm also has an effective defence mechanism against predators. From a small yellow sac near its tail, it can release a yellow fluid that smells and tastes unpleasant.
"That's why some people believe the manure earthworm isn't a very good bait for fishing," says Pommeresche.
Earthworms have muscles and claws
Earthworms have no skeleton. Instead, the fluid inside their bodies, together with their muscles, stiffens the body and allows them to move, Pommeresche explains.
What they do have is an abundance of muscles. This is how these soft-bodied creatures are able to force themselves down into the soil and continue moving deeper and farther.
Circular muscles make the worm thin. Longitudinal muscles contract the body, making it shorter and thicker. These muscles appear as rings around the body and are especially powerful near the front end.
Tiny bristles help the earthworm move forward and prevent it from slipping backward.
If you or a bird try to pull an earthworm out of the ground, it will also use these bristles like claws to grip the soil tightly, giving it a chance to avoid being eaten.
Eight longitudinal rows of tiny bristles help the earthworm move.(Photo: Reidun Pommeresche)
Peaceful creatures
Earthworms do not have teeth, but they do have a gizzard, much like birds.
This muscular organ contains grains of sand that grind up what the earthworm eats: plant debris, microorganisms, and soil. What comes out is rich in both plant nutrients and soil carbon.
Earthworms can live for many years. Common earthworms as old as 12 years have been found in the wild.
In captivity, they have lived for more than 30 years. Other earthworm species reproduce more quickly and do not live as long. They typically reach about four years of age.
"But many animals want to eat these protein-rich worms, so in the wild, a couple of years is considered a long life for an earthworm," says Pommeresche.
