Earthworm

Powerful Little Earth Movers

Just as bees are crucial to pollinating flowers, earthworms are crucial to having those flowers grow in the first place. These worms aerate the soil and allow oxygen, water, and nutrients to enter, which then enter the roots of the plants. Their castings also get nutrients into the soil, and they can add 10 tons of their castings to the soil in a year.

Other earthworms might not live in the soil, but they help break down materials that go into manure and compost. They’re also good decomposers of dead animals. It would be hard to maintain gardens and farms without them.

3 Incredible Facts!

Here are some facts about this essential invertebrate:

• Earthworms writhe when people pick them up because the pressure causes their muscles to contract, and the very salt in the person’s skin is toxic to them.
• The longest ever found was a member of the Amynthas mekongianus species. This 10-foot long worm was found on the muddy banks of southeast Asia’s Mekong River.
• There are about 180 species of earthworm in the United States and Canada, and about 60 of them are invasive.

You can check out more incredible facts about earthworms at this link.

Classification and History

Because there are so many species of earthworms, there are many genus and species names. All, however, belong to the phylum Annelida, which means “little rings” in Latin, to the Class Clitellata, and to the Order Opisthopora. There is some dispute about subclasses, suborders, and families, but overall there are about 7,000 species. Only 150 species are common to different areas of the world. The rest are specific to their locale.

Fossil evidence indicates that earthworms originated on the sea floor well over 500 million years ago. It is thought that they began as anemone-like organisms then became Facivermis yunnanicus in the Cambrian period when other complex animals evolved in the sea. This worm-like animal had five pairs of spiny legs up front and a swollen end.

There are three categories of earthworm:

• Those that don’t burrow but live in leaf litter or compost to eat decomposing organic matter;
• Those that feed on and burrow horizontally into shallow layers of soil; and
• Those that dig deep, permanent vertical burrows into the soil and come to the surface to eat plant material.

Appearance

These worms have some of the most identifiable anatomies of all invertebrates. Who hasn’t seen the long segmented, slimy pink, gray, or brown cylinder (pinched at both ends) that is an earthworm? But if you look at it through a magnifying glass, you might see that its segments, save its front and back, are covered with rows of tiny hairs called setae. These help steady the animal as it moves and anchor it when it mates. The number of segments the worm has depends on its species.

The mouth is also protected from ingesting unwanted soil by a flap called a prostomium. The prostomium helps the worm feel and sense where it is. Some worms use this structure to grab onto vegetation and bring it into their burrow to eat. By the way, the worm doesn’t have teeth, although it has a strong mouth full of muscles to push its food into its digestive system.

Earthworms lack ears and eyes, but they can still sense their environment. They can sense light through structures called the Light cells of Hess, and receptors that can sense vibrations, touch, taste, and smell.

A person will also notice that the worm has swelling over a few of its segments near the front. This is the clitellum, and it houses the animal’s reproductive system. The back of the body is often cylindrical but it can be different shapes depending on the species. The cylinder’s last segment, the periproct, has the anus.

Behavior

Basically, earthworms dig through the soil, rotting leaves, manure, or compost. They do this through contractions of the muscles in their segments in ways that resemble the contractions of the human digestive system. The movement even has the same name: peristalsis.

The worm uses brute force to move dirt or debris. A baby worm can move soil that’s 500 times its weight out of the way. As it moves, its skin is protected both by a cuticle and by mucus. The mucus contributes to a soft clicking noise the worm makes as it burrows.

Earthworms can also regenerate their segments from the head end like many lizards can regenerate their tail if it’s lost. Indeed, one species, Eisenia fetida can grow into two worms if it’s cut in two. Speaking of E. fetida, this worm gives off a bad smell if it’s roughed up, and other worms taste bad to would-be predators.

Habitat

These animals are found in places that are warm, dark, and moist. This can be soil, rotting vegetation, or compost. They are found all over the world, and some species have become invasive. Although earthworms like it warm and wet, it seems they don’t like it too warm. There are fewer of them in tropical climates than in temperate zones. They do best when the temperature is between 59 and 86 degrees Fahrenheit.

Diet

These animals can be said to be omnivorous, for they eat both plant and animal matter. Basically, they eat the very stuff they are moving through, whether it be soil, manure, or decaying plant material. Their digestive system is very simple. It is just a straight tube from the mouth to the anus, without the bends or curves, or coils that are found in other animals. When the food is digested, what’s left comes out in little pellets called castings.

The worm doesn’t use all of the nutrients in its food, so much of it, like nitrogen, is present in the castings. This makes castings just the thing for fertilizing soil. For a complete list of foods earthworms eat, check out our “What Do Earthworms Eat: Their Diet Explained” page.

Predators and Threats

The number of animals that like to eat earthworms is too long to list here, but they include robins and other insectivorous birds. Frogs and toads snap them up in the blink of an eye. They are eaten by turtles, salamanders, slugs, snails, snakes, leeches, foxes, jackals, Malayan civet cats, moles, hedgehogs, and hog badgers. They are parasitized by nematodes and flukes and mites. Cluster fly larvae eat newly hatched worms.

Of course, humans impale them on hooks to catch fish and feed them to their reptilian pets. Climate change may also pose a threat to the worms if the pH or temperature of their habitat is drastically changed. For now, they are abundant and their conservation status is not evaluated.

Reproduction and Life Cycle

The earthworm is usually a hermaphrodite, which means it has both male and female organs. As a hermaphrodite, it is both father and mother to its babies at the same time. A few species can reproduce without their eggs being fertilized. This is called parthenogenesis. There are even some species that have to mate anyway for even parthenogenetic reproduction to occur.

Earthworm reproduction is complex and a bit sticky. First, they mate on the surface, which means they usually mate at night. Their reproductive organs are stored in their ninth to their fifteenth segment. These segments store either sperm or eggs or sacs that store sperm, and they have to come into contact for babies to be made.

However, the way this is done is ingenious. First, the two worms join nose to tail and exchange sperm. After they’ve done this, the clitellum starts to secrete a slimy tube that forms a ring around the worm. When this happens, the worm wriggles back out of the ring. The tube picks up the worm’s eggs, and as the earthworm continues to wriggle out of the ring, it picks up the sac that has collected sperm, and the sperm fertilizes the eggs. When the worm backs completely out of the ring, the ring closes up into sort of a cocoon, which is hidden in the soil.

The fertilized eggs inside develop and hatch out into little worms after about three weeks to continue the life cycle. Baby worms do not undergo any sort of metamorphosis such as that seen in amphibians and insects. They are tiny replicas of their parents and simply get bigger.

By the way, if there’s not much food or if becomes too cold or dry, the life cycle can be put on hold. Earthworms are capable of estivation, which means they basically shut down their life processes until conditions improve. These invertebrates can live eight years, but most don’t live nearly that long. Many don’t even manage to hatch out of their cocoons.

Population

These worms are notoriously abundant, so in those places where they’re invasive, it is difficult to get rid of them. The one thing humans can do is not transport species of earthworms to places where they are not native. This can be tough because it’s hard to tell them apart. Scientists can only be sure of their classification through minuscule differences in anatomy, such as the number of segments or whether the male pores are in front of or behind the female pores.