Among the animals that inhabit the soil, a very well-known one is often found: “the earthworm”, a worm classified as “annelid”, that is, with a body formed by a series of rings.
For many, earthworms are unpleasant beings; for others, a tremendously valuable animal, for being an extraordinary plowman and a fantastic natural fertilizer factory, thanks to its ability to move, aerate, and modify the soil.
There are not only earthworms, but also water worms (both marine and freshwater), preferring moist environments.
About 3,500 species of earthworms have been studied, some adapted to breeding, giving life to a product of great interest: worm humus. And as if that were not enough, they are an indispensable food for other species, and bait for fishing.
Keep reading and discover their characteristic traits, habitat, what they eat, how they reproduce, and the types of earthworms that exist.
What are Earthworms?
They are “annelid worms”, that is, their body is divided into various rings or segments, classified as oligochaetes “from the Greek oligo (scarce) and chaeta (hair)”, because of the tiny rows of hairs that run through their body, useful as gripping elements during movement.
Their origin is aquatic, so terrestrial species still completely depend on moisture to survive; they have nocturnal habits and an estimated lifespan of 5 years.
Did you know that…? Two earthworms (42,000 and 30,000 years old) found in the frozen subsoil of Siberia, were successfully revived by a group of scientists in Moscow, becoming the oldest living beings on earth.
Characteristics of the Earthworm
External Morphology
- Soft body, cylindrical, elongated towards the ends, with a shiny surface and divided into circular rings or segments, varying between 80 and 150 (earthworm), or 95 (red worm).
- They have bilateral symmetry (one half of the body is the same as the other).
- They do not have parapods (feet). Instead, the body is surrounded by hairs or bristles (chaetae) used for movement, and their arrangement depends on the species. Normally, they display two pairs of bristles on the belly and two pairs on the sides of the body.
- The first segment of the earthworm is its head, reduced, on whose end the mouth appears; it does not have teeth, ears, or eyes, but they are very sensitive to light.
- On the surface of the body, small orifices (genital, excretory, and sensory pores) are observed that communicate the internal part with the exterior.
- It has a wider segment, called “clitellum” that stands out from the rest of the body, and fulfills reproductive functions.
- The outer wall of the body is covered by a transparent and thin membrane that secretes a lubricating substance (mucus) that contains nitrogen, an extremely important nutrient for the plant world.
- The last segment contains the anus or pygidium.
- Variable size, ranging from 1 millimeter (aquatic species) to 3 meters in length (the giant earthworm of Australia).
- Depending on the species, it shows light brown, violet, yellow, pale pink, reddish-gray, or bright red coloring.
Did you know that…? Sunlight is very annoying and harmful to earthworms, and if they are exposed to it for a few minutes, they can die.
Internal Morphology
The general body cavity of the earthworm is called the “coelom”; it is a hollow space between the outer wall and the digestive system, filled with a foul-smelling fluid, blood vessels, and nerve cords.
Like its external part, the internal part of the earthworm is also divided into rings, formed by circular and longitudinal muscles that allow its movement when it contracts and stretches.
Digestive System
The digestive system of the earthworm consists of the mouth, pharynx, esophagus, crop, gizzard, intestine, and anus. The crop and the gizzard serve the function of the stomach; the crop stores the food (sac) and the gizzard grinds it.
What Do Earthworms Eat?
Any decaying organic matter becomes food for earthworms! Mainly dead leaves, grasses, vegetables, dead worms, animal feces, bacteria, or organic remains. Aquatic species also feed on small crustaceans.
When feeding, it can consume the equivalent of 90% of its body weight, and the discarded remains become an excellent organic fertilizer, rich in nutrients for the soil and plants (humus).
Nervous System
It consists of a pair of cerebral ganglia connected to a central nerve cord that receives information (moisture, temperature, brightness, touch sensation…) through sensory cells (in each ring), and taste cells (in the mouth).
Circulatory System
The circulatory system of the earthworm is closed and very developed, composed of five main blood vessels, one dorsal located above the digestive tube, and four ventral or secondary, interconnected to irrigate blood throughout the body.
How Many Hearts Does an Earthworm Have?
It has five pairs of hearts!, or aortic arches, structures that surround the esophagus and work together to distribute blood in the body.
Muscular System
Earthworms have a layer of circular muscles, and another thicker layer formed by longitudinal muscles, both layers allow the movement of their body as it contracts and stretches.
Did you know? Earthworms have been observed capable of moving stones more than fifty times their weight!
Excretory System
Formed by a large number of organs, called “nephridia” distributed throughout the body and connected to the outside by small orifices (nephridiopores), which allow the drainage of the generated wastes.
Respiratory System
The earthworm breathes through the pores of its skin (cutaneous respiration), so it must remain moist at all times for the gas exchange to occur.
Certain aquatic species have small structures that protrude from the body wall and act as simple gills.
How do earthworms breathe?
As they do not have lungs, they use the pores (small holes) in the tissues of their skin; the hemoglobin present in their blood takes in oxygen through these pores and expels carbon dioxide, making it necessary to maintain moisture during the process.
Reproductive system
Earthworms are hermaphrodites, meaning each individual carries both sexes. The male reproductive system consists of two pairs of tiny testicles and a funnel that contains the sperm, which communicates with the outside through the male genital pore.
The female reproductive apparatus includes a pair of ovaries, and a funnel with small egg sacs that communicate with the oviducts (tube that communicates the funnel with the outside) to exit through the female genital pore.
How do earthworms reproduce?
Despite being hermaphrodites (having both sexes), they do not fertilize themselves, so they need to mate with another to procreate. A pair of earthworms is attracted by the secretions of their body, wrapped in a mucous capsule and releasing both eggs and sperm.
Reproduction can be sexual or asexual. In sexual reproduction, fertilization is cross, meaning each earthworm receives the sperm from the other and keeps it in its female genital apparatus until the moment of fertilization.
After mating, each earthworm forms a small cocoon (ootheca), thanks to the liquid secreted by the clitellum (thicker part of the body); there, the sperm and oocytes (eggs) are stored.
Subsequently, the cocoon is deposited in moist soil, and it opens two or three weeks later, to give rise to new earthworms.
Asexual reproduction occurs when the earthworm’s body segments or divides into several parts; each of these can regenerate the body of the earthworm.
Did you know? You can tell if an earthworm is fertilized by observing the part of the clitellum (ring or thicker segment of its body), if it is swollen, it will soon lay a cocoon from which several earthworms will be born!
Types of earthworms
Depending on the habitat they occupy, earthworms can be terrestrial, freshwater, and marine.
There are worms that, despite also being called earthworms, do not belong to the annelids, such as the intestinal worms Ascaris lumbricoides (nematodes) and Taenia or tapeworm (platyhelminths).
Earthworm
It inhabits almost all regions of the planet (except Antarctica); lives in moist and loose soils, and is continuously moving from the surface layers to the deep, digging galleries while stirring and ventilating the soil, or under rocks.
It abounds in orchards and gardens, and is considered one of the most important soil organisms, increasing the decomposition of organic matter and the recycling of nutrients.
Depending on the area of the soil where they live and their diet, they can be subdivided into: epigeic, endogeic, and anecic.
Epigeic Earthworms
These are species of small size and bright red or brown coloration, which inhabit the surface of the soil (organic horizon), at a depth varying between 5 to 30 centimeters.
They are adapted to the variable conditions of the surface and feed on organic matter (dry leaves, manure). They rarely form burrows.
The most well-known epigeic species is the “California redworm or striped worm (Eisenia foetida),” of Eurasian origin, but owes its name to the locality of California, where its benefits in the production of organic fertilizer (humus) were discovered.
Its adaptation to a wide range of temperatures, the ability to feed on any organic waste, its rapid reproduction, and the intensive way it works to decompose them, classify the California redworm as one of the species of greatest interest.
The soil that passes through its body is transformed, and its excrements reach 5 times more nitrates, 7 times more Phosphorus, 11 times more Potassium, 2 times more Calcium, and 2 times more Magnesium than common soil!
Other varieties of epigeic earthworms include: Lumbricus castaneus, Helodrilus oculatus, Dendrodrilus rubidus, Dendrobaena octaedra…
What is worm humus?
Worm humus is the best fertilizer for agriculture, coming from the digestion process of certain species of worms (epigeic), which recycle organic matter in their intestine and transform it into fertilizer.
This natural process carried out by the worm is recreated in worm farms, using the technique of vermicomposting (worms present in the compost), controlling the conditions of light, temperature, and humidity.
The result is an ecological fertilizer, called “worm humus,” the best organic fertilizer for crops, rich in microbial flora, nitrogen, phosphorus, and potassium.
The California red worm (Eisenia foetida) is the most used; the African red worm (Eudrilus eugeniae), the Taiwanese red worm (Perionyx excavatus), and the nocturnal red worm (Lombricus rabellus) are also employed.
Did you know that…? In ancient Egypt, the qualities of the earthworm in soil fertilization were quite well known, and they were so carefully protected that if anyone dared to export a single worm out of the kingdom, they could be punished, even with the death penalty!
Endogeic Earthworms
They live permanently inside the soil, at a depth of 50 centimeters, in horizontal galleries (tunnels) that they excavate with their body; they feed mainly on the organic matter of the soil, or that which is dragged by rain or other organisms.
Their coloration is pale, varying between pink, gray, and light green. As they never leave the gallery where they live, it fills up with their feces, supplying nutrients to the soil.
Varieties: Allolobophora chlorotica, Octolasion lacteum, Apporetodea rosea…
Anecic Earthworms
They are large in size and display dark brown colors; they live in burrows or vertical galleries, in a U-shape, that can extend several meters deep.
They feed on plant remains and soil, which they find on the surface when they come out at night and drag into the gallery, and play a fundamental role in the aeration and conditioning of the soil, transforming the properties of the earth.
The characteristic specimen of this category is the common earthworm (Lombricus terrestris), which, thanks to its vertical excavations, facilitates the flow of water through the soil and the transport of nutrients to deep layers.
Also noteworthy is the genus of giant earthworms (Megascolecidae), with species like the Gippsland giant earthworm in Australia measuring 3.7 meters long.
Other varieties include: Didymogaster sylvaticus, Lumbricus friendi, Aporrectodea longa…
Did you know that…? An experiment carried out by researchers at Wageningen University, in the Netherlands, revealed that earthworms are capable of reproducing in soil with characteristics identical to that on the planet Mars.
Freshwater Worm
Lives in the mud, associated with freshwater habitats such as ponds, wetlands, riverbanks, swamps, wells, groundwater… They are very thin, their size varies between 1 millimeter and 3 centimeters long, and they feed on bacteria and organic remains.
Certain species are moss-like in shape and reddish in color, which allows them to capture food while moving in a striking manner.
Varieties: Tubifex, Naidinae, Olavius algarvensis, Lumbriculus variegatus…
Marine Worm
Very few worm species are marine. Commonly they are whitish in color; they are included in the families “Enchytraeidae and Tubificidae” and are abundant in cold regions, from the coast to areas of great depth.
Varieties: Branchiura, Enchytraeus albidus, Ainudrilus angustivasa…
Do worms harm plants?
Despite the countless benefits that worms contribute to the soil and ecosystems, some believe that their activity can have negative effects on plants, such as an excessive level of nitrogen, causing more fragile plants, or damage to their roots, stopping their growth.
Others indicate that they only represent a problem when there is no control over the population. In any case, the positive arguments about their contributions of nutrients and essential minerals for the correct development of plants outweigh the negatives.
Did you know that…? The Greek philosopher Aristotle named the worms “the intestines of the earth,” due to their multiple benefits and their mobility in the soil.