OLIGOCHAETES: CHARACTERISTICS, NUTRITION, REPRODUCTION, HABITAT - BIOLOGY - 2025

The oligoquetos or worms are segmented worms of the phylum Annelida, Clitellata class, with few setae or bristles, which are external appendages tiny rod - shaped used for locomotion. They comprise about 6,000 species grouped into about 25 families.

The oligochaetes have an internal cavity (coelom) partitioned in the form of numerous consecutive chambers. This segmentation determines portions called metamers, of more or less identical structure, this being a characteristic found in annelids, arthropods and chordates, including vertebrates.

Earthworm (Lumbricus terrestris)

The metamerization of the body represents an adaptive advantage, since it allows for specialization in the different portions of the animal. In the body the head that contains a brain is distinguished, followed by a trunk formed by up to 800 segments that culminates with the anus.

In general, their bodies are covered by a moist cuticle with epithelium that presents glandular and sensitive cells. They also have longitudinal and circular muscle layers, which allow them to move.

Your ganglia, nerves, blood vessels, muscles, and gonads are metamerized. While the digestive system is the exception, as it is not segmented, they are mainly terrestrial with some representatives of freshwater and marine.

One of the best known representatives of the oligochaetes is the earthworm (Lumbricus), which is often used as a model of the subclass.

Body and locomotion

The metamers are observed on the outside of the cylindrical body, as rings that internally divide it through septa. These septa generate the segmentation of your coelom, which is the fluid-filled internal cavity. There is also a segmentation of the coelom into left and right compartments.

In the anterior segments of the body of the oligochaetes there are specialized structures of the nervous, digestive, circulatory and reproductive systems.

Externally, the cylindrical body of the oligochaetes is surrounded by two sets of segmented muscles, one of them arranged longitudinally along the body and the other encircling each segment.

Movement generally involves anchoring through the ketae -which are presented in pairs- and the forward elongation of the part anterior to this anchored segment, thanks to the contraction of the muscles that surround the segments.

Then the front ketae are fixed and the longitudinal muscles are contracted, releasing the posterior segments that are drawn forward.

Digestive system

Its non-metamerized digestive system is a straight tube that constitutes the body axis, located in the center of the coelom and supported by longitudinal mesenteries and partitions that cross the length of the body.

The worm's mouth connects to a muscular pharynx. It then presents a crop where it stores what is ingested and later is a gizzard, where it grinds its food with the use of soil particles.

The remaining intestinal tube digests the ingested food with the help of the secreted enzymes, until it reaches the rectum that precedes the anus.

Excretory system

This system fulfills the functions of filtration, reabsorption and secretion of internal fluids. It is made up of a pair of metanephridiums for each segment (except for the segment of the head that lacks these structures), which are convoluted ducts that lead to an external lateral pore called nephridiopore, where waste substances are expelled into the environment.

Circulatory system

The circulatory system has vessels arranged longitudinally throughout your body. One glass is usually located on the back and two on the belly.

In the case of earthworms, they also have five pairs of hearts or discrete and contractile dilations of the blood vessels, which connect the dorsal and the greater ventral vessels. Through irregular contractions, the hearts force the movement of the blood.

Within the vessels circulates the red hemolymph that contains hemoglobin and cells similar to white blood cells, called free amoebocytes.

Respiratory system

Respiration is generally carried out through the skin by simple diffusion, as most do not have developed respiratory organs. However, external gills can be found in some aquatic species.

Nervous system

Its nervous system is made up of an anterior ganglionic mass called the brain, from which two nerves originate that form two longitudinal cords lateral to the intestine, called the ventral medulla.

In addition to this centralized nervous system, oligochaetes have sensory cells that perform functions as tactile, taste, light receptors (photoreceptors) and moisture detectors (hygro-receptors). Through touch receptor cells they can respond to vibrations in the ground.

Moisture receptors are very sensitive cells and are found in the first anterior segments, where in turn are abundant light sensitive cells. The latter are also presented on the back of the body.

Figure 1 Schematic of the anterior part of an Oligochaete (Modified from

Nutrition

Oligochaetes feed on vegetation, decomposing organic materials and debris. Earthworms, for example, ingest soil that passes through their digestive tract and subsequently excrete crumbled and enriched matter.

Since earthworms also aerate the soil when feeding and this favors soil fertility for plant growth, earthworms are considered to play an important role in soil maintenance and nutrient circulation.

Reproduction

Earthworms are hermaphroditic, meaning that both male and female reproductive organs are present in the same individual.

Some can also reproduce by parthenogenesis, a special type of reproduction based on the development of unfertilized female sex cells, from which a new individual is generated.

When they mate, they place their heads in opposite directions and their ventral surfaces come into contact, joining through mucous secretions from their cliteli, which are thickened bands of the epidermis.

Before separating, both exchange sperm that they deposit in the partner's receptacles. Finally, two or three days later, the clitellus of each one secretes a mucous band or cocoon, which will house the mature ovules and the sperm received from the couple.

Once the eggs are fertilized by sperm, the fertilized eggs are imbued in a capsule or cocoon, which is released to the outside. Future worms will be born from the cocoon.

Copulation of earthworms.

Habitat

Oligochaetes colonize a wide variety of habitats: terrestrial, freshwater and marine. They can constitute up to 90% of the biomass of soil invertebrates, in addition to being pillars in the construction of ecosystems, since they provide aeration and fertilizers to this matrix.

The biogeography of oligochaetes has been studied extensively and has contributed to the development of theories about the evolution of our planet, such as plate tectonics and vicarious biogeography.

Biotechnological applications and diverse uses

There are numerous biotechnological applications of oligochaetes (specifically earthworms). Some of its uses are as follows:

• In the production of fertilizer or humus, liquid (also called foliar because it is applied to the leaves of the plants) or solid (to be applied to the soil).
• As a protein source for animal and human food (earthworm meal).
• As bioindicators of contamination, in tests to measure the acute toxicity of chemical substances such as pesticides (specifically, the Eisenia foetida species is often used in these tests).
• In the recovery and rescue of affected and / or degraded soils.

Some curiosities

Aristotle was one of the first people to study the role of earthworms in turning the soil; aptly calling them: "the Intestines of the Earth."

At the end of the 19th century, Charles Darwin wrote about the extreme importance of earthworms, in his last work: "The formation of vegetable mold through the action of earthworms."

Darwin developed aspects such as the importance of these worms in the decomposition of dead plants and animals that reach the soils, in the continuous rotation and maintenance of the soil structure, aeration, drainage and fertility of the same.

Prior to the publication of Darwin's work, earthworms were commonly considered pests of soil-dwelling crops.

However, Darwin's views on the benefits of earthworms were later supported and expanded. Notably, many of Darwin's observations were so advanced that nearly half a century passed before many of them were confirmed.

References

1. Brusca, RC & Brusca, GJ (1990). Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusetts. USES
2. Chang, C.-H., Rougerie, R., & Chen, J.-H. (2009). Identifying earthworms through DNA barcodes: Pitfalls and promise. Pedobiologia, 52 (3), 171–180.
3. Darwin, C. (1881). The Formation of Vegetable Mold Through the Action of Worms with Observations on Their Habits, Murray, London. Taken from darwin-online.org.uk
4. Pop, AA, Wink, M., & Pop, VV (2003). Use of 18S, 16S rDNA and cytochrome c oxidase sequences in earthworm taxonomy (Oligochaeta, Lumbricidae). Pedobiologia, 47 (5-6), 428–433.
5. Qiu, JP, (1999). Earthworms and their application in environment protection. I. Earthworms and their functions in ecosystem. J. Shanghai Agri. Coll. 17, 227-232.
6. Sales D., F. (1996). Worm meal, protein alternative in the tropics and types of food. Folia Amazónica, Vol. 8 (2), 77-90.