LEECHES: CHARACTERISTICS, RESPIRATION, REPRODUCTION - BIOLOGY - 2025

The leeches are animals Hirudinea class. It is a class belonging to the Annelida phylum whose members are known worldwide as bloodsuckers. They are characterized by having a flattened and segmented body with suction cups that allow them to attach to the host.

This class was first described in 1818 by Jean Baptiste Lamarck. Likewise, some specimens belonging to this class constitute known parasites of some vertebrates such as fish, reptiles and amphibians.

Hirudinean specimen. Source: NOAA Great Lakes Environmental Research Laboratory

In the field of medicine, Hirudineans have been used in special procedures such as bleeding. Similarly, the substances that they synthesize and that are present in their saliva have numerous applications, such as analgesics and anticoagulants, among others.

Taxonomy

The taxonomic classification of the Hirudineans is as follows:

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Annelida.

- Class: Clitellata.

- Subclass: Hirudinea.

characteristics

The Hirudineans are multicellular eukaryotic organisms, which means that their genetic material (DNA) is found enclosed in the cell nucleus, forming the chromosomes. Likewise, they present cells that have undergone a differentiation process and have specialized in various functions.

During its embryonic development, three germ layers appear: ectoderm, endoderm and mesoderm. From these layers all the organs that will make up the adult animal are formed.

These animals have variable colors: there are black, brown and even red. They are also characterized by presenting a structure known as the clitellus, which actively participates in the reproduction process.

They are heterotrophic individuals, since they are not capable of synthesizing their own nutrients, but instead feed mainly on other living beings or their fluids.

Regarding symmetry, the Hirudineans present bilateral symmetry, that is, if a line is drawn along the median plane of the body, two exactly equal halves are obtained.

Morphology

The hirudíneos have a flattened body in the dorsal-ventral direction, with a size ranging from 5mm to 45 cm.

Like all members of the phylum Annelida, the Hirudineans have a segmented body. Depending on the species, the number of segments will vary. There are 15, 30 and 34 segments. Each segment is known as a metamer.

Its body is divided into three areas or zones: head, trunk and pygidium. Likewise, the external segmentation does not match the internal one, since each metamer internally encompasses several rings.

One of the characteristic elements of the Hirudineans are the suckers. They present two, one at the cephalic pole level, in the mouth, and a posterior one, at the opposite end of the animal. The suction cups are useful for both feeding and locomotion.

Hirudinea fixed by its two suction cups. Source: Pixabay.com

Contrary to what happens with the other annelids, the bodies of the Hirudineans do not present any type of prolongation. They don't have podiums or podiums.

On both sides of the body they present a series of pores that are the mouth of the metanephridiums. Those pores are called nephridiopores.

Animal wall

The body wall of the Hirudineans is made up of several layers:

- Cuticle: it is the outermost coverage. It is the typical anelidian cuticle. It has protective purposes and is produced by the epidermis. It has epithelial cells, including glandular and sensory cells.

- Basement membrane: it is located below the cuticle. It is very thin.

- Circular and longitudinal muscles: made up of muscle fibers whose function is to contract and relax to promote the movement of the animal.

Coelom

It is a typical cavity of animals called coelomates. This has different configurations, depending on the animal group. In this sense, in the Hirudineans the coelom is quite small and is filled with a type of tissue called botryoidal or cellenchyme.

Likewise, in the Hirudineans, the coelom is limited to being a set of narrow channels.

Digestive system

The digestive system of the Hirudineans is made up of a complete tube that covers the entire length of the animal.

This tube is divided into functional areas such as the mouth, pharynx, esophagus, crop (stomach), intestine and rectum. It is important to note that the crop has some sac-like extensions that are known as blind. These are of great importance, as they provide the animal with the ability to store large amounts of food.

Enlargement of the mouth of a hirudinean. Source: Erin Hayes-Pontius

The digestive system is divided into regions: Stomodeus (anterior), made up of the mouth, jaws (in species that have them), pharynx and esophagus; a middle region, made up of the intestine; and finally the proctodean (posterior), which contains the rectum and anus.

In the different orders that make up this subclass, the configuration of the digestive system can vary. For example, there are some that have strong jaws, as well as others whose pharynx is so strong that it can crush animals, or pharynx specialized in suction.

In the interior of the digestive tract, there are several bacterial species that are intended to help the Hirudinians in the digestion and degradation of proteins. This is thanks to the synthesis of specialized enzymes for this purpose.

Nervous system

The Hirudinean nervous system is primitive and is basically made up of neuronal groupings known as ganglia.

They present two cerebral ganglia, two ventral nerve cords, a periesophageal collar, and metameric ganglia. The neurons that make up the nervous system are of the follicular type.

They also have primitive but functional sensory organs. Among these are the eyes, which are cup-shaped and have the function of capturing visual stimuli. They also have free nerve endings that serve as tactile and vibration receptors.

Reproductive system

As is well known, members of the hirudine subclass are hermaphrodites. This means that the same specimen has male and female reproductive organs.

The female reproductive system is made up of a pair of ovaries that are located in some species of bags called ovisacos. Ducts called oviducts emerge from each oviscus, which lead to a primitive vagina. This vagina opens to outer space through a pore that is located in most specimens in segment 11.

On the other hand, the male reproductive system is made up of several pairs of testicles. From each testicle comes a duct, the efferent canal. These efferent channels form a larger duct, the vas deferens. There are two vas deferens. These in turn fuse in the animal's midline to form a broad structure, known as the seminal vesicle.

A duct, the ejaculatory duct, starts from the seminal vesicle, which leads to another enlargement called atrium, which opens to the outside through the male genital pore.

Excretory system

It is made up of excretory organs called metanephridiums. They are located in pairs, in number between 15 and 18 pairs.

These present a nephridian vesicle and open to the outside through a nephridiopore.

Circulatory system

The Hirudineans have an open circulatory system. They also have two hearts and two blood vessels that carry blood in opposite directions. One of them towards the head and the other towards the opposite pole of the animal.

Both vessels are kept connected through a lacuna or vascular sinus. Likewise, the animal has a highly developed blood plexus at the level of the most superficial layer of the animal.

Breathing

The type of respiration that Hirudineans present is cutaneous. This is done through simple diffusion through the skin.

It is important to note that the skin of the Hirudineans has a wide network of blood vessels. It is through the blood that gases such as oxygen (O ₂) and carbon dioxide (CO ₂) circulate.

Through passive transport, specifically diffusion, gases diffuse through the animal's skin into the blood vessels. The basis of the diffusion process is that a substance passes through a membrane down a concentration gradient. The latter means that the substance passes from a place where it is highly concentrated to another where it is poorly concentrated.

Taking this into account, oxygen passes from the outside of the animal into the blood vessels, while carbon dioxide diffuses in the opposite direction.

It should be mentioned that there are species in which the type of respiration is not cutaneous but branchial. This is because they inhabit aquatic ecosystems as fish parasites.

Thanks to this, they underwent certain modifications that allowed them to develop gills, through which they can take oxygen directly from the water.

Feeding

Hirudineans are heterotrophic organisms, some of which are predators of small invertebrates. Likewise, there are some Hirudineans who are blood-sucking, that is, they feed on blood.

For food, species that are predatory, capture their prey using their jaws. Later they gobble them up in their entirety. In the case of species that feed on blood, they attach to the host through their anterior suction cup and begin to suck the blood.

Usually the host is unaware because hirudinea secretes an analgesic substance that does not allow the victim to feel pain.

Exemplary sucking blood from the body surface. Source: GlebK

In their digestive tract, Hirudineans do not secrete endopectidase enzymes, so they do not have the ability to break down the proteins in their food. However, this obstacle is overcome thanks to the presence of bacteria (Aeromonas liquefasciens), which carry out this work.

It is important to note that the digestive tract has a large number of so-called blind spaces in which the animal can store part of the nutrients it ingests. In the case of Hirudineans who feed on blood, they can suck as much blood as 5 or 6 times their body weight.

Once the nutrients are absorbed and enter the animal's circulation, the waste substances are excreted through the anus.

Reproduction

Hirudineans reproduce only sexually. This reproduction requires the union of the female and male gametes. Fertilization is internal and can occur through the copulation of two different specimens or by other mechanisms.

In the first case (copulation), two specimens are aligned in such a way that the female and male pores meet and come into contact. Immediately the penis of one of the specimens is inserted into the vagina of the other, thus depositing the sperm. The sperm find the eggs and fertilize them.

Another mechanism of reproduction is hypodermic impregnation. In species that have this type of reproduction, sperm are stored in spermatophores. For the reproductive process, two specimens are attached to each other by means of the previous suckers, and later the spermatophores are released in the area of ​​the partner's clitellum.

Finally the sperm pass through the wall of the animal and travel to the ovaries through various channels. Fertilization eventually occurs. While all this occurs, a cocoon is produced at the level of the animal's clit, which is where the fertilized eggs are to be stored and developed. In some species there is only one egg per cocoon, while in other species there is more than one.

If the species is terrestrial, these cocoons are deposited on the ground, while if we speak of aquatic hirudineans, the cocoon can be deposited on the seabed or in one of the many algae that exist.

The development of the individual is direct, that is, an individual emerges from the cocoon that presents the characteristics of an adult hirudinean, with the exception that it is much smaller.

Classification

The hirudinea subclass is divided into two infraclasses: Acanthobdellida and Euhirudinea. The latter is in turn classified into two orders: Arhynchobdellida and Rhynchobdellida.

-Acanthobdellida

It is made up of exclusive parasitic species of fish. This is a very interesting group of organisms because it has primitive characteristics that bring them closer to the oligochaetes, such as ketae in the anterior segments and a metameric coelom surrounding the intestinal canal.

However, they also have certain characteristics that resemble the Hirudineans, such as the parasitic lifestyle.

-Euhirudinea

They are considered the true leeches.

Arhynchobdellida

This group of organisms presents us with a proboscis. They are found in freshwater habitats and can be amphibians. Most of these species feed on small invertebrates, but there are also others that are hematophagous. These may or may not have teeth.

Rynchobdellida

They are leeches that do not have jaws, but do have proboscis. Depending on the species, they can inhabit freshwater or marine ecosystems. They also have an anterior sucker that in some is well developed and in others it is barely defined. Its most outstanding feature is its very well developed proboscis. They are parasitic in life.

Representative species

Hirudo medicinalis

The most representative species of the hirudinea class is Hirudo medicinalis. This belongs to the order Arhynchobdellidae. It is not easy to distinguish from the rest of the Hirudineans, however it is widely used in the medical field, both for bleeding and for the usefulness of the substances it synthesizes. It is also known by the name of sangonera or bloodsuckers.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
4. Manrique, M., Ortega, S. and Yanguas, P. (2008). The leech, a worm in the history of health. Nursing index. 17 (4)
5. Oceguera, A. and León, V. (2014). Biodiversity of leeches (Annelida: Eurhirudinea) in Mexico. Mexican journal of biodiversity. 85.
6. Vera, C., Blu, A. and Torres, M. (2005). Leeches, parasites present yesterday and today. Chilean journal of infectology. 22 (1).