QUELICERATES: CHARACTERISTICS, CLASSIFICATION, HABITAT, REPRODUCTION - BIOLOGY - 2024

The chelicerates are a subphylum of the phylum Arthropoda, whose members are characterized by presenting a first pair of appendages called chelicerae. It was first described in 1901 by the German zoologist Richard Heymons. This is a fairly broad group of organisms, the most recognized members of which are arachnids, such as spiders and scorpions.

Many of the chelicerates have venom glands, in which they synthesize toxins that allow them to paralyze and neutralize their prey in order to feed on them. In general, the species of this subphylum are found all over the planet.

Specimens of chelicerates. Source: Eurypterids Pentecopterus Vertical.jpg: Patrick LynchPycnogonid.jpg: Steve ChildsLimulus polyphemus (aq.). Jpg: Hans HillewaertGasteracantha cancriformis 2.jpg: Mike Kullen

Taxonomy

The taxonomic classification of chelicerates is as follows:

• Domain: Eukarya
• Animalia Kingdom
• Phylum: Arthropoda
• Subphylum: Chelicerata

characteristics

Like all members of the Eukarya domain, chelicerates are characterized by presenting cells in which DNA is packaged within the cell nucleus, conforming to chromosomes. Likewise, they are made up of different cell types, which allows us to affirm that they are multicellular organisms.

Each of the types of cells that make them up has been associated and specialized in specific functions, thus shaping the various tissues that make up the animal.

Likewise, chelicerates are organisms that have bilateral symmetry, that is, if an imaginary line is drawn along its longitudinal plane, two exactly equal halves are obtained.

The chelicerates, during their embryonic development, present the three germ layers known as ectoderm, mesoderm and endoderm. The importance of these layers lies in the fact that different types of cells and tissues develop from them that will eventually make up the adult individual.

Chelicerate animals are dioecious, which means that the sexes are separate. There are female individuals and male individuals.

Some of the species that make up this subphylum have glands that synthesize a toxic substance, a poison. This poison is used to hunt their prey or to defend themselves from possible predators.

There are species in which the venom is so powerful that it can even kill an adult human being.

Morphology

Like all individuals members of the phylum Arthropoda, chelicerates have a segmented body, specifically in two tagmas, which are known as prosoma and opistosome. The prosome is what in other groups is called the cephalothorax, while the opisthosoma is the abdomen.

Likewise, they have a protective covering that is mainly made up of a polysaccharide called chitin.

-Prosome

It is made up of an acron, which is located before the segments, and six segments.

Likewise, several of the appendages that characterize chelicerates emerge from the prosome. Among these are the chelicerae; these constitute the first pair of appendages of the animal.

Cheliceros

The chelicerae is the characteristic element of this subphylum. Depending on the species, they have different functions, such as inoculating the poison to possible prey (arachnids). In such a way that in some species chelicerae are associated with venom-synthesizing glands.

The chelicerae are made up of pieces that are known by the name of artejos. Depending on the amount, shape and arrangement of these, there will be three types of chelicerae:

• Scissors: these resemble a clamp. They are made up of two pieces.
• Pincer: they are made up of three pieces and are shaped like a pincer. They are typical of species such as the horseshoe crab.
• In pocket knife: they are made up of two knives and are similar to folding knives. It is the most abundant type of chelicerae among the different species of spiders.

Types of chelicerae. (A) In razor. (B) In scissors. (C) Forceps Source: Xavier Vázquez Likewise, another type of appendix is ​​found in the prosome, the pedipalps. These are the second pair of appendages of the chelicerates. They are made up of a total of six pieces.

Those arteries that comprise it are, from the most distal to the most proximal: tarsus, tibia, patella, femur, trochanter and coxa. Through the latter the pedipalp articulates with the body of the animal. Likewise, depending on the species, the pedipalps will have different morphology.

Likewise, apart from the chelicerae and pedipalps, in the prosoma there are also four pairs of legs, which have the only function of the animal's locomotion.

-Opistosoma (abdomen)

It shapes the rest of the animal's body. It is made up of approximately 12 segments, the last of which is known as telson. Generally, no appendix is ​​detached from this part of the body.

Its importance lies in the fact that inside there are structures belonging to the different organic systems, causing important functions related to circulation and reproduction, among others, to be carried out.

At the level of the second segment of the opistosome there is an orifice, the gonopore. This is part of the reproductive system of the animal. In male individuals, it is where the sperm are released, while in the case of female sex, it is where the sperm is received for the fertilization process to occur. Similarly, in the female the gonopore is the site through which she lays the eggs.

-Internal anatomy

Digestive system

The digestive system of chelicerates is quite simple. It is divided into three parts: stomodeum, mesodeo and proctodeo. In general, the digestive tract is narrow.

The stomodeus is made up of the oral cavity, which communicates with the esophagus, which is a thin and thin tube, provided with musculature. After the esophagus is the mesodeum, which contains a large number of digestive glands that secrete enzymes of this type.

The mesodeum makes its way to the proctodeum, which is the final portion of the digestive tract. The proctodeum ends in an opening, the anus. This is where the substances that make up digestion waste are released.

Respiratory system

The respiratory system is variable, depending on the habitat of the animal. Within the chelicerates, there are aquatic and terrestrial ones.

In the case of aquatic chelicerates, their respiratory system is made up of a system of gills. These are nothing more than a set of highly vascularized lamellae, which filter and retain the oxygen present in the water that circulates through them. Likewise, when they capture oxygen, they give up carbon dioxide.

On the other hand, in terrestrial chelicerates, such as spiders and scorpions, the respiratory system is made up of the so-called book lungs. Importantly, this system has nothing to do with the lungs of terrestrial vertebrates.

Book lungs are organs that are made up of invaginations of the integument, whose organization resembles the pages of a book. This arrangement increases the surface area through which gas exchange takes place.

As for the quantity, it varies depending on the species. There are chelicerates that have only one pair of book lungs, while there are others that have up to four pairs.

Finally, the book lungs communicate with the outside through holes known as spiracles, which open to the outside on the surface of the animal's body.

Excretory system

It is made up of the Malpighi tubes and a series of nephridiums. Both structures have the ability to filter waste from the blood, to release it directly at the level of the intestine, to be excreted in a solid way as part of the stool.

Regarding the substances that they excrete, they are nitrogen compounds mainly in the form of ammonia or ammonia.

Aquatic chelicerates can excrete through the gills, while terrestrial chelicerates, not having a wide availability of water, are forced to convert their waste to the solid state. In this way, they can be disposed of as feces.

Circulatory system

The circulatory system of chelicerates is of the open or lagoon type. Blood circulates through an internal cavity known as a hemocele. The liquid that circulates through the chelicerate is the hemolymph.

Likewise, the main organ of the chelicerate circulatory system is a tubular heart that has a dorsal position. This has the function of pumping the hemolymph.

Nervous system

The chelicerate nervous system is made up of two nerve cords in a ventral position. These cords have a ganglion in each of the animal's segments.

Likewise, the brain is made up of the fusion of a pair of preoral ganglia. From this the ventral nerve cords mentioned above are detached.

Now, with regard to the sense organs, chelicerates present on the surface of their exoskeleton some extensions that look like a kind of hair whose function is to capture any tactile stimulus.

Likewise, at the level of the head they present compound eyes and ocelli that have the peculiarity of being pigmented. The latter are located in the middle of the head and the eyes in a lateral position.

Likewise, chelicerates present chemoreceptors that capture stimuli related to smell and taste.

Habitat and distribution

Chelicerates are widely distributed throughout the globe. The arthropods have been a group that have had great success colonizing various environments, and within these the chelicerates have not been left behind.

Members of the Chelicerata subphylum are present in a wide variety of ecosystems, both terrestrial and aquatic. There are species that live in brackish and fresh waters, as well as in places with extreme climatic conditions, such as in places with constant snow, in large desert areas like the Sahara or in the most recognized deserts of the American continent.

Classification

The chelicerates subphylum is classified into three classes:

-Arachnida: made up of organisms that lack antennae and have four pairs of legs, in addition to a pair of chelicerae and a pair of pelipalps. Spiders, mites and scorpions belong to this group.

-Merostomata: made up of animals that do not have pedipalps, they have a fairly wide body and 5 pairs of legs. They are also purely aquatic organisms.

-Pycnogonida: made up of aquatic animals that are characterized by having the ability to blend in with the environment in which they develop, which is the seabed. They are characterized by having four pairs of legs that are sometimes very long and thick.

Feeding

The chelicerates group have different food preferences. Most are carnivores, some are herbivores, parasites, detrivores, and even blood-sucking.

Due to the characteristics of the anatomy of their digestive system, chelicerates are prevented from ingesting large portions of food. Because of this, they have been forced to develop another way of eating.

The chelicerates present an external type digestion. This means that the processing and degradation of food occurs outside the animal's body and not inside, as would be the norm. This is so mainly in carnivores.

When the chelicerates identify a prey, they capture it with their pedipalps and immediately inoculate digestive enzymes. These will act directly on the tissues of the prey, degrading them, until obtaining a kind of shapeless mush.

Now yes, the animal is able to ingest that resulting liquid substance. Once inside, the absorption of nutrients occurs at the level of the midgut or midgut. Finally, the components that are not absorbed are sent to the proctodeum to be released as fecal waste.

On the other hand, in animals that are herbivores, their feeding process is simpler, since they feed on the pollen of the plants, as well as their nectar.

Reproduction

The type of reproduction seen in chelicerates is sexual, that is, it involves the fusion of male and female sexual gametes.

In terrestrial species, internal fertilization is observed, which can be direct or indirect. While in aquatic species fertilization is external. Likewise, there are oviparous species and others that are ovoviviparous.

The process in terrestrial-type species is as follows: when it is a direct fertilization, the male directly introduces the sperm into the female's body so that they fuse with the female gametes.

In the case of indirect fertilization, the male releases a structure known as a spermatophore that contains the sperm. The female picks it up and introduces it into her body for fertilization to occur.

Once fertilization has occurred, the eggs are formed. In species that are oviparous, the female releases the eggs to the external environment, while in ovoviviparous species, the egg remains within the female's body.

Finally, after the development time has elapsed, the eggs hatch. Here there can be two cases: a larva can emerge from the eggs that will need to undergo transformations until it becomes an adult; or an individual emerges that presents the characteristics of the adults of the species, only small in size.

Representative species

The chelicerates subphylum is very diverse. It covers more than 70,000 species distributed in the three classes that make it up. Among the most representative species we can mention:

Limulus polyphemus

It is the well-known horseshoe crab. It belongs to the class Merostomata. They have a fairly resistant exoskeleton, in addition to having five pairs of legs and spending most of their lives buried in the sand.

Limulus polyphemus. Source: © Hans Hillewaert

Lactrodectus mactans

It is the well-known black widow spider. It is characterized by a red spot that appears at the level of the abdomen, an unequivocal sign for its identification. It synthesizes a poison that is highly toxic and that can trigger reactions in humans that range from mild to certain complications that can lead to death.

Androctonus crassicauda

They are very poisonous scorpions that are characterized by having a dark-colored body, which can range from brown to black. They are mainly located in dry areas such as the Middle East.

References

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