ACANTHOCEPHALUS: CHARACTERISTICS, NUTRITION, REPRODUCTION - BIOLOGY - 2024

The Acanthocephala (Acanthocephala) are obligate parasites invertebrates, whose main characteristic is the presence of a evaginable proboscis, armed with bones, which allows them to be fixed to the intestinal mucosa guests. The name Acanthocephala comes from the Greek acanthus, which means thorn, and kephale, which means head.

The phyllum Acanthocephala is widely distributed worldwide and includes more than 1,300 species, distributed in four classes (Archiacanthocephala, Eoacanthocephala, Palaeacanthocephala and Polyacanthocephala).

Corynosoma wegeneri. Source: Dr. Neil Campbell, University of Aberdeen, Scotland, UK

They are macroscopic animals whose body size varies from a few millimeters, in some fish parasites, to more than 60 cm in the case of Gigantorhynchus gigas. Females are generally larger than males.

Acanthocephalus have complex life cycles, involving various vertebrate and invertebrate hosts. These cycles are known for less than a quarter of the described species.

The adult parasite is harbored in vertebrates, while the larval forms are harbored in invertebrates. Fish represent the main definitive hosts, although they can parasitize amphibians, reptiles, birds, and mammals.

Possibly the ancestors of these rare organisms were species that parasitized marine arthropods during the Cambrian period. From these, their cycles became increasingly complex, including arthropod predators.

General characteristics

In its early development, three layers of embryonic tissue are recognized (endoderm, ectoderm and mesoderm), which is why they are called triploblastic.

They are organisms shaped like worms (vermiforms), whose body is not segmented. Its pseudocoelom (blastocelic cavity) is fluid-filled and may be compartmentalized by ligamentous sacs.

They have a reversible proboscis with a mechanical function for fixation in the host. They present a hydraulic system called “lemniscus” that allows them to extend the proboscis. They do not have a digestive system.

They have a simple nervous system, with a ventral cerebroid ganglion in the proboscis receptacle, and a pair of lateral longitudinal nerves. In addition, they have a genital ganglion in the most distal area.

Except for some species, the protonephridiums are absent. The excretory system is only present in the members of a family, where it appears as two protonephridia that flow into the reproductive system.

The sexes are separated into different individuals, that is, they are dioecious organisms. Their eggs have three or four membranes.

They have larval stages. The Acantor form is a spindle-shaped spindle-shaped larva, which has hooks in its anterior part. In the acantela form the proboscis, the proboscis sac and the reproductive organs are notable.

They also have an encysted form known as cystacanth. This is formed when the acantela is embedded.

Morphology

Your body is divided into two regions. The anterior part or prosome is made up of a hollow structure known as a proboscis. It has a spiny portion and a non-spiny neck. The number, shape, and size of the proboscide hooks have taxonomic value in this group.

The posterior part or metasoma, where the different systems are housed, is connected to the prosome by two elongated structures that are called lemniscus, which are folds formed by the inner wall of the body in the pseudocoelom.

The body wall is differentiated into a cuticle (outermost layer), a syncytial-type epidermis with channels, or fluid-filled lagoons, and a muscular, innermost layer.

They have circular and longitudinal musculature. Thanks to the retractor proboscis muscle, this structure can be invaginated into a muscular sac called the proboscis receptacle.

As a result of the adaptive process to the obligatory parasitic life, there is a significant reduction and modification of their systems. The organs are located within an open cavity known as a blastocoelom. This is partially segmented, with ligaments structurally similar to the mesentery.

Nutrition

Acanthocephalus do not have a mouth to eat food. Her digestive system is extremely modified, her digestive tract is completely absent.

The cuticle protects the organism from the enzymatic action of the host's digestive system, and at the same time presents characteristics that are permeable to the nutrients contained in the intestinal environment.

Nutrients such as sugars, triglycerides, amino acids, and nucleotides are absorbed through the body's integument. Thus, they feed directly on the nutritional substances present in the intestine of the host organism, as in the case of flatworms known as tapeworms, from the Cestode group.

Reproduction

Acanthocephaluss are dioecious organisms that present internal fertilization. Your male reproductive system is made up of a pair of testicles, two vas deferens, two seminal vesicles (dilated ejaculatory duct), and two accessory seminal (cementum) glands. The penis is located in a posterior position.

In the cement glands of the male, the fertilization cap is produced, which closes the female orifice once fertilization has occurred.

The female reproductive system is made up of a pair of ovaries, a uterus, and an oviduct. The ovaries disintegrate to form numerous ovarian masses, which are located in the pseudocele and in the ligament sacs.

A uterine bell, which communicates with the uterus, acts as a selector structure, allowing only the passage of mature eggs.

Lifecycle

The life cycles in the different species of acantocephalus are complex. In these invertebrate and vertebrate hosts are involved.

The adult male and female inhabit the vertebrate host, which acts as the definitive host. In the intestine of the vertebrate host (fish, amphibians, birds and mammals) copulation occurs, producing eggs.

Life cycle of Moniliformis moniliformis (Acanthocephala: Archiacanthocephala: Moniliformida). Source: modified from Division of Parasitic Diseases and Malaria team

The eggs develop in the acantocephalus to form embryonated eggs, which contain acanthor larvae. These are deposited by the parasite in the intestine of the vertebrate host.

The eggs are released into the external environment with the host's feces. Once in the environment, the eggs can be ingested by an invertebrate (usually a crustacean or a mollusk), which will act as an intermediate host.

In the intermediate host, the larvae develop in the digestive tract and then penetrate its walls, reaching the body cavity or coelom, where it becomes acantela. In the coelom, the acantocephalus larvae encyst, taking the form of a cystacanth.

When the invertebrate is consumed by a vertebrate, the latter ingests the cystacanths with it. The cystic form becomes activated and transforms into an infective phase.

Once inside the gut of the final vertebrate host, the acantocephalus evacuates its proboscis and attaches to the host. Then your reproductive system develops. From there, fertilization can occur and a new cycle can begin.

Alterations in the behavior of invertebrates

An interesting aspect, associated with the complex life cycles of the acantocephalus, is that their action on the intermediate invertebrate host can modify some physiological parameters, which result in alterations in their behavior that make them more susceptible to predation by the definitive host. vertebrate.

In the case of the acantocephalic species that parasitize a freshwater crustacean, the action of the parasite causes the crustaceans, instead of swimming towards the bottom of the body of water in the presence of the predator, swim towards the light, holding tightly on the aquatic vegetation. This increases the chances of predation by ducks and other vertebrates.

The aberrant behavior may be associated with a modification in the serotonin release processes, which cause reproductive behavior associated with mating to develop.

On the other hand, terrestrial crustaceans such as scale insects that normally move and take refuge in humid and dark places, when they are parasitized they move in illuminated and open spaces. This makes them vulnerable to predatory birds.

It has been calculated that 30% of the mealybugs captured by the birds are infected by the acantocephalus, while only 1% of the individuals present in the environment are infected.

Pathology and medical importance

Acanthocephalic infections in humans are rare, but can occur through ingestion of raw fish or crabs infected with polymorphids. They can also occur through accidental infections with acantocephalus that parasitize rats or pigs.

The traumatic action of the proboscis can cause pain, due to its deep penetration, which generates local damage and inflammation in the place where the parasite is fixed. This lesion can become infected with pathogenic organisms such as batteries. Sometimes they can perforate the intestine, causing peritonitis in the host. In these cases, the parasites must be removed by surgery.

M. mmoniliforis has been reported as an accidental parasite of humans, being a frequent parasite in rodents and domestic carnivores such as cats and dogs.

At least two species, Macracanthorhynchus hirudinaceus and Moniliformis moniliformis, are of veterinary medical interest, and can occasionally infect humans.

The first one parasitizes domestic and wild swine, such as pigs and peccaries, and some primates, such as monkeys, which are predators of coleopterans in the larval stage.

In the intestine the parasite competes with the host for food resources.

Bioindicators

Acanthocephalus have been considered as environmental indicators, due to their ability to concentrate heavy metals.

The concentration of heavy metals in these parasites is a thousand times higher than those found in the tissues of their host. Furthermore, it has been found that the parasitized hosts present lower concentrations of metals than those individuals of the same species that are not parasitized by acantocephalus.

References

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