KETOGNATHS: CHARACTERISTICS, HABITAT, REPRODUCTION, FEEDING - BIOLOGY - 2025

The chaetognaths are a group of marine animals which are characterized by having an elongated body shaped torpedo. They are quite controversial, since even specialists discuss some of their characteristics to classify them phylogenetically correctly.

They were described by the German zoologist Karl Leuckart in 1854. These animals have managed to survive on the planet for quite some time, since the first records that are had of their existence date from the Paleozoic era, specifically the Cambrian period.

Examples of ketognaths. Source: Various authors. Compilation by me.

This phylum is made up of two classes: Sagittoidea and Archisagittoidea. In these classes there are a total of 20 genera, which are made up of approximately 120 species. These are ubiquitous, since they are distributed throughout all the seas of the globe.

characteristics

Ketognaths are animals with a transparent body that stand out because they present in all their cells their genetic material packed and enclosed within the cell nucleus, delimited there by a membrane.

They are also multicellular organisms because they are made up of different types of cells, each one specialized in different functions, such as the secretion of substances, nutrition or reproduction.

Ketognaths are hermaphrodites. They reproduce in a sexual way, with internal fertilization and direct development, in addition to being oviparous. Likewise, they present bilateral symmetry, which means that they are made up of two exactly equal halves.

Taxonomy

The taxonomic classification of ketognaths is as follows:

Domain: Eukarya.

Animalia Kingdom.

Superfilo: Spiralia.

Phylum: Chaetognatha.

Morphology

Ketognaths are small in size, ranging from 1 cm to 12 cm. They also have an elongated body, torpedo-shaped and translucent, although some species have reddish, orange or pink colors.

- External anatomy

The body of ketognths is made up of three areas or regions: head, trunk and tail.

Head

It is clearly differentiated from the rest of the body by a series of structures that characterize it. In the first place, it presents a kind of hooks, also known as hooks, which are arranged in 2 rows on the lateral edges of the head. Its function is related to the capture of prey.

In the head is an opening, the mouth. This is surrounded by structures called denticles that contribute to the chewing of food. They also have small compound eyes.

Between the head and the trunk is the neck, which is short in length. This constitutes the point of origin of a cap-type integument fold, which serves as protection for the head when it is retracted.

Trunk

It is the longest portion of the body of ketognaths. It has two pairs of fins, a pair of anterior position and another of posterior position. These do not have any type of musculature and are supported by organs called homalopterygiums, which are a kind of soft rays.

Towards its posterior region are the orifice of the anus and the openings that correspond to the female genital canal.

Representation of a ketognath. Source: Apokryltaros

Tail

It is the shortest part of the body of the ketognath. Internally, it is fully occupied by the testicles of the animal. It has a caudal fin, as well as two laterally positioned extensions known as digítelas.

- Internal anatomy

The body is delimited by a wall that has several layers. From the inside out, the following can be mentioned: longitudinal muscles, nervous plexus, basement membrane, epidermis and cuticle. The latter fulfills the function of protecting the animal.

Ketognaths have several cavities of the coelomatic type. In the head, this cavity is called the procele and is odd. The trunk has the mesocele that stands out for being even. And finally, the tail is the metacele, being also a pair.

These animals have a digestive system, a nervous system and reproductive organs. However, they do not have a respiratory system, an excretory system, or a circulatory system.

Digestive system

It's pretty straightforward. It is made up of the mouth, which leads to the oral cavity. It is immediately followed by the pharynx, which is where the greatest amount of digestive enzymes are produced.

After the pharynx is the intestine, which is the site of absorption. Finally, the digestive tract culminates in the anus, which is the hole through which digestion waste is released.

Nervous system

It is superficial in location. Presents at the level of the head a neuronal accumulation, the cerebrospinal ganglion, from which some nerve fibers emerge that are directed towards the different structures of the animal. Besides the cerebroid ganglion, there are others such as the vestibular ganglia and the ventral ganglion.

Reproductive system

The male reproductive system is located in the tail. It is made up of the testicles (1 pair) that have ducts, through which they release sperm. These flow into the seminal vesicles.

On the other hand, the female reproductive system has two ovaries that are located in the trunk. From these there are ducts (oviducts) that have a structure known as a seminal receptacle. Finally, the oviducts flow into the vagina, which opens to the outside through the genital pore.

Habitat and distribution

The members of the Chaetognatha phylum are purely aquatic animals. However, they do not do well in all ecosystems of this type, but are found exclusively in marine-type ecosystems.

Within marine ecosystems, ketognaths are more abundant in those where salinity levels are lower. This is why it can be said that the favorite habitat of this type of animal is marine spaces with waters with a low salt content.

Reproduction

The type of reproduction observed in ketognaths is sexual. In this, for a new individual to develop, the contact, union and fusion of sex cells are necessary. Sexual reproduction is advantageous over asexual, since it is closely related to genetic variability.

Ketognaths are hermaphroditic animals. This means that the same individual has male reproductive organs and female reproductive organs. In this sense, it is understandable to believe that these animals are self-fertilized. However, this is not the case, at least not on a regular basis.

One individual fertilizes another. Although on some occasions there may be self-fertilization.

Reproduction in ketognaths is characterized because fertilization is internal, development is direct and they are oviparous.

Before fertilization occurs, these individuals exhibit some courtship rites that have not yet been fully elucidated by specialists.

For fertilization to take place, what happens is that two individuals come into contact and one of them releases a spermatophore anywhere on the other individual's trunk. This contains the sperm.

The spermatophore has the ability to dissolve the outer layer of the body (cuticle) so that the sperm can penetrate the trunk and thus reach the eggs to fertilize them.

After fertilization, the laying of the eggs comes. Not all species of ketognaths lay their eggs in the same way. Some place them one by one, some in groups and others in rows.

Finally, when the appropriate time has elapsed and the individual has developed correctly, an animal emerges from the eggs that has the characteristics of an adult ketognath. Hence, the development is direct, since the individuals that hatch from the eggs do not go through larval stages.

Feeding

These animals are carnivores, frequently feeding on small animals such as some invertebrates, such as copepods and even some jellyfish.

Ketognaths are very efficient predators. The moment it perceives any prey, the animal instinctively ejects its head from the hood and secures it with the hooks it has there for this purpose.

It immediately engulfs the prey, which makes it practically whole. Food enters the mouth and passes into the pharynx, where it is subjected to the action of digestive enzymes that are secreted there.

Subsequently, in the intestine is where the absorption of processed nutrients occurs mostly. The waste product of digestion, which is not required by the body, is sent to the anus, to be released abroad.

References

1. Bone, Q. and Kapp, H. (1991) The biology of the Chaetognaths. Oxford University Press. London
2. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Palma, S. (2001). Bibliographic index on aquatic biodiversity in Chile: Quetognatos (Chaetognatha). Maritime Science and Technology. 24.
6. Simonetti, J., Arroyo, A., Spotorno, A. and Lozada, E. (1995). Biological diversity of Chile. CONICYT.