AGNATOS: CHARACTERISTICS, RESPIRATION AND FEEDING - BIOLOGY - 2025

The agnatos comprise a group ancestral animal vertebrates that do not have jaws. Within the agnates we find the now extinct ostracoderms, and the living species popularly known as lampreys and witch fish.

Although witchfish do not have vertebrae, and lampreys only present these structures in a rudimentary state, they are included in the subphylum Vertebrata because they have a skull and other diagnostic characteristics of the group.

Mouth of a lamprey.

Source: I, Drow male

Witchfish and lampreys appear to be closely related species due to their apparent external resemblance, similar to an eel. However, they are so different from each other that taxonomists include them in separate classes.

A group of ostracordemos gave rise to a lineage of jawed organisms, known as gnathostomes.

characteristics

Agnatos include a group of 108 species, divided into two classes. The first one is the Mixini class, to this group some 70 species of "witch fish" belong. The second class is Petromyzontida with 38 species of lampreys.

Curiously, the group is usually characterized by the structures that they do not have - and not by the ones it does present.

Members of both classes lack jaws, an attribute that gives the group its name. It is important to note that agnates have a mouth, but lack mandibular structures derived from the branchial arches.

In addition, they lack internal ossification, scales, and even fins. These animals are eel-shaped and exhibit pore-like gill openings. All living species have only one nostril.

The fossil history of witchfish and lampreys begins in the Carboniferous, but surely the group appeared much earlier, within the Cambrian or even earlier.

Taxonomy

According to the classification proposed by Nelson (2006) used by Hickman (2010), agnathates are found within the Phylum Chordata. Below we will describe the most important characteristics of each class:

Myxini class

Habitat and ecology

The so-called witch fish or mixines are animals that live strictly in marine ecosystems. Some species of this class are well known, such as the North American Atlantic witchfish Myxine glutinosa and the Pacific witchfish Eptatretus stoutii.

Certain species have declined thanks to overfishing, as demand for their leather has increased in the market.

Osmotic composition of the body

The internal osmotic composition of the hagfish body is another particular aspect of the class. Body fluids are in osmotic equilibrium with seawater, a typical characteristic of invertebrates but not vertebrates.

In other vertebrates, seawater has a concentration of salts about two-thirds greater than the internal composition of the animal. This implies that in the mixines there is no net flow of water, outside or inside the fish.

Highlight features

The mixins do not have any type of appendages - call them fins or even appendages. They have a single terminal opening (the nostril), the water enters through this single nasal opening, passes through a duct, up to the pharynx and the gills.

The vestibular apparatus (or ear) is an organ involved in the balance of the animal, and includes a single semicircular canal. There is no vertebra-like element around its notochord.

The circulatory system is simple and is made up of the heart, made up of a venous sinus, atrium, and a ventricle. There are accessory hearts. The digestive system is even simpler: they do not have a stomach or a spiral valve. They also do not have cilia in the gastrointestinal tract.

The anterior portion of the dorsal cord is thickened in a differentiated brain. They have 10 pairs of cranial nerves and do not have a cerebellum.

The senses of sight these animals are poorly developed. In fact, they are practically blind. To counteract this disadvantage, the sense of smell and touch is sharp and allows them to locate their prey.

On the other hand, mixins have the ability to "roll up" their body and form a knot. They do this behavior to capture or to escape.

Mucus production

Mixins are characterized by the production of a mucous or gelatinous substance. The production of this mucus is triggered when the animal is disturbed. The stimulus of the disturbance triggers the exudation of a substance similar to milk which, in contact with sea water, becomes an extremely slippery substance.

The presence of this fluid allows the witch fish to be so slippery that it becomes virtually impossible to capture prey.

Class Petromyzontida

Habitat and ecology

Half of the living lampreys exhibit parasitic life habits. Although some species live in the ocean, all of them lay their eggs in bodies of fresh water (so they have to move there).

Lampreys use their oval-shaped mouths to adhere to rocks and maintain a stable position. Parasitic lampreys use this same system to tear the skin of their prey, open the blood vessels of the animal and feed on its fluids.

Osmotic composition of the body

Contrary to mixins, lampreys have a system that regulates their osmotic and ionic composition.

Highlight features

Like witchfish, they are eel-like organisms with bare skin. They have a fin that is located in the middle of the body. However, they do not have even fins or any other type of limb. The notochord is prominent and is accompanied by individual blocks of cartilage (these are the rudimentary vertebrae).

The circulatory system is made up of a heart with a venous sinus, atrium, and ventricle. In the anterior portion of the nerve cord there is a differentiated brain and, unlike the mixins, there is a small cerebellum. They have 10 pairs of cranial nerves. The digestive system does not have a differentiated stomach

The ear - or vestibular apparatus - is made up of two semicircular canals. Like witch fish, these animals lack bone and scales. The eyes are well developed in adult specimens.

Classification in Cyclostomata

It has been suggested to group these two classes of living species under the name of Cyclostomata (a term that refers to the rounded mouth opening of lampreys and mixins). However, when we analyze this order from a cladistic perspective, we find that the group is paraphyletic.

Lampreys have a number of characteristics (rudimentary vertebrae, extrinsic eye muscles, two semicircular canals, and a cerebellum) that are unique to jawed animals, the gnathostomes.

However, when molecular methodologies are applied to elucidate the phylogenetic relationships of the group, it has been concluded that, indeed, lampreys and witchfish form a monophyletic group.

This grouping, which is not consistent with that found when morphological characters are taken into account, is not supported by most zoologists. Therefore, the phylogenetic hypothesis of cyclotomes needs to be revised.

Breathing

Respiration in agnates occurs through gills. Specifically by internal gills, which have lamellae. These develop on the walls of the bursa of the pharynx. The gills of agnates are called "bagged."

Reproduction

Mixines

The current literature does not have much information regarding the reproductive biology of the hagfish. Females are known to outnumber males in proportion, about 1 male for every 100 females.

A single individual possesses both ovaries and testes, but only one class of gonads is functional. For this reason, witchfish are not hermaphrodites in the strict sense. Fertilization is external.

Females produce a low number of eggs (about 30) that are large - 2 to 7 centimeters, depending on the species. Larval stage is not known.

Lampreys

Lampreys have separate sexes and external fertilization. They ascend to bodies of fresh water for reproduction. Marine forms are anadromous (that is, they leave the ocean, where they spend most of their adult life, and travel to fresh water to reproduce).

The males build nests, where the eggs will be attached and covered by sand. The adults die shortly after laying the eggs.

After about two weeks, the eggs hatch, releasing the larval stage of the lampreys: the ammocete larva. The larva and the adult form differ so much in their characteristics that the first classifications considered them as distinct species.

The ammocete larva has an incredible resemblance to the amphoxus (cephalochord), and possesses the diagnostic characteristics of chordates at first glance.

Feeding

Mixins are carnivorous animals that feed on living or dying prey. Within its diet we find a diversity of annelids, mollusks, crustaceans and also fish.

Witchfish have a tooth-like structure and a structure reminiscent of a muscular tongue to capture their prey.

In the case of lampreys, they may or may not exhibit a parasitic lifestyle. The lampreys that are so have a keratinized mouth structure that allows them to adhere to their prey, the fish. Anchored by these sharp "teeth," lampreys can feed on the bodily fluids of their prey.

In non-parasitic species, their digestive tracts degenerate when they are adults - so these forms do not feed. The lamprey dies in a short time, after it completes the reproductive process.

In contrast to the adult form, the ammocete larva feeds on suspended particles.

References

1. Audesirk, T., Audesirk, G., & Byers, BE (2003). Biology: Life on Earth. Pearson education.
2. Curtis, H., & Barnes, NS (1994). Invitation to biology. Macmillan.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
4. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
5. Parker, TJ, & Haswell, WA (1987). Zoology. Chordates (Vol. 2). I reversed.
6. Randall, D., Burggren, WW, Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan.