MYXINI: CHARACTERISTICS, HABITAT, FEEDING, RESPIRATION - BIOLOGY - 2025

The Myxini or mixines are primitive marine vertebrate fishes, belonging together with the lampreys to the agnate group, because they are the only living vertebrate organisms that lack jaws.

They have an elongated shape like eels, with a size that can vary from 15 to 140 cm. They have fleshy tentacles around the mouth, which are called barbels and serve a sensory function.

Mixines. By NOAA (http://www.photolib.noaa.gov/htmls/expl2939.htm), via Wikimedia Commons

The agnate fish appeared approximately 470 million years ago and for more than 100 million they were the only vertebrates that inhabited the earth. Today most are extinct.

The mixins are part of the Agnatha superclass, which due to its simplicity is usually taken as representatives of the first stages of evolution in vertebrates.

Specialized eating habits, parasites for lampreys and scavengers for hagfish, may be the predominant reason why they are the only survivors of agnate.

Historically, there are differences between scientific currents in terms of their classification, whether to place them as true vertebrates or not, currently the dispute continues.

The oldest fossil of the Mixin group found, dates from about 300 million years ago.

General characteristics

Mixins are the most primitive vertebrate animals. They lack even fins, jaws, and eyes (some species have vestigial eyes).

They have a bony skull, but the skeleton is cartilaginous without the development of vertebrae, so it is quite rudimentary. The nervous system cord is not protected by cartilage.

They live on the seabed with nocturnal habits, they prefer cold waters with temperatures below 22 ° C, in tropical waters they are located in the deepest areas.

They feed primarily on dead, sick or trapped animals. By ingesting decomposing matter they play a fundamental role in the food chain, fulfilling the recycling of nutrients.

Respiration is carried out by filtration of seawater through gills arranged in bags, and they also have the ability to breathe through the skin at great depths.

They have the most primitive renal system among vertebrates, so body fluids show the same concentration of seawater where they live.

Regarding the gender relationship, it is estimated that in the populations there is a proportion of 100 female individuals for each male specimen.

With respect to the fishing industry, they are not a commercial interest group, their catches are made accidentally in the fishing gear used mainly on the seabed, for the exploitation of other species that are in the same habitat.

Taxonomy

The Myxini class is composed of a single order with a single family, which is made up of 5 genera and approximately 75 species.

The taxonomic classification is as follows:

Animalia Kingdom

Edge: Chordata

Subphylum: Vertebrata

Superclass: Agnatha

Class: Myxini

Order Myxiniformes

Family Myxinidae

Genders:

Eptatretus (49)

Myxine (22)

Nemamyxine (2)

Neomyxine (1)

Notomyxine (1)

The two most important genera of the mixins are Eptatretus, comprised of some 49 species that inhabit galleries dug into the ocean floor, and Myxine, represented by 22 species that live in temporary burrows or associated with muddy sediments.

The longest species is the Eptatretus goliath, which reaches sizes of up to 140 cm, and the smallest is the Myxine pequenoi with measurements less than 18 cm.

Morphology

Adult individuals are generally around 50 cm long, their bodies are elongated and have no development of a dorsal fin.

The skin is bare, without the presence of scales. Its coloration is variable depending on the species, identifying mixines in pink, blue, gray, black, white or spots.

The eyes can be absent or degenerated, without muscles or optic nerves, and even partially covered by the thick skin of the trunk. The ocular system is so little evolved that it does not allow them to visualize detailed images, only in some cases they are able to detect light.

The mixins are distinguished by excreting a large amount of mucus and protein threads, this is done through exclusive glands of these organisms that are distributed throughout the body.

This process has been extensively studied, mainly due to the particular characteristics of the mucus produced, and its possible various commercial uses, if it can be manufactured artificially.

In nature, the generation of the substance is associated with a means of defense of the animal against predators, which is used in conjunction with the movements that allow it to coil itself, which facilitates its release when it is captured.

Habitat

The mixins are distributed in the marine waters of temperate zones around the world, being found in most of the oceans, with the exception of the Red Sea, the Arctic and the Antarctic.

They are benthic species, that is, they live on the seabed, being located mainly in caves and areas of loose substrate such as sand or mud.

Mixines. By Linda Snook.Credit: NOAA / CBNMS. (NOAA Photo Library: sanc1691), via Wikimedia Commons

Individuals normally remain buried for protection, leaving only the head region out of the sediment.

They are observed in a wide range of depths, reporting species up to 1,600 meters deep.

Feeding

Mixines are considered practically blind, so they detect food through an efficient system of smell and touch, made up of six tentacles located around the mouth.

They are nocturnal predators that eat primarily carrion, dead or dying animals such as fish and large invertebrates (polychaete worms), and occasionally other invertebrates that live near the seabed, such as annelids, mollusks and crustaceans.

They support food thanks to two horny and serrated plates that close like clamps, fulfilling the function of jaws, later they extend a long tongue that presents the peculiarity of having teeth, which they use to tear off pieces of tissue.

Once they adhere to the meat of the prey, they can tie a knot from their tail that slides to the front, in order to exert greater mechanical force and extract larger pieces.

Finally they pierce the body they are ingesting, devouring the meat and entrails from the inside out.

Their esophagus is ciliated and they lack a stomach. Once the food reaches the intestine, it is assimilated by a mucous substance that surrounds them, which is secreted by the walls of the intestine.

The remains that are not digested in the intestine, are expelled wrapped within the mucous material. Because the metabolism is quite slow, they are able to survive for months without eating any food.

Circulatory system

Mixin have a main heart that is divided into two chambers, the atrium and the ventricle. Additionally, they have two auxiliary hearts or rudimentary driving valves, which are located throughout their body in the branchial and caudal region.

Blood fluid has nucleated cells, but respiratory pigments are very similar to that of invertebrates.

When you breathe, the blood is pumped into the body and oxygenated in the capillaries of the gills, circulates throughout the body through the aortas and later returns to the gills again thanks to the veins.

Reproduction

The reproductive process is considered to be little known, due to its deep-sea habitat making it difficult to study.

Although the ovaries and testes may be in the same individual, they are not functionally hermaphrodites. The young have both gonads but when they reach sexual maturity, they behave as a single sex, so they are considered a species of separate sexes.

The mechanisms that establish the choice of sex in organisms have not been determined, although it is speculated that it could be influenced by the proportion of sexes in the area.

They also have the ability to change sex throughout their lives. Fertilization is external on silt bottoms. The female releases groups of 23 to 30 eggs no larger than 3 cm in size and oval in shape.

The incubation period does not exceed two months, after which a baby with a size between 4 to 5 cm hatches, with the same phenotype as the adult. As there is no larval phase, development is direct without metamorphosis, unlike lampreys where this phase plays a very important role in their life cycle.

Breathing

The respiration process in the mixins is carried out by the aspiration of the seawater through the only nostril they present, and later this is expelled through the branchial channels.

In the branchial sacs, oxygen is transported to the blood vessels and carbon dioxide leaves the body by diffusion. Depending on the species, the gill openings can vary in number, from one to 14 on each side of the body.

The mixins also developed cutaneous respiration, as an adaptation to the low concentrations of environmental oxygen found in the great depths where they usually live.

References

1. Bessonart, M. and A. Rodríguez. (2007). Agnatos and Chondrichthyans. Science Faculty. University of the Republic, Uruguay. 14 pp.
2. Campbell, N. and J. Reece. (2007). Biology. Editorial Panamericana. 1351 pp.
3. Guisande, C. et al. (2013). Sharks, rays, chimeras, lampreys and mixinids from the Atlantic coast of the Iberian Peninsula and the Canary Islands. Díaz de Santos editions. 227 pp.
4. Martín C. and I. Sobrino. (2011). Current agnates. Similarities and differences. Sevilla University. Recovered from bioscripts.net
5. Padilla, F. and A. Cuesta. (2003). Applied zoology. Díaz de Santos editions. Madrid Spain. 468 pp.
6. Sanz, F. (2009). Nutrition and feeding in fish farming. Volume I. Spanish Aquaculture Observatory Foundation. 803 pp.