OPHIUROIDS: CHARACTERISTICS, HABITAT, REPRESENTATIVE SPECIES - BIOLOGY - 2025

The ophiuroids are a group of animals that belong to the class Ophiuroidea of ​​the phylum Echinoderms. Their appearance is quite striking, since they consist of a central disk from which the arms are detached, which can be found branched or rigid in appearance.

Externally they closely resemble starfish. They also constitute the group of echinoids that contains the largest number of species, with approximately 2,000 described.

Specimens of ophiuroids. Source: NOAA / NOS / NMS / FGBNMS; National Marine Sanctuaries Media Library

The origin of the ophiuroids dates back to the Paleozoic era, specifically the Ordovician period, since the first fossils collected come from this. Due to this, it is possible to affirm that these organisms have been really successful as far as adaptation is concerned, since they have managed to survive several processes of mass extinction.

Taxonomy

The taxonomic classification of ophiuroids is as follows:

-Domain: Eukarya.

-Animalia Kingdom.

-Filo: Echinodermata.

-Subphylum: Eleutherozoa.

-Class: Ophiuroidea.

characteristics

The ophiuroids are eukaryotic and multicellular organisms, whose cells have the particularity of conserving totipotency. This means that they can be transformed into any type of cell according to the individual's requirements.

Likewise, they are triblastic, due to the fact that the three germ layers are present in their embryonic development: ectoderm, mesoderm and endoderm. Continuing with the embryological part, they are coelominated and deuterostomized.

These animals present a type of pentaradial symmetry, in which their organs are arranged around a central axis. They move through a system of tube feet similar to that of other echinoderms, with the difference that they do not have suction cups or blisters.

They are dioecious, which means that the sexes are separate. They do not present sexual dimorphism, since both males and females are physically similar.

They also reproduce in a sexual and asexual way. Its fertilization is external and its development indirect. It should be noted that they are oviparous.

Morphology

- External anatomy

The members of the Ophiuroidea class have a rounded body, which is flattened. The arms are detached from this, which are characterized by being thin and long.

The body has two surfaces, one aboral and the other oral. On the aboral side it has several plates. One of them has a central position and the others are arranged around it. These plates are overlapped, which means that they are superimposed on each other, like the tiles of a roof. No hole is observed on this surface.

Example of ophiuroid. Their arms covered with thorns are observed. Source: Bernard DUPONT from FRANCE

On the oral surface, the most striking structure is the mouth. This is surrounded by about five jaws. It also presents the oral shields, under one of which is the madreporito.

The arms are formed by structures called ossicles that are articulated with each other and are covered by plates. Likewise, the arms of some species have spines.

- Internal anatomy

Nervous system

It's pretty straightforward. It is made up of a nerve ring located in the central disc. From this nerve fibers emerge that are distributed towards the arms. It is generally one pair for each arm.

Digestive system

The digestive system that these individuals present is incomplete. They have an inlet opening (the mouth) and no outlet opening.

The mouth is made up of several jaws (5), which allow entry to the oral cavity. Immediately after this is a small tube, the esophagus. This is characterized because its cells are ciliated, which facilitates the transit of food.

Finally the esophagus empties into a blind stomach, which is quite wide, occupying almost the entire cavity of the central disc. The stomach also has a series of lateral lobes.

Circulatory system

It is divided into two parts: a small ring of blood vessels (oral hemal lagoon) and a larger ring (aboral hemal lagoon), almost the same diameter as the central disc. The latter emits blood vessels that supply organs such as the gonads and the stomach.

Habitat and distribution

The ophiuroids are purely aquatic and marine animals. This means that they live in bodies of brackish water such as the oceans and seas.

Regarding the geographical distribution of these organisms, they are found mainly in the tropical areas of the Atlantic Ocean, from which it can be deduced that they prefer areas where temperatures are warm.

They are particularly abundant where there are large coral reefs, such as the Gulf of Mexico.

The depth at which they can be found is quite variable, since species have been found at only 1 meter deep, as well as others have been found at more than 300 meters.

Reproduction

In ophiuroids, the two types of reproduction that exist can be observed: sexual and asexual. The basic difference between the two is that sexual involves the fusion of sex cells and therefore the interaction of two individuals, while asexual reproduction only requires one parent, since it does not involve the combination of genetic material.

Asexual reproduction

There are various mechanisms through which asexual reproduction can occur. In the case of ophiuroids, the type of asexual reproduction observed is fragmentation. This consists of the individual losing one of her arms and part of her central disk.

From these fragments it is possible for a new individual to regenerate. This is because the cells of the ophiuroids retain an extremely important property: totipotency. Totipotent cells are those that can transform and develop any type of tissue.

In such a way that, when it is detached, the cells of that fragment are activated and begin to undergo a differentiation process, transforming into the different tissues that are required to generate another individual.

Sexual reproduction

It is the type of reproduction that is observed more frequently in ophiuroids. Fertilization is external, as it occurs outside the female's body.

The process is as follows: gametes are produced in the gonads that are found inside the bursae. When the time is right, those gametes are released outside, where they meet and fuse. From there an egg is formed, which can be incubated within the bursae.

When the required time has elapsed, a small larva hatches from the egg, known by the name of Ofiopluteus. This is of free life, until it begins to undergo the metamorphosis that will turn it into a small star, when it is deposited on the substrate.

Nutrition

Ophiuroids are considered heterotrophic organisms. Taking into account their diet, these individuals can be: predators, scavengers or suspensivores.

Predators capture their prey with the help of their arms, especially the spines that they contain and the gelatinous substance, similar to the mucus that they secrete.

Thanks to this, the prey remain attached to them. Later, they flex their arms until they carry their small prey to their mouths. Among the most common prey are polychaetes, some mollusks and crustaceans.

In the case of those that are suspensivores, they wave their arms so that the different food particles that are in the currents remain attached to them. Immediately, with the help of the secreted mucus, they form a kind of food bolus, which is slowly transported along the aboral surface of the arm, until it reaches the mouth.

There are also those that are scavengers, who feed on dead organic matter, that is, the remains of animals that are in a state of decomposition.

From the mouth, the food moves towards the esophagus and there, thanks to the cilia, it is transported to the stomach where the process of digestion and absorption of nutrients occurs. Finally, digestion wastes are eliminated through the mouth, since the digestive system of ophiuroids does not have an anus.

Representative species

Gorgonocephalus arcticus

This species is characterized by having several arms that are widely branched. They also have a skeleton that is made up of magnesium carbonate. It belongs to the order Phrynophiurida.

Astrophyton muricatum

It is part of the order Phrynophiurida. It is particularly abundant on the north coast of South America (Caribbean Sea) and on the east coast of Mexico.

It has 5 arms that in turn branch into several branches. It has nocturnal habits, since during the day it remains sessile, with its arms closed around it for protection. At night it tends to spread out and move in search of food.

Astrophyton muricatum specimen with its arms retracted. Source: NOAA

Astroboa nuda

It has habits very similar to those of Astrophyton muricatum. During the day it compacts its arms around its disk, forming a ball and at night it unfolds them to fulfill its feeding process. It has a rather plant appearance and is pale in color.

References

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