BRACHIOPODS: CHARACTERISTICS, MORPHOLOGY, CLASSIFICATION - BIOLOGY - 2025

The brachiopods are a phylum of animals that originated from the Cambrian period and had its peak until the Ordovician. At present they are considered as a vestigial group; there are only about 335 known species.

They are characterized by presenting two shells, similar to bivalve mollusks; the difference of this group is that their shells are uneven. In addition, in bivalves its plane of symmetry is located where both shells meet, while in brachiopods it is perpendicular to the union of the two shells.

Example of a brachiopod. Source: Didier Descouens

Likewise, they have a peduncle through which they remain fixed to the substrate. They are found in purely marine environments, especially in very deep places. Species of brachiopods have been recorded that, instead of attaching to an inanimate surface, they do so to algae.

Taxonomy

The taxonomic classification of brachiopods is as follows:

• Kingdom: Animalia
• Superfilo: Brachiozoa
• Phylum: Brachiopoda

General characteristics

They are triblastic and coelomed

Brachiopods are triblastic organisms. This means that during their embryonic development they present the three germ layers: ectoderm, mesoderm and endoderm. From these, the various organs that will make up the adult individual are generated.

Similarly, they have coelom, a cavity that is formed from the mesoderm. In brachiopods it is divided into 2 parts: mesocele and metacele.

They are protostomados

In protostome animals, during the period of embryonic development, the blastopore first gives rise to the mouth. Some have an anus (such as inarticulates), while others do not (such as articulated)

Longevity

The different species of brachiopods that exist do not have a standard lifespan. They can live from 3 to 30 years, in some cases even longer.

Behavior

In their adult phase, the vast majority of brachiopods are sessile in life. They are fixed to the substrate through their peduncle. In the larval stage they are free and can float freely.

Feeding

The feeding process is pretty straightforward. The shells are opened by different mechanisms in articulated and inarticulated. Lolophore cilia create currents through which phytoplankton are drawn to the animal. Food passes through a structure known as the brachial sulcus, towards the mouth.

Digestion takes place in the so-called digestive gland, which, through various contractions and relaxations, introduces food and excretes waste in the form of feces. The fecal balls are expelled out of the animal by opening and closing the shells.

Morphology

The main characteristic of brachiopods is that they are made up of two valves, placed in such a way that one goes up and the other goes down. Its size is variable, there are from 5mm to more than 80mm. Even fossils have been found that measure 38 cm.

Valves

The valves or shells are secreted by the mantle. This is nothing more than a crease in the wall of the body. These shells are covered by an extremely thin layer, made up of material of organic origin, known as the periostraque.

Likewise, between the two shells there is a cavity that is known as the paleal cavity. Inside this a typical structure of brachiopods is located, called lophophore.

Loptophore

The loptophore is an organ that can have a horseshoe or crown shape, characterized by being covered by a large number of extensions or cilia. It is located near the mouth of the animal.

The function of this organ has to do with the feeding of the animal. When they vibrate, they cause currents to form in the water that undoubtedly attract possible food particles. It catches them and introduces them into the oral cavity to be processed.

The lolophore is attached to a structure known as the brachidium. The brachidium is an extension of one of the leaflets.

The leaflets open and close thanks to the action of the adductor (to close) and abductor (to open) muscles.

Morphology of a brachiopod. Source: Muriel Gottrop and TaraTaylorDesign

Similarly, brachiopods have a peduncle through which they can be attached to the substrate. Despite being of strong and solid consistency, the peduncle has the characteristic of being hollow.

With regard to the material that constitutes the valves of the brachiopods, there are two types. In articulated brachiopods, the shell is made of calcium carbonate, whereas in non-articulated brachiopods, there are shells composed of calcium phosphate with chitin.

Internal anatomy

Brachiopods have specialized systems: circulatory, digestive, excretory, and nervous.

Circulatory system

It is a mixed system, since it has closed vessels and some lagoons. It has a central vessel and other lateral vessels.

Similarly, the coelom plays an important role in the circulation process.

Digestive system

It has specialized structures: mouth, esophagus, stomach, intestine, rectum and anus. In the case of the articulated ones, the digestive tract is blind, that is, they do not present an anus.

The digestive glands and the hepatopancreas flow into the stomach.

Excretory system

It presents the metanephridiums, which are organized in pairs. There are 1 or 2 pairs. These lead to the metacele.

It also has nephridiopores, which open to the outside on each side of the mouth.

Nervous system

The nervous system is quite rudimentary. The nerve fibers are concentrated around the esophagus. From the supraesophageal lymph node mass, nerves are given off to the mantle and the loptophore. Similarly, there is a periosophageal ring from which the nerves emerge for all the remaining organs.

Habitat

This type of organism is found exclusively in marine habitats. However, they are not abundant in places with many waves or currents. So typical places where brachiopods are most likely to be found include: crevices and caves, rocky ledges, ocean floor, and slopes of continental shelves.

Likewise, it is pertinent to mention that through their peduncle, they are attached to the substrates. Some also prefer to sink into shallow water sediments. Similarly, they are more abundant in marine places where temperatures are quite low.

Reproduction

The type of reproduction that is observed in brachiopods is sexual. None of the known species reproduce asexually. As is known, sexual reproduction involves the union of sex cells or gametes, female and male.

Brachiopods are dioecious, which means that the sexes are separate. There are female and other male individuals. In very few if not any species hermaphroditic individuals can be observed.

Likewise, the fertilization observed in brachiopods is external. This type of fertilization is carried out outside the female's body.

Gametes, ovules and sperm, develop in the gonadal tissue that is derived from the peritoneum of the metacele. Once the gametes are mature enough, they remain free in the metacele and are released to the outside through the nephridia.

Fertilization and embryonic development

Already abroad, both gametes fuse in the process of fertilization, forming the zygote. Later, the zygote undergoes its maturation and development process until it reaches the larval stage. All brachiopods develop the free larvae stage.

It is important to note that there are some species of brachiopods, specifically of the articulated type, that are of the incubator type. In these species, the females incubate the fertilized eggs until they reach the larval form and are released.

Once fertilization occurs and the zygote is formed, it undergoes the segmentation process, which is of a total and equal type. Likewise, the symmetry of these organisms is radial. Eventually, a structure known as a celloblastula is formed which subsequently undergoes gastrulation.

Through the gastrulation process, the archenteron is formed. The coelom originates from here, through two processes, depending on the type of brachiopod.

Articulated

In this type of brachiopod, the coelom is produced through a process known as enterocelia.

Finally, the larva is divided into three lobes: anterior, peduncular and mantle. Likewise, the edges of the mantle are folded back along the peduncle.

Unarticulated

In the inarticulate, the coelom is produced by schizocelia.

Later, the larvae formed are similar in appearance to adult individuals. The difference is that the peduncle is retracted in the cavity of the mantle and both the lobes of the lobule and the body have a disproportionate size, they are very large.

Classification

Brachiopods are classified into two classes: Articulata and Inarticulata.

Articulata Class

Individuals in this class have the following characteristics:

• Composed of approximately 290 species, distributed in three orders: Rhynchonellida, Terebratulida and Thecidedina.
• The digestive tract does not have an anus.
• Their shells are made of calcium carbonate.
• They have a peduncle, but it is not muscled.
• The lolophore features internal support elements
• The shells are joined by a system of pits and teeth.

Examples of brachiopods. Source: Luis Ruiz Berti

Class Inarticulata

Inarticulate brachiopods have the following characteristics:

• It is made up of approximately 45 species, distributed in two orders: Lingula and Acrotretida.
• They have a digestive tube with anus.
• The shells of the inarticulate are composed of calcium phosphate.
• Despite the fact that some species lack a peduncle, in those that do have one, they present an intrinsic musculature.
• The lolophore is internal and does not have any type of support.
• The shells of the inarticulate are united only through the action of muscles.

References

1. Boucot A., Johnson, J. and Talent, J. (1969). Early Devonian Brachiopod Zoogeography. The Geological Society of America.
2. Brusca, R. and Brusca, G. 2005. Invertebrates. McGraw Hill, Interamericana.
3. Curtis, H., Barnes, N., Schnek, 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. Moore, RC; Lalicker, CG; Fischer, AG (1952). Invertebrate Fossils. Mcgraw-Hill College
6. Ushatinskaya, GT (2008). "Origin and dispersal of the earliest brachiopods". Paleontological Journal 42 (8): 776-791