Bryozoans characteristics, morphology, reproduction, nutrition

2047
Philip Kelley

The bryozoans They are a group of animals that are also known as ectoprozoans. Etymologically, the word bryozoan means "moss animal", which combines perfectly with its appearance, since they look like small plants stuck to the substrate..

These animals are particularly abundant, with more than 5,500 species having been described and it is believed that many more remain to be discovered. They are typical of aquatic environments and, like brachiopods, belong to the group of lofoforates. As such, they have a characteristic element in their anatomy: the loptophore, which helps them capture food..

Graphic representation of various types of bryozoans. Source: Ernst Haeckel [Public domain]

Thanks to their appearance, they are normally confused with other organisms that make life in the seas such as algae and corals. In fact, their physical characteristics have been a headache for taxonomists, not knowing exactly where to locate them for a long time..

Article index

  • 1 Taxonomy
  • 2 Features
  • 3 Morphology
    • 3.1 - External anatomy
    • 3.2 - Internal anatomy
  • 4 Habitat and distribution
  • 5 Classification
    • 5.1 Phylactolaemata
    • 5.2 Gymnolaemata
    • 5.3 Stenolaemata
  • 6 Playback
    • 6.1 Asexual reproduction
    • 6.2 Sexual reproduction
  • 7 Nutrition
  • 8 Featured species
    • 8.1 Myriapora truncata
    • 8.2 Pentapora fascialis
    • 8.3 Tricellaria inopinata
    • 8.4 Bugula neritina
  • 9 References

Taxonomy

The taxonomic classification of bryozoans is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Superfilo: Spiralia

-Lophotrochozoa

-Lophophorata

-Phylum: Bryozoa.

Characteristics

Bryozoans are animals that are not found individually, but forming colonies. The size of these colonies is variable, there are small ones, as well as there are species that form colonies of up to 30 cm. These colonies are sessile and attach to hard substrates such as rocks, although they also attach to the shells of some animals..

Likewise, bryozoans enter into the classification of multicellular coelomed animals. This means that they are made up of various cell types, which specialize in a wide range of functions. These include nutrition, absorption, and reproduction..

Taking into account the embryonic development, the animals of this phylum are considered triblastic, since they present at some point in their formation, the three germ layers: endoderm, mesoderm and ectoderm. The importance of these layers lies in the fact that it is from them that the tissues of the adult organism are formed.

Like many animals, bryozoans have an internal cavity known as a coelom. This contains the internal organs of the animal.

When it comes to nutrition, bryozoans are considered heterotrophic animals. This is because even though they look like plants, they are not. Therefore, they do not carry out the photosynthesis process, but must feed on other living beings or substances made by others..

They also have bilateral symmetry, so if an imaginary line is drawn along the longitudinal plane of the animal's body, two exactly equal halves are obtained..

Finally, bryozoans are hermaphrodites, that is, they have female and male reproductive structures. They reproduce sexually and asexually, with internal fertilization and indirect development.

Morphology

- External anatomy

Bryozoans are mainly made up of two parts: the cystid or teak and the zooid, also known as polipid..

Cistido

The cystid is not a single structure, but is made up of the union of two parts: a cover that is rigid, calcareous and the wall of the animal's body. The latter is made up of two areas: the exocyst, made up of the union of two layers, the cuticle and the covering; and the endocyst, made up of the innermost layer (somatopleura) and the epidermis.

As their names allow to infer it, the endocyst is internal and the exocyst presents a more external position. It is important to note that the calcareous layer is synthesized and secreted by the epidermis.

Zooid

It has approximate measurements of 1 mm. It constitutes the soft parts of the animal. It is protected by the cystid. Its most distinctive element is the loptophore.

This is a structure that is retractable and is made up of several tentacle-like arms. The number of these is variable, although generally it does not exceed 20. The function of the lophophore is related to nutrition. The body presents on one of its sides the opening that corresponds to the anus of the animal.

Image of a bryozoan in which the loptophore is appreciated. Source: TheAlphaWolf [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

- Internal anatomy

Inside the body, bryozoans have three cavities: procele, mesocele, and metacele. The first is found only in those species that develop in freshwater habitat..

Of the three cavities, the largest is the metacele. Because bryozoans are not isolated from each other, but forming colonial structures, they communicate with each other. In this sense, the metacele of one communicates with the metacele of another, through a conduit known as rosette plates. This communication plays an important role during sexual reproduction.

Bryozoans do not have a respiratory, excretory, or circulatory system. Respiration and excretion occur through a simple process of passive transport, diffusion, that occurs through the body wall.

Digestive system

It is the dominant apparatus of the bryozoans. It occupies practically the entire interior of the animal's body and is made up of a tube that is U-shaped..

It begins with the mouth, which opens in the center of the loptophore. Immediately after this comes a widening of the tube, the pharynx. This is followed by a very short esophagus, which opens into the stomach.

The stomach is divided into three parts: the cardia, cecum, and pylorus. After the stomach there is a small intestine and finally the rectum, which culminates in the anal opening..

Nervous system

The nervous system of bryozoans is quite rudimentary and simple. It consists of a single nerve ganglion that is located above the esophagus and is attached to a ring that continues around the pharynx.

Nerve fibers emerge from this ganglion and are distributed throughout the animal's body..

Habitat and distribution

Bryozoans are animals that are limited solely and exclusively to aquatic environments. Within these, they are mainly found in salt water. Only a very small percentage of the species that make up this phylum are found in freshwater bodies.

They are generally found forming colonies, which can be found fixed to a substrate, covering large areas, or they can appear as erect, branched or foliar colonies..

In terms of distribution, bryozoans can be found in all the oceans of the planet. This regardless of the temperature, since they have managed to adapt to different temperature levels. In such a way that there are them in tropical waters of warm temperature and in cold waters also.

Classification

The bryozoa phylum is made up of three classes: Phylactolaemata, Gymnolaemata and Stenolaemata.

Phylactolaemata

A group of bryozoans that are unique to freshwater habitats. Organisms of this class usually live in colonies, in which all members are exactly the same: clones of the individual that gave rise to the colony. This class consists of a single order: Plumatellida.

Gymnolaemata

These are considered to be the most common group of bryozoans. They can be found in both fresh and salt water, although they are more abundant in the latter. They form large, sessile colonies.

Likewise, the consistency of these is varied, there are flexible like gelatin and rigid calcareous. It is made up of two orders: Cheilostomata and Ctenostomata. Also an order that is extinct: Cryptostomata.

Stenolaemata

It is a group of bryozoans dating from the Paleozoic Era. They are organisms that are clearly found on the seabed. Furthermore, they have a limited polymorphism. This class is made up of seven orders, of which six are extinct and only one survives to this day: Cyclostomatida.

Reproduction

Bryozoans are hermaphrodites. They also present a phenomenon known as proterandria. This consists of the organism going through two stages, first one in which it is male and a second in which it is female..

Likewise, in bryozoans there are two types of reproduction: asexual and sexual. The latter involves the fusion of sex cells, while the asexual does not.

Asexual reproduction

In this type of reproduction, individuals are obtained exactly the same as the first one that originated the colony. In bryozoans, the observed asexual reproduction mechanism is budding. Colonies are formed by this process.

Budding consists of the formation of a bud somewhere on the surface of the animal. The cells in that bud undergo a series of cell divisions until a new individual is formed..

Likewise, in some bryozoans a special budding process occurs that allows them to survive adverse environmental conditions. Bryozoans produce a kind of capsule that is filled with cells. This generally occurs in the summer and fall. They are highly resistant to the harsh conditions of the winter season and in the spring they give rise to new bryozoans.

Sexual reproduction

As mentioned, bryozoans group together to form colonies. These are made up of both female and male individuals.

Sperm pass from one individual to the other through the conduit that communicates them (rosette plates) for fertilization to occur. Although self-fertilization may also occur.

Fertilization occurs within the cyst. Once the eggs are fertilized, they remain inside the coelomatic cavity. These can be incubated or expelled to the outside.

The development of bryozoans is indirect, since a larva emerges from the fertilized eggs that has the ability to move freely through the water by swimming. Finally it is fixed to some substrate and begins to generate a new colony through budding..

Nutrition

Bryozoans are suspensivorous animals, which means that they feed on suspended particles in water currents. They also feed on zooplankton.

To be able to feed, the loptophore is an essential element. This is responsible for redirecting the water currents towards the animal's mouth. Another element that contributes and facilitates feeding is the mucus secreted by the tentacles of the loptophore, which captures the food and moves it towards the mouth..

Food is not swallowed immediately, but rather accumulates in the oral cavity. Once there are enough, the pharynx dilates and food passes into the esophagus. From here they go to the cardia, where they experience a large number of movements, which help to mix and grind.

Subsequently, the food passes to the cecum, where it is subjected to enveloping movements, a product of the cilia there. The food particles that are not absorbed are compacted into small balls of feces, which are released through the anus.

Featured species

Myriapora truncata

This species forms arborescent colonies with a resistant consistency, with a large number of pores on their surface. The coloration that they can present is between orange and red. Its habitat is the Mediterranean Sea and it is located mainly in poorly lit places, such as crevasses..

Myriapora truncata. Source: Parent Géry [Public domain]

Pentapora fascialis

The colonies that this species forms have flat branches that are irregular in shape. It is of an intermediate color between pink and orange and is kept fixed to the substrate by structures known as encrustants..

These species have been found at depths of up to 100 meters and can grow to more than 15 cm. It is also known as 'elkhorn', due to the resemblance it has with the antlers of these animals.

Tricellaria inopinata

This type of bryozoan forms erect columns that are attached to hard substrates such as pieces of wood, rocks, algae and even some invertebrates such as the shells of some mollusks. It has a whitish color.

It is found in the Pacific Ocean and in the north of the European continent. Its growth is fast, so it covers the substrates in very short periods of time.

Bugula neritina

This bryozoan forms colonies that can reach 15 cm in length. They have a bushy and at the same time branched appearance. It is very abundant on the coasts of the Pacific Ocean. It fixes on hard substrates such as some algae, other bryozoans, and some invertebrate animals, especially their shells. They are usually dark in color, be it red or brown.

References

  1. Bock, P., Gordon. (2013). Phylum Bryozoa Ehrenberg 1831. Zootaxa 3703 (1).
  2. Capetillo, N. (2011). The wonderful bryozoans. The bohío special supplement. 1 (2).
  3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
  4. Hickman, C. P., Roberts, L. S., Larson, A., Ober, W. C., & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill
  5. Massard, J. and Gemer, G. (2008) Global diversity of bryozoans (Bryozoa or Ectoprocta) in freshwater. Hydrobiology 595
  6. Ryland, J. (2005). Bryozoa: an introductory review. Swansea University.

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