The Phylum Bryozoa is undoubtedly the largest and most common of the lophophore phyla. This phylum contains about 4,000 living species and is one of the major animal phyla.
Despite this, it is less widely recognized than other groups of animals. This article examines this animal group’s anatomy, physiology, individual morphologies, ecology, and phylogenetic classification.
Table of Contents
What is the Phylum Bryozoa?
They are colonial, sessile (attached to the substrate) creatures of tiny size (approximately 0.5 mm) that are generally covered by a protective covering that leaves an aperture through which the lophophore, a group-specific organ, emerges.
The Phylum Bryozoa is a simple creature in which coelom occupies the majority of the body cavity; no organs for respiration, excretion, or circulation are detected, which may be owing to the size of these organisms. The digesting machinery, which is U-shaped, may be seen.
There are three classes of the phylum bryozoa: Phylactolaemata, Gymnolaemata, and Stenolaemata.
The majority of today’s bryozoans, virtually all of which are marine, are classified as Gymnolaemata. The animals in the class Phylactolaemata are all freshwater organisms with only around 50 species, while the organisms in the class Stenolaemata include a few modern marine species and over 500 fossil species.
Morphology of the Phylum Bryozoa
Individual colonies have varying morphologies depending on the species and, more importantly, which class they belong to. Some are mostly box or oval shaped, with some being tubular (depending on the species).
In general, they are protected by a protective covering (zoecium), which may be composed of chitin and proteins (as in the class Gymnolaemata, order Anasca) or a calcium carbonate cover on which a cuticle is found (as in the class Gymnolaemata, order Ascophora); the thickness of this calcareous layer varies greatly between species.
Individuals dedicated to feeding (autozooids) in all species, independent of cover, have an opening through which the lophophore will emerge. Some of the more advanced animals even have a cover to shield this entrance.
The digestive system, muscle fibers, and the funiculus, which is a tube of peritoneal tissue dedicated mostly to food distribution, are located inside the protective cover, while the coelom occupies the majority of the hole.
Because of the presence of the lophophore, the phylum bryozan is classified as a lophophores. It is a crown of tentacles of various forms that is preserved in the animal’s cavity and evolved for eating; it is normally circular with a series of tentacles with cilia and a canal in the middle through which food is forced towards the base of the lophophore where the mouth is placed.
Longitudinal muscle fibers, nerves, and a little part of coelom are found within the tentacles. The anus, which is also seen in the lophophore, is placed at the end of the digestive system.
The lophophore of freshwater filactolemates is horseshoe-shaped rather than round. The muscle fibers that traverse the coelom help to evert the lophophore and return it to its cavity. The nervous system is made up of a ring from which nerves emerge, most of which travel to the tentacles but also to other sections of the organism.
Characteristics of the Phylum Bryozoa
The Phylum Bryozoans are common creatures found in coastal seas, where they grow on rocks, shells, seaweed, and other surfaces.
While some species create stoloniferous colonies, the majority develop encrusting or arboreal colonies that are based on the fusion of neighboring zooids. The dorsal surface is on the substrate or other zooids, whereas the ventral surface is uncovered and known as the frontal surface.
There are species with little calcified that form arboreal colonies, such as Bugula, in which the zooids are fused in a double row; however, the vast majority are encrusting species, glued to the substrate on the dorsal part while the ventral or frontal part is exposed and on which the opening of the lophophore can be found.
Erect foliaceous colonies can also be generated by a layer of zooids or two layers united by the dorsal portion of the zooids.
The Phylum Bryozoans communicate with one another via pores in their walls. The passage of coelomic fluid via these holes has been reported in certain species, although they are generally hindered by specific cells, some of which are associated to the funiculus, and it has been demonstrated that lipids flow through the funiculus from the feeding zooids.
While their structure is basic, it has been shown that at least some species (Electra, Membranipora) have a colonial neural system; nerves from one individual would flow through the pores and connect in some way with the nerves of another zooid. In the event of danger, this neural system can retract the lophophores.
Colonies of the phylum bryozoans are often polymorphic, meaning they contain zooids of various morphologies. The type zooids are those that can feed, known as autozooids, and colonies are largely made up of these zooids. Individuals who have been altered are referred to as heterozooids.
Some of the phylum bryozoans are adapted for colony attachment in the form of stolons, attachment disks, rhizoid structures, and so on; these are extremely small individuals with just the body wall and funicular tissue visible inside. The most sophisticated bryozoans, the cheilostomates, have adapted heterozooids for colony protection. The avicularia and vibraculae are as follows:
Avicularia: They are somewhat smaller than autozooids and have a highly developed operculum that serves as a jaw to protect the colony from assaults by small creatures and larvae of other species that may attach themselves to the colony. These avicularia can be sessile or pedunculate, in which case they perform quick motions like a bird’s peck. They have also been discovered in some Bugula species to act as a defense against bigger creatures such as amphipods and polychaetes whose appendages would be stuck in the jaws.
Vibraculae: In this example, the operculum is transformed into a thread or bristle that cleans the colony of debris and larvae from neighboring colonies as it moves.
Physiology of the Phylum Bryozoa
The Phylum Bryozoans eat by utilizing their lophophore to capture food particles floating in water. The lophophore resembles a crown of tentacles and is formed by increases in coelomic fluid pressure caused by muscle movements inside the organism.
In organisms with soft or flexible coverings, these muscles are attached to these walls and, upon contraction, cause the pressure of the coelomic fluid to rise, whereas in animals with rigid calcareous coverings, there is a chitinous membrane on the front wall to which the muscles are attached, and, upon contraction, this covering invaginates, causing the pressure of the coelomic fluid to rise, causing the lophophore to emerge.
Lophophore eversion in cyclostomates is caused by a distinct process, which is currently being explored. It is known that muscles are employed, but the body wall is not distorted. The lophophore retractor muscles, which are attached to the lophophore and contract to force the lophophore into the body cavity, are responsible for retraction.
The lophophore is made up of tentacles, which have cilia, and the movement of these cilia is what pulls food particles towards the mouth. They generate a current that drives the water towards the funnel’s base, where it is forced out between the tentacles.
There are several current research publications investigating the prey size of the phylum bryozoans, the majority of which are between 2 and 10 m in length. Many species are capable of twisting the lophophore-formed funnel to find additional food.
Cilia in the pharynx process food, which is then sent to the stomach, which takes up the majority of the digestive tract. Digestion occurs here, both extracellular and intracellular.
Respiration is produced by the body wall itself, as there is no mechanism specialized to respiration. The coelomic fluid is responsible for the movement of gases, food, and waste. The funiculus transports nutrients between people, at least in part.
Fertilization of the Phylum Bryozoa
Most of the Phylum Bryozoans are hermaphrodites, meaning they have both sexes in the same zooid, and are generally proterandrous (male organs develop before female organs), however there are few dioecious species with distinct male and female zooids.
Fertilization has only been observed in a few anasci, and the sperm is delivered through the apices of some of the tentacles.
The supraneural pore is a pore in the back that allows eggs to be discharged. Only a few species discharge their eggs into the sea, whereas the majority of marine and freshwater species incubate them, so the egg exits via the pore and enters a chamber where it develops.
Cyphonaute larvae are formed from the eggs that grow in the water. The young of filactolemates develop in an embryonic sac that arises on the body’s surface.
Fertilization in ascomorphs has not yet been witnessed, but it is thought to occur relatively shortly after the egg enters that chamber. Most cheilostomata have these chambers, which are known as ovicells.
They are usually found at the distal end of a zooid, at the boundary between two contiguous zooids, and consist of a globular-shaped ooecium formed by two layers, the inner one called entooecium and the outer one called ectooecium, both of which can be calcified or one or both of which can be membranous; usually the frontal area of the ectooec
Although certain species’ reproduction is known to be seasonal, most species’ reproduction happens all year.
The embryos of the phylum bryozoans grows cilia and becomes a free-swimming larva. Some species generate cyphonaute larvae that have been modified. Once released, the larvae swim towards the light, which appears to encourage species distribution since they would travel to the surface where water displacements are larger.
This migratory stage is brief, and they quickly cling to the substrate. When the larva reaches the substrate, it investigates it with its cilia and continues to crawl ahead until it finds a better site for attachment. A initial zooid called ancestrula is created, which soon grows into a functioning zooid with feeding capability and is generally considerably distinct in shape from succeeding zooids.
Because the reproductive cycles of the phylum bryozoans are so diverse, it is impossible to give a general rule; however, in our latitudes, it is normal that at the beginning of spring, coinciding with the increase in daylight hours, there is an important somatic growth that culminates with the species’ reproduction.
Colony formation of the Phylum Bryozoans
After attachment, the larva undergoes metamorphosis, giving rise to the colony’s first member, the ancestrula, which is morphologically distinct from the other zooids. Gemmation produces heterozooids from it. This process varies by species; in general, buds are generated that give rise to new individuals.
Zooids are short-lived and are replaced via regeneration. Their body compresses dramatically, their tissues become disordered, and they can even be phagocytized; the stomach tissue turns into a brownish mass known as the brown body.
A bud grows in the wall of this animal, giving rise to a new individual capable of storing the brown body inside. Individual degeneration is logically caused by age, but environmental circumstances appear to be quite important as well.
Phylactolemates reproduce asexually via statoblasts during colony development. Buds emerge on the funiculus and convert into a stiff membrane containing undifferentiated, yolk-laden germ cells.
Evolution of the Phylum Bryozoa
Most of the phylum bryozoans are phylactolemates. Primitive features include tube-shaped zooids, an aperture at the anterior end, a horseshoe-shaped lophophore, and the lack of heterozooids.
The Cambrian period has the first known fossil of the phylum bryozoa. The cheilostomates, the most numerous marine group today, evolved during the Jurassic period.
They are creatures with high adaptation success; they attach themselves to many sorts of hard surfaces, often drilling through them to do so.
They are widely recognized for their abundance on ship hulls and the resulting economic concerns. Antifouling agents do not often function with these species, which attach themselves to the substrate by chemical perforation and, once attached, encourage the subsequent adhesion of algae and other organisms.
The existence of spines and the increasing complexity of the same, as well as the development of heterozooids as complex as the avicularia, are characteristics that imply evolution, as are the reduction of the membranous frontal region and its replacement by calcareous surface. As a result, the ascophores are unquestionably the most developed, followed by the anasci.
The Phylum Bryozoans are not highly selective in their attachment to homogenous substrates, but when the substrate is rich in diversity, bryozoans exhibit a notable preference for various types of substrate, depending on the species. Most species appear to favor mobile substrates for adhesion, such as algae.
This appears plausible, because on a mobile substrate, access to replenished water with more suspended particles that serve as food is higher.
The Phylum Bryozoans are also frequent in locations with water currents, where feeding of these creatures is encouraged. Temperature-salinity differences appear to be connected to differences in the depth range in which bryozoans are found, with depth being just an associated component.
Sea urchins and certain fishes prey on the phylum bryozoans, but their principal predators are small, highly specialized invertebrates like nudibranchs and pycnogonids. Adalaria proxima, a well-known example, consumes the phylum bryozoan Electra pilosa epiphyte of Fucus serratus.
Classification of the Phylum Bryozoa
- Phylactolaemata class. Freshwater with cylindrical zooids and a lophophore in the form of a horseshoe. The zooids have not calcified. Individuals have a continuous coelom. There is no polymorphism.
- The Stenolaemata class. Bryozoans from the sea. Zooids are tubular, having walls that have already calcified and bonded between them. The orifices terminate.
- Gymnolaemata is a class. The majority is marine. Because of polymorphism. Lophophore in a circle.
- Order Ctenostomata. Colonies stoloniferous or compact. With non-calcified exoskeleton. The orifice through which the lophophore exits does not have an operculum.
- Order Cheilostomata. Box-shaped zooids. With calcareous walls. The orifice through which the lophophore exits has an operculum (except in the genus Bugula). They may have avicularia, vibraculae or both. Eggs are usually hatched in ovicells (special structures to perform this function).
- Suborder Anasca. The front wall is membranous.
- Suborder Ascophora. The frontal wall is calcified.
