33.7: Bryozoans (Bryozoa) and Brachiopods (Brachiopoda)

Phylum Bryozoa

Bryozoa, also known as Ectoprocta, is a major invertebrate phylum, whose members, the bryozoans, are tiny, aquatic, and mostly sessile and colonial animals. Also known as moss animals or sea mats, the colonial species of bryozoans generally build collective stony skeletons of calcium carbonate that are superficially similar to coral.

Bryozoans have a distinctive feeding organ called a lophophore found only in two other animal phyla, Phoronida (phoronid worms) and Brachiopoda (lamp shells). Characterized by some as a "crown" of ciliated tentacles, the lophophore is essentially a tentacle-bearing ribbon or string that is an extension (either horseshoe-shaped or circular) surrounding the mouth.

Tiny as individuals, but clearly visible and significant in their large colonies, bryozoans play an important role in sediment stabilization and binding, as well as key roles in food chains, consuming microorganisms and in turn being prey for fish and other animals. Their bi-level functionality is apparent in the food chain where their harvesting of phytoplankton as a source of nutrition for their own maintenance, growth, and multiplication makes the nutrients from the phytoplankton available also to those fish and other animals that prey on the bryozoans. Additionally, while building their colonial structures in which they thrive, the byozoans serve the larger ecosystem by stabilizing sediments.

Bryozoans are found in marine, freshwater, and brackish environments. They generally prefer warm, tropical waters but are known to occur worldwide. There are about 5,000 living species, with several times that number of fossil forms known. Fossils are known from the early Ordovician period about 500 million years ago (mya).

Ecology

Although most species of Bryozoa live in marine environments, about 50 species inhabit freshwater. Some marine colonies have been found at 8,200 meters below the surface, but most bryozoans inhabit shallower water (Waggoner and Collins 1999). 

Bryozoans attach to a variety of solid substrates, including rocks, shells, wood, sand grains, and blades of kelp, although some colonies form on sediment (Waggoner and Collins 1999). Bryozoan colonies also encrust pipes and ships, becoming a nuisance. Freshwater bryozoans may attach to tree roots and aquatic plants.

Most extant (living) bryozoans are typically immobile, sessile, and colonial. Nonetheless, whether there are one or a few exceptions, bryozoans are characteristically colony-forming animals. Many millions of individuals can form one colony. The colonies range from millimeters to meters in size, but the individuals that make up the colonies are tiny, usually less than a millimeter long. In each colony, different individuals assume different functions. Some individuals (the autozooids) gather up the food for the colony, while others (the heterozooids) depend on them for food and contribute to the colony in other ways. Some individuals (the kenozooids) are devoted to strengthening the colony and still others (the vibracula) to cleaning the colony.

Bryozoans are suspension feeders, preying on phytoplankton, including diatoms and unicellular algae and being preyed upon by fish and sea urchins (Waggoner and Collins, 1999). 

Anatomy

Figure \(\PageIndex{1}\): Costazia costazi, a coralline bryozoan.. (CC-BY Wikipedia Katya Swarts)

Bryozoan "skeletons" are a collective production growing in a variety of shapes and patterns: Mound-shaped, lacy fans, branching twigs, and even corkscrew-shaped. Their "skeletons" have numerous tiny openings, each of which is the home of a minute animal called a zooid.

Each bryozoan zooid has a coelomate body (having a true body cavity) with a looped alimentary canal or gut, opening at the mouth and terminating at the anus.

Bryozoans feed with a specialized structure called a lophophore, which looks like a "crown" of tentacles and has been more precisely characterized as essentially a tentacle-bearing ribbon or string in the shape of a horseshoe or closed ring surrounding the mouth (Smithsonian 2007; Luria et al. 1981). Bryozoans have no defined respiratory or circulatory systems due to their small size. They do, however, have a hydrostatic skeletal system and a simple nervous system.

The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water that drives water, together with entrained food particles (mainly phytoplankton), toward the mouth. The digestive system has a U-shaped gut, and comprises a pharynx, which passes into the esophagus, followed by the stomach. The stomach has three parts: the cardia, the caecum, and the pylorus. The pylorus leads to an intestine and a short rectum terminating at the anus, which opens outside the lophophore. In some groups among the bryozoa, notably some members of the bryozoan order ctenostome, a specialized gizzard may be formed from the proximal part of the cardia.

The gut and lophophore, which comprise most of the organs and tissues of an individual zooid, are also the principal components of what is called the "polypide." Cyclical degeneration and regeneration of the polypide is characteristic of marine bryozoans. After the final cycle of polypide degeneration and regeneration, the skeletal aperture of the feeding zooid may become sealed by the secretion of a terminal diaphragm. In many bryozoan colonies, only the zooids within a few generations of the growing edge are in an actively feeding state; older, more proximal zooids (for example, in the interiors of bushy colonies) are usually dormant. Gaseous exchange occurs across the entire surface of the body, but particularly through the tentacles of the lophophore.

Bryozoans can reproduce both sexually and asexually. All freshwater bryozoans, as far as is known, are hermaphroditic (meaning they are both male and female), and most marine bryozoans as well (Smithsonian 2007). They may produce sperm and eggs at the same time (simultanenous hermaphrodites), or the male sexual organs may mature before the female organs (protandric hermaphrodites).

Asexual reproduction occurs by budding off new zooids as the colony grows, and is the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. A colony formed in this way is composed entirely of clones (genetically identical individuals) of the first animal, which is called the ancestrula.

Distinguishing features of Bryozoans, and Brachiopods

Bryozoans, and brachiopods strain food out of the water by means of a lophophore, a "crown" of hollow tentacles. Bryozoans form colonies consisting of clones called zooids. Brachiopods, generally thought to be closely related to bryozoans and phoronids, are distinguished by having shells rather like those of bivalves. All three of these phyla have a coelom, an internal cavity lined by mesothelium. Some encrusting bryozoan colonies with mineralized exoseletons look very like small corals. However, bryozoan colonies are founded by an ancestrula, which is round rather than shaped like a normal zooid of that species. 

Phylum Brachiopoda

Brachiopods are shelled, filter-feeding marine organisms (Figure \(\PageIndex{2}\)) that inhabit the seafloor and come in various shapes and sizes. They have been around since the  Cambrian with incredible diversity during the Paleozoic Era

 (Figure \(\PageIndex{3}\)). Brachiopods are still around today, but their diversity is greatly diminished.

Figure \(\PageIndex{2}\) – Examples of brachiopods. Image credit: Digital Atlas of Ancient Life, CC BY-NC-SA.

Figure \(\PageIndex{3}\) – Diversity of Brachiopoda genera. Image credit: Paleobiology Database, CC BY.

Superficially, brachiopods may look like bivalves, but the two are not related. One of the biggest differences between brachiopods and bivalves lies in their symmetry. Both have bilateral symmetry, but the plane of symmetry in brachiopods is vertical rather than horizontal (Figure \(\PageIndex{3}\)). This means that the left half of a brachiopod is a mirror image to the right half. In bivalves, the plane of symmetry is horizontal, and the upper half is a mirror image of the lower half.

Figure \(\PageIndex{4}\) – Differences in symmetries between a brachiopod (left) and a bivalve (right). Image credit: Jaleigh Q. Pier, CC BY-SA.

Brachiopod shells have two valves that are distinct in both size and shape (Figure \(\PageIndex{5}\)). The brachial valve is usually the smaller of the two valves and has a ridge, or fold, down the middle of the valve. The pedicle valve is usually larger than the brachial valve and has a valley down the middle. It also has a hole through which the pedicle passes. The pedicle is a fleshy, stalk-like feature which some groups use to attach themselves to hard rocky seafloor. It is rarely preserved, but brachiopod shells will often have a pedicle opening preserved along the hinge-line that varies in shape from rounded to triangular.

Figure \(\PageIndex{5}\) – Brachiopod morphology. Image credit: Jaleigh Q. Pier, CC BY-SA.

 

Phylum Bryozoa adapted from Bryozoa. (2008, November 19). New World Encyclopedia, . Retrieved 03:29, November 14, 2023 from https://www.newworldencyclopedia.org/p/index.php?title=Bryozoa&oldid=858749 CC-BY-SA.

Phylum Brachiopoda adapted from https://geo.libretexts.org/Bookshelves/Geography_(Physical)/The_Story_of_Earth_-_An_Observational_Guide_(Hauptvogel_and_Sisson)/01%3A_Labs/1.08%3A_Fossils. CC-BY-NC-SA