Bryozoans are tiny colonial animals that generally build stony skeletons of calcium carbonate, superficially similar to coral (although some species lack any calcification in the colony and instead have a mucilaginous structure.) Bryozoa are also known as moss animals or moss animacules (which is the literal Greek translation of bryozoa) or as sea mats. They generally prefer warm, tropical waters, but are known to occur worldwide. There are about 8,000 living species, with several times that number of fossil forms known.
Bryozoans are 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 (the zooids) are tiny, usually less than a millimeter long. In each colony, different individuals assume different functions. Some individuals gather up the food for the colony (autozooids), others depend on them (heterozooids). Some individuals are devoted to strengthening the colony (kenozooids), and still others to cleaning the colony (vibracula). There is only a single known solitary species, Monobryozoon ambulans, which does not form colonies.
Bryozoan skeletons grow 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. They also have a coelomate body with a looped alimentary canal or gut, opening at the mouth and terminating at the anus. They feed with a specialized, ciliated structure called a lophophore, which is a crown of tentacles surrounding the mouth. Their diet consists of small microorganisms, including diatoms and other unicellular algae. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish. Bryozoans do not have any defined respiratory, or circulatory systems due to their small size. However, they do have a simple nervous system and a hydrostatic skeletal system. Several studies have been undertaken on the crystallography of bryozoan skeletons, revealing a complex fabric suite of oriented calcite or aragonite crystallites within an organic matrix - see for example Hall et al. (2002).
The tentacles of the bryozoans are ciliated, and the beating of the cilia creates a powerful current of water which drives water together with entrained food particles (mainly phytoplankton) towards the mouth. The gut is U-shaped, and consists of a pharynx which passes into the esophagus, followed by the stomach, which 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, notably some ctenostomes, a specialized gizzard may be formed from the proximal part of the cardia. Gut and lophophore are the principal components of the polypide. Cyclical degeneration and regeneration of the polypide is characteristic of marine bryozoans. After the final polypide degeneration, the skeletal aperture of the feeding zooid may become sealed by the secretion of a terminal diaphragm. In many bryozoans only the zooids within a few generations of the growing edge are in an actively feeding state; older, more proximal zooids (e.g. in the interiors of bushy colonies) are usually dormant.
Because of their small size, bryozoans have no need of a blood system. 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 bryozoans, as far as is known, are hermaphroditic (meaning they are both male and female). 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 this way is composed entirely of clones (genetically identical individuals) of the first animal, which is called the ancestrula.
Fossil bryozoans are found in rocks beginning in the early Ordovician. They were often major components of Ordovician seabed communities and, like modern-day bryozoans, played an important role in sediment stabilization and binding, as well as providing sources of food for other benthic organisms. During the Mississippian (354 to 323 million years ago) bryozoans were so common that their broken skeletons form entire limestone beds. Bryozoan fossil record comprises more than 1,000 described species. It is plausible that the Bryozoa existed in the Cambrian but were soft-bodied or not preserved for some other reason; perhaps they evolved from a phoronid-like ancestor at about this time.
Bryozoans are important members of sclerobiont (organisms which dwell on hard substrates such as shells and rocks) communities in the fossil record and in the Recent. For a review of sclerobiont evolution, history and ecology, see Taylor & Wilson (2003).
Most fossil bryozoans have mineralized skeletons. The skeletons of individual zooids vary from tubular to box-shaped and contain a terminal aperture from which the lophophore is protruded to feed. No pores are present in the great majority of Ordovician bryozoans, but skeletal evidence shows that epithelia were continuous from one zooid to the next.
With regard to the bryozoan groups lacking mineralized skeletons, the statoblasts of freshwater phylactolaemates have been recorded as far back as the Permian, and the ctenostome fossils date only from the Triassic.
One of the most important events during bryozoan evolution was the acquisition of a calcareous skeleton and the related change in the mechanism of tentacle protrusion. The rigidity of the outer body walls allowed a greater degree of zooid contiguity and the evolution of massive, multiserial colony forms.