The Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a lophophore, a "crown" of tentacles used for filter feeding. The bryozoa are classified as the marine bryozoa (Stenolaemata), freshwater bryozoa (Phylactolaemata), and mostly-marine bryozoa (Gymnolaemata), a few members of which prefer brackish water. Most marine bryozoans live in tropical waters, but a few are found in oceanic trenches and polar waters. 5,869living species of bryozoa are known. Originally all of the crown group Bryozoa were colonial, but as an adaptation to a mesopsammal (interstitial spaces in marine sand) life or to deep-sea habitats, secondarily solitary forms have since evolved. Five genera are solitary: Aethozooides, Aethozoon, Franzenella, Solella, which together make up the family Aethozoidae, and Monobryozoon, which has a statocyst-like organ with a supposed excretory function.
The terms Polyzoa and Bryozoa were introduced in 1830 and 1831, respectively. Soon after it was named, another group of animals was discovered whose filtering mechanism looked similar, so it was included in Bryozoa until 1869, when the two groups were noted to be very different internally. The new group was given the name "Entoprocta", while the original Bryozoa were called "Ectoprocta". Disagreements about terminology persisted well into the 20th century, but "Bryozoa" is now the generally accepted term.
Colonies take a variety of forms, including fans, bushes and sheets. Single animals, called zooids, live throughout the colony and are not fully independent. These individuals can have unique and diverse functions. All colonies have "autozooids", which are responsible for feeding, excretion, and supplying nutrients to the colony through diverse channels. Some classes have specialist zooids like hatcheries for fertilized eggs, colonial defence structures, and root-like attachment structures. Cheilostomata is the most diverse order of bryozoan, possibly because its members have the widest range of specialist zooids. They have mineralized exoskeletons and form single-layered sheets which encrust over surfaces, and some colonies can creep very slowly by using spiny defensive zooids as legs.
Each zooid consists of a "cystid", which provides the body wall and produces the exoskeleton, and a "polypide", which holds the organs. Zooids have no special excretory organs, and autozooids' polypides are scrapped when they become overloaded with waste products; usually the body wall then grows a replacement polypide. Their gut is U-shaped, with the mouth inside the crown of tentacles and the anus outside it. Zooids of all the freshwater species are simultaneous hermaphrodites. Although those of many marine species function first as males and then as females, their colonies always contain a combination of zooids that are in their male and female stages. All species emit sperm into the water. Some also release ova into the water, while others capture sperm via their tentacles to fertilize their ova internally. In some species the larvae have large yolks, go to feed, and quickly settle on a surface. Others produce larvae that have little yolk but swim and feed for a few days before settling. After settling, all larvae undergo a radical metamorphosis that destroys and rebuilds almost all the internal tissues. Freshwater species also produce statoblasts that lie dormant until conditions are favorable, which enables a colony's lineage to survive even if severe conditions kill the mother colony.
Predators of marine bryozoans include sea slugs (nudibranchs), fish, sea urchins, sea spiders, crustaceans, mites and starfish. Freshwater bryozoans are preyed on by snails, insects, and fish. In Thailand, many populations of one freshwater species have been wiped out by an introduced species of snail. Membranipora membranacea, a fast-growing invasive bryozoan off the northeast and northwest coasts of the US, has reduced kelp forests so much that it has affected local fish and invertebrate populations. Bryozoans have spread diseases to fish farms and fishermen. Chemicals extracted from a marine bryozoan species have been investigated for treatment of cancer and Alzheimer's disease, but analyses have not been encouraging.
Earliest mineralized skeletons of bryozoans are known from the early Cambrian. Early fossils are mainly of erect forms, but encrusting forms gradually became dominant. It is uncertain whether the phylum is monophyletic. Bryozoans' evolutionary relationships to other phyla are also unclear, partly because scientists' view of the family tree of animals is mainly influenced by better-known phyla. Both morphological and molecular phylogeny analyses disagree over bryozoans' relationships with entoprocts, about whether bryozoans should be grouped with brachiopods and phoronids in Lophophorata, and whether bryozoans should be considered protostomes or deuterostomes.
Description
Distinguishing features
Bryozoans, phoronids 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 that are typically about long. Phoronids resemble bryozoan zooids but are long and, although they often grow in clumps, do not form colonies consisting of clones. 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 exoskeletons 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. On the other hand, the founding polyp of a coral has a shape like that of its daughter polyps, and coral zooids have no coelom or lophophore.
Entoprocts, another phylum of filter-feeders, look rather like bryozoans but their lophophore-like feeding structure has solid tentacles, their anus lies inside rather than outside the base of the "crown" and they have no coelom.
+ Summary of distinguishing features Bryozoa(Ectoprocta) Other lophophorates Other Lophotrochozoa Similar-looking phyla Phoronida Brachiopoda Annelida, Mollusca Entoprocta Corals (class in phylum Cnidaria) Coelom Three-part, if the cavity of the epistome is included Three-part One per segment in basic form; merged in some taxa none Formation of coelom Uncertain because metamorphosis of larvae into adults makes this impossible to trace Enterocoely Schizocoely not applicable Lophophore With hollow tentacles none Similar-looking feeding structure, but with solid tentacles none Feeding current From tips to bases of tentacles not applicable From bases to tips of tentacles not applicable Multiciliated cells in epithelium Yes no Yes not applicable Position of anus Outside base of lophophore Varies, none in some species Rear end, but none in Siboglinidae Inside base of lophophore-like organ none Colonial Colonies of clones in most; one solitary genus and family Sessile species often form clumps, but with no active co-operation Colonies of clones in some species; some solitary species Colonies of clones Shape of founder zooid Round, unlike normal zooids not applicable Same as other zooids Mineralized exoskeletons Some taxa no Bivalve-like shells Some sessile annelids build mineralized tubes; most molluscs have shells, but most modern cephalopods have internal shells or none. no Some taxa
Types of zooid All bryozoans are colonial except for one genus, Monobryozoon, and family, Aethozoidae. Individual members of a bryozoan colony are about long and are known as zooids, since they are not fully independent animals. All colonies contain feeding zooids, known as autozooids. Those of some groups also contain non-feeding heterozooids, also known as polymorphic zooids, which serve a variety of functions other than feeding; colony members are genetically identical and co-operate, rather like the organs of larger animals. What type of zooid grows where in a colony is determined by chemical signals from the colony as a whole or sometimes in response to the scent of predators or rival colonies.
The bodies of all types have two main parts. The cystid consists of the body wall and whatever type of exoskeleton is secreted by the epidermis. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. The latter is always absent in freshwater species. The body wall consists of the epidermis, basal lamina (a mat of non-cellular material), connective tissue, muscles, and the mesothelium which lines the coelom (main body cavity) – except that in one class, the mesothelium is split into two separate layers, the inner one forming a membranous sac that floats freely and contains the coelom, and the outer one attached to the body wall and enclosing the membranous sac in a pseudocoelom. The other main part of the bryozoan body, known as the polypide and situated almost entirely within the cystid, contains the nervous system, digestive system, some specialized muscles and the feeding apparatus or other specialized organs that take the place of the feeding apparatus.
Feeding zooids
The most common type of zooid is the feeding autozooid, in which the polypide bears a "crown" of hollow tentacles called a lophophore, which captures food particles from the water. In all colonies a large percentage of zooids are autozooids, and some consist entirely of autozooids, some of which also engage in reproduction.
The basic shape of the "crown" is a full circle. Among the freshwater bryozoans (Phylactolaemata) the crown appears U-shaped, but this impression is created by a deep dent in the rim of the crown, which has no gap in the fringe of tentacles. The sides of the tentacles bear fine hairs called cilia, whose beating drives a water current from the tips of the tentacles to their bases, where it exits. Food particles that collide with the tentacles are trapped by mucus, and further cilia on the inner surfaces of the tentacles move the particles towards the mouth in the center. The method used by ectoprocts is called "upstream collecting", as food particles are captured before they pass through the field of cilia that creates the feeding current. This method is also used by phoronids, brachiopods and pterobranchs.