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Phylum of colonial aquatic invertebrates called moss animals

Automatic taxobox
| fossil range = small|Contested Cambrian records (Pywackia, Protomelission
| subdivision ranks = Classes
| subdivision = See text. | synonyms = Ectoprocta (Nitsche, 1869) (formerly subphylum of Bryozoa)
| synonyms ref = 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. Most marine bryozoans live in tropical waters, but a few are found in oceanic trenches and polar waters. The bryozoans are classified as the marine bryozoans (Stenolaemata), freshwater bryozoans (Phylactolaemata), and mostly marine bryozoans (Gymnolaemata), a few members of which prefer brackish water. 5,869living species 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. Solitary species has been described in four genera; Aethozooides, Aethozoon, Franzenella and Monobryozoon). The latter having 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, pycnogonids, 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. Mineralized skeletons of bryozoans first appear in rocks from the Early Ordovician period, making it the last major phylum to appear in the fossil record. This has led researchers to suspect that bryozoans arose earlier but were initially unmineralized, and may have differed significantly from fossilized and modern forms. In 2021, some research suggested Protomelission, a genus known from the Cambrian period, could be an example of an early bryozoan, but later research suggested that this taxon may instead represent a dasyclad alga. 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.

All bryozoans are colonial except for one genus, Monobryozoon. 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; 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) 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. 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 lophophore and mouth are mounted on a flexible tube called the "invert", which can be turned inside out and withdrawn into the polypide, connects the mesothelium covering the gut with that lining the body wall. The wall of each strand is made of mesothelium, and surrounds a space filled with fluid, thought to be blood. and pull themselves through the sediments.

Some authorities use the term avicularia (plural of avicularium) to refer to any type of zooid in which the lophophore is replaced by an extension that serves some protective function, consist only of the body wall and funicular strands crossing the interior, The shapes of colonies vary widely, depend on the pattern of budding by which they grow, the variety of zooids present and the type and amount of skeletal material they secrete. Many species form colonies which consist of sheets of autozooids. These sheets may form leaves, tufts or, in the genus Thalamoporella, structures that resemble an open head of lettuce. Large colonies of encrusting species often have "chimneys", gaps in the canopy of lophophores, through which they swiftly expel water that has been sieved, and thus avoid re filtering water that is already exhausted. They are formed by patches of non feeding heterozooids. New chimneys appear near the edges of expanding colonies, at points where the speed of the outflow is already high, and do not change position if the water flow changes. Some freshwater species secrete a mass of gelatinous material, up to in diameter, to which the zooids stick. Other freshwater species have plant like shapes with "trunks" and "branches", which may stand erect or spread over the surface. A few species can creep at about per day.

The phylum was originally called "Polyzoa", but this name was eventually replaced by Ehrenberg's term "Bryozoa". in which the anus lies outside the "crown" of tentacles. After the discovery of the Entoprocta , in which the anus lies within a "crown" of tentacles, the name "Bryozoa" was promoted to phylum level to include the two classes Ectoprocta and Entoprocta. However, in 1869 Hinrich Nitsche regarded the two groups as quite distinct for a variety of reasons, and coined the name "Ectoprocta" for Ehrenberg's "Bryozoa". Nevertheless, some notable scientists have continued to regard the "Ectoprocta" and Entoprocta as close relatives and group them under "Bryozoa". However, the change would have made it harder to find older works in which the phylum was called "Bryozoa", and the desire to avoid ambiguity, if applied consistently to all classifications, would have necessitated renaming of several other phyla and many lower level groups. In practice, zoological naming of split or merged groups of animals is complex and not completely consistent. Works since 2000 have used various names to resolve the ambiguity, including: "Bryozoa", Some have used more than one approach in the same work. The common name "moss animals" is the literal meaning of "Bryozoa", from Greek βρυόν ('moss') and ζῷα ('animals'), based on the mossy appearance of encrusting species. Until 2008 there were "inadequately known and misunderstood type species belonging to the Cyclostome Bryozoan family Oncousoeciidae." Modern research and experiments have been done using low vacuum scanning electron microscopy of uncoated type material to critically examine and perhaps revise the taxonomy of three genera belonging to this family, including Oncousoecia, Microeciella, and Eurystrotos. This method permits data to be obtained that would be difficult to recognize with an optical microscope. The valid type species of Oncousoecia was found to be Oncousoecia lobulata. This interpretation stabilizes Oncousoecia by establishing a type species that corresponds to the general usage of the genus. Fellow Oncousoeciid Eurystrotos is now believed to be not conspecific with O. lobulata, as previously suggested, but shows enough similarities to be considered a junior synonym of Oncousoecia. Microeciella suborbicularus has also been recently distinguished from O. lobulata and O. dilatans, using this modern method of low vacuum scanning, with which it has been inaccurately synonymized with in the past. A new genus has also been recently discovered called Junerossia in the family Stomachetosellidae, along with 10 relatively new species of bryozoa such as Alderina flaventa, Corbulella extenuata, Puellina septemcryptica, Junerossia copiosa, Calyptotheca kapaaensis, Bryopesanser serratus, Cribellopora souleorum, Metacleidochasma verrucosa, Disporella compta, and Favosipora adunca.

The traditional view is that the Bryozoa are a monophyletic group, in which the class Phylactolaemata is most closely related to Stenolaemata and Ctenostomatida, the classes that appear earliest in the fossil record.]] In 2009 another molecular phylogeny study, using a combination of genes from mitochondria and the cell nucleus, concluded that Bryozoa is a monophyletic phylum, in other words includes all the descendants of a common ancestor that is itself a bryozoan. The analysis also concluded that the classes Phylactolaemata, Stenolaemata and Gymnolaemata are also monophyletic, but could not determine whether Stenolaemata are more closely related to Phylactolaemata or Gymnolaemata. The Gymnolaemata are traditionally divided into the soft bodied Ctenostomatida and mineralized Cheilostomata, but the 2009 analysis considered it more likely that neither of these orders is monophyletic and that mineralized skeletons probably evolved more than once within the early Gymnolaemata. Bryozoans' relationships with other phyla are uncertain and controversial. Traditional phylogeny, based on anatomy and on the development of the adult forms from embryos, has produced no enduring consensus about the position of ectoprocts. In the opinion of Ruth Dewel, Judith Winston, and Frank McKinney, "Our standard interpretation of bryozoan morphology and embryology is a construct resulting from over 100 years of attempts to synthesize a single framework for all invertebrates," and takes little account of some peculiar features of ectoprocts.

When entoprocts were discovered in the 19th century, they and bryozoans (ectoprocts) were regarded as classes within the phylum Bryozoa, because both groups were sessile animals that filter fed by means of a crown of tentacles that bore cilia. From 1869 onwards increasing awareness of differences, including the position of the entoproct anus inside the feeding structure and the difference in the early pattern of division of cells in their embryos, caused scientists to regard the two groups as separate phyla, Brachiopods were also assigned to the "Tentaculata", which were renamed Lophophorata as they all use a lophophore for filter feeding. The Lophophorata are usually defined as animals with a lophophore, a three part coelom and a U shaped gut. Bryozoan's tentacles bear cells with multiple cilia, while the corresponding cells of phoronids', brachiopods' and pterobranchs' lophophores have one cilium per cell; and bryozoan tentacles have no hemal canal ("blood vessel"), which those of the other three phyla have. "Total evidence" analyses, which used both morphological features and a relatively small set of genes, came to various conclusions, mostly favoring a close relationship between lophophorates and Lophotrochozoa. A study of the mitochondrial DNA sequence suggests that the Bryozoa may be related to the Chaetognatha.

Most species are filter feeders that sieve small particles, mainly phytoplankton (microscopic floating plants), out of the water. In some species the first part of the stomach forms a muscular gizzard lined with chitinous teeth that crush armored prey such as diatoms. Wave like peristaltic contractions move the food through the stomach for digestion. The final section of the stomach is lined with cilia (minute hairs) that compress undigested solids, which then pass through the intestine and out through the anus. The different bryozoan groups use various methods to share nutrients and oxygen between zooids: some have quite large gaps in the body walls, allowing the coelomic fluid to circulate freely; in others, the funiculi (internal "little ropes") The developing embryo relies on egg’s yolk, extraembryonic nutrition (matrotrophy) or both. In ctenostomes the mother provides a brood chamber for the fertilized eggs, and her polypide disintegrates, providing nourishment to the embryo. Stenolaemates produce specialized zooids to serve as brood chambers, and their eggs divide within this to produce up to 100 identical embryos. The cleavage of bryozoan eggs is biradial, in other words the early stages are bilaterally symmetrical. It is unknown how the coelom forms, since the metamorphosis from larva to adult destroys all of the larva's internal tissues. In many animals the blastopore, an opening in the surface of the early embryo, tunnels through to form the gut. However, in bryozoans the blastopore closes, and a new opening develops to create the mouth. Species that brood their embryos form larvae that are nourished by large yolks, have no gut and do not feed, and such larvae quickly settle on a surface. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. Plumatella emarginata produces both "sessoblasts", which enable the lineage to control a good territory even if hard times decimate the parent colonies, and "floatoblasts", which spread to new sites. New colonies of Plumatella repens produce mainly "sessoblasts" while mature ones switch to "floatoblasts".

Most marine species live in tropical waters at depths less than However, a few have been found in deep sea trenches, especially around cold seeps, and others near the poles. The great majority of bryozoans are sessile. Typically, sessile bryozoans live on hard substrates including rocks, sand or shells. Encrusting forms are much the commonest of these in shallow seas, but erect forms become more common as the depth increases. Colonies of the species Alcyonidium disciforme, which is disc shaped and similarly free living, inhabit muddy seabeds in the Arctic and can sequester sand grains they have engulfed, potentially using the sand as ballast to turn themselves right side up after they have been overturned. Some bryozoan species can form bryoliths, sphere shaped free living colonies that grow outward in all directions as they roll about on the seabed. In 2014 it was reported that the bryozoan Fenestrulina rugula had become a dominant species in parts of Antarctica. Global warming has increased the rate of scouring by icebergs, and this species is particularly adept at recolonizing scoured areas. The phylactolaemates live in all types of freshwater environment – lakes and ponds, rivers and streams, and estuaries Some ctenostomes are exclusively freshwater while others prefer brackish water but can survive in freshwater.

Marine species are common on coral reefs, but seldom a significant proportion of the total biomass. In temperate waters, the skeletons of dead colonies form a significant component of shell gravels, and live ones are abundant in these areas. Other predators on marine bryozoans include fish, sea urchins, pycnogonids, crustaceans, mites and starfish. In general marine echinoderms and molluscs eat masses of zooids by gouging pieces of colonies, breaking their mineralized "houses", while most arthropod predators on bryozoans eat individual zooids. In freshwater, bryozoans are among the most important filter feeders, along with sponges and mussels. Freshwater bryozoans are attacked by many predators, including snails, insects, and fish. In Thailand the introduced species Pomacea canaliculata (golden apple snail), which is generally a destructive herbivore, has wiped out phylactolaemate populations wherever it has appeared. P. canaliculata also preys on a common freshwater gymnolaemate, but with less devastating effect. Indigenous snails do not feed on bryozoans. Several species of the hydroid family Zancleidae have symbiotic relationships with bryozoans, some of which are beneficial to the hydroids while others are parasitic. Modifications appear in the shapes of some these hydroids, for example smaller tentacles or encrustation of the roots by bryozoans. The bryozoan Alcyonidium nodosum protects the whelk Burnupena papyracea against predation by the powerful and voracious rock lobster Jasus lalandii. While whelk shells encrusted by the bryozoans are stronger than those without this reinforcement, chemical defenses produced by the bryozoans are probably the more significant deterrent. In the Banc d'Arguin offshore Mauritania the species Acanthodesia commensale, which is generally growing attached to gravel and hard substrate, has formed a facultative symbiotic relationship with hermit crabs of the species Pseudopagurus cf. granulimanus resulting in egg size structures known as bryoliths. Nucleating on an empty gastropod shell, the bryozoan colonies form multilamellar skeletal crusts that produce spherical encrustations and extend the living chamber of the hermit crab through helicospiral tubular growth. Some phylactolaemate species are intermediate hosts for a group of myxozoa that have also been found to cause proliferative kidney disease, which is often fatal in salmonid fish, and has severely reduced wild fish populations in Europe and North America. Marine bryozoans are often responsible for biofouling on ships' hulls, on docks and marinas, and on offshore structures. They are among the first colonizers of new or recently cleaned structures. Freshwater species are occasional nuisances in water pipes, drinking water purification equipment, sewage treatment facilities, and the cooling pipes of power stations. A group of chemicals called bryostatins can be extracted from the marine bryozoan Bugula neritina. In 2001 pharmaceutical company GPC Biotech licensed bryostatin 1 from Arizona State University for commercial development as a treatment for cancer. GPC Biotech canceled development in 2003, saying that bryostatin 1 showed little effectiveness and some toxic side effects. In January 2008 a clinical trial was submitted to the United States National Institutes of Health to measure the safety and effectiveness of Bryostatin 1 in the treatment of Alzheimer's disease. However, no participants had been recruited by the end of December 2008, when the study was scheduled for completion. More recent work shows it has positive effects on cognition in patients with Alzheimer's disease with few side effects. About of bryozoans must be processed to extract of bryostatin, As a result, synthetic equivalents have been developed that are simpler to produce and apparently at least as effective.