Bryozoa

moss animals

(Also: bryozoans)

Features
By Jeremy Wright

Diversity

Phylum Bryozoa (or Ectoprocta ), commonly known as “moss animals”, includes over 5,000 currently recognized species (with over 5,000 additional, extinct forms known) of sessile, almost exclusively colonial (only one solitary species, Monobryozoon ambulans , is known), coelomate organisms that superficially resemble soft coral polyps. This resemblance is due to the presence of a ring of cilia-lined tentacles, called a lophophore, which these species use to generate currents that assist in feeding on diatoms and other planktonic organisms. Bryozoans have traditionally been placed into three classes: Phylactolaemata , Stenolaemata , and Gymnolaemata , which includes orders Ctenostomata and Cheilostomata . Though the majority of bryozoan species are marine, fresh and brackish water forms are also known. Colonies usually grow on rocky substrates, but many other solid surfaces are used as well, from the shells and exoskeletons of other invertebrates to floating chunks of Antarctic ice. While the size of an individual bryozoan zooid is quite small, averaging half a millimeter in length, total colony sizes can range from one centimeter to over a meter across. Colonies range in appearance, from gelatinous blobs, to bushy or tree-like forms, there are also encrusting species that excrete mineralized exoskeletons and greatly resemble small corals.

Geographic Range

Bryozoans are found in freshwater, brackish and marine ecosystems throughout the world, from all depths and latitudes.

Habitat

Bryozoans are sessile and colonial, typically settling on hard substrate including sand grains, rocks, and shells, as well as on blades of kelp or other algae, although some species settle on softer sediment. Colonies are lophopodid (covered in a protective gelatinous layer, which individuals protrude) or plumatellid (typically erect or prostrate). Some are stoloniferous (individual zooids arise separately from each other along horizontal stolons), or non-stoloniferous (zooids are adjacent and compacted with colonies, taking a variety of forms including encrusting, arborescent and discoidal). Colonies may be relatively small, a few centimeters across or a centimeter high, but colonies as large as 3 feet across or 4 inches high, comprised of a million zooids, are possible for some species.

Systematic and Taxonomic History

The taxonomic classification of Bryozoa is complex and historically problematic. The name Bryozoa was first used by Christian Gottfried Ehrenberg in 1831, to refer to a group of organisms previously referred to as Polyzoa by J. Vaughan Thompson, just one year before. The name Ectoprocta was then coined for this phylum by Nitsche in 1869, when it was discovered that organisms previously included in Bryozoa differed in their anatomy and physiology, the ectoproct anus is located outside the perimeter of the lophophore, while the other group, Entoprocta , have their anus located inside the lophophore. The use of molecular phylogenetic analyses has revealed that Ectoprocta and Entoprocta , along with Cycliophora (an obscure phylum of sac-like organisms found on the mouthparts of lobsters) form a clade of related organisms. The name Bryozoa is now commonly applied to Ectoprocta and Entoprocta , collectively, while the name Polyzoa has recently been applied to the higher taxon containing all three lineages.

Molecular phylogenetic analyses have supported the monophyly of Ectoprocta , as well as that of its three traditionally accepted classes, with the freshwater Phylactolaemata apparently representing a basal taxon to the sister groups Stenolaemata and Gymnolaemata . Gymnolaemata has traditionally been divided into two orders, Ctenostomata and Cheilostomata , based on soft and hard body forms, respectively. However, molecular analyses have indicated that these two orders are non-monophyletic, and hard body forms have evolved multiple times in Gymnolaemata .

Bryozoa , along with Phoronida and Brachiopoda , have historically been grouped into Lophophorata , due to the presence of a lophophore in all three phyla. Previously considered basal deuterostomes , they are now classified as protostomes and considered to be more closely related to trochozoans (mollusks, annelids, etc.), in the superphylum Lophotrochozoa (occasionally referred to as Spiralia , a clade containing all organisms developing via spiral cleavage, the equivalency of these two names is a continuing matter of debate).

Physical Description

An individual organism within a colony is called a zooid, and is made up of a cystid and a polypide. The cystid is the outer casing (the chitinous, calcified or gelatinous zoecium, secreted by the zooid), and the attached body wall. The polypide is comprised of the lophophore and viscera. The lophophore extends through the cystid orifice, and it may be covered with an operculum. Individual zooids are small; the largest known species grows to 4 mm. An epidermis and peritoneum underlay their zoeciums. There may or may not be longitudinal and circular muscles under these layers. Bryozoans are capable of withdrawing their lophophores into their zoeciums in order to avoid predation (other anti-predator adaptations include surface spines or production of toxic chemicals in some species). In some species, the ciliated tentacles of the lophophore are arranged in a horseshoe shape, while in others they are arranged circularly. In species with the first pattern, there is a food groove at the base of the lophophore, leading to the mouth. In those with a circular arrangement, each tentacle has one ciliated frontal tract and two ciliated lateral tracts. The cilia create a feeing current, which flows toward the mouth, they also direct particles toward the mouth, changing the direction of their stroking in order to do so. The gut is U-shaped, beginning at the mouth and terminating in an anus located within the lophophore ring. There are no special excretory organs; when the polypide of a zooid accumulates an overabundance of waste chemicals, it is replaced by a new one, which is grown from the body wall.

Zooids within a colony may be polymorphic and specialized. All colonies have autozooids, which are responsible for feeding and digestion; the rest of the zooids in the colony are known as heterozooids and cannot feed. Some zooids, known as kenozooids, are greatly reduced and used for attachment to substrates. Others, known as varicularia, have sharp, well-developed opercula (avicularia) to defend the colony. There also may be vibracula, which have a flagellular operculum used for cleaning, or ooecia, which are specialized for brooding eggs.

The degree to which zooids are connected to each other within a colony varies. In colonies of class Phylactolaemata , all of the zooids have a continuous metacoel, each with a funiculus (tissue cord) extending from the end of its gut to its body wall. Species within other classes are connected to lesser degrees. Stenolaemates have interzooidal pores allowing some exchange of coelomic fluid. Gymnolaemates living in stoloniferous colonies have septa separating the zooids, along the stolons, and a stolonal funiculus connecting each individual’s funiculus to the stolon through pores in the septa. Those living in non-stoloniferous colonies have walls that are packed tightly together, with pores between the walls. Food and waste materials are distributed from individual zooids throughout an entire colony.

  • Sexual Dimorphism
  • sexes alike

Development

Bryozoans are hermaphroditic. Eggs may be brooded within gonozooids, or embryo sacs. Cleavage is radial, holoblastic and nearly equal, creating a coeloblastula. Development may be indirect or mixed; in all cases there is a free-swimming dispersal form. Phylactolaemate species develop from coeloblastulae into a cystid stage and then a ciliated polypide. Stenolaemate embryos bud, creating secondary and tertiary embryos (polyembryony). Gymnolaemates undergo gastrulation by delamination, with one of each pair of daughter cells becoming endoderm and/or mesoderm. Many larvae of free spawning bryozoan species are flattened and triangular, with a functional gut; these are known as cyphonaute larvae. Embryos of species that brood eggs do not have a digestive tract and are planktonic for only a short time. Some freshwater bryozoans may produce statoblasts, masses of cells surrounded by chitinous valves, which lie dormant, surviving temperature extremes and even desiccation, until conditions change.

All bryozoan larvae are positively phototaxic and many have pigment spots, which may be light sensitive. They later become negatively phototaxic, swim to the bottom, and settle. Once on the bottom, they rely on chemical and tactile cues to determine suitability of the area and, if appropriate, a sticky material is secreted. At that point, the metamorphosed larva becomes an ancestrula, beginning a new colony.

Reproduction

Bryozoans are hermaphroditic; some are simultaneous (all Phylactolaemata species) and others are protandric. A few species are dioecious; in these species, colonies most often include both male and female zooids. Gonads are transient and gametes are released first into the metacoel, before migrating to the mesocoel. Sperm are typically released through the tentacles, while eggs may be released to the water or an external brooding area through a supraneural pore or intertentacular organ, found between the bases of the tentacles.

A bryozoan colony begins with a single individual, known as an ancestrula. Ancestrulas are sexually produced, but colonies grow through asexual reproduction. Breeding is somewhat regulated by water temperatures and levels of sunlight: rising temperatures and increased light trigger phytoplankton growth which, in turn, triggers budding and, to a lesser extent, sexual reproduction. Species may free-spawn or, more often, females will brood eggs for at least a short time. Larvae of brooding species settle much more quickly following hatching, as their larval forms cannot feed.

Most species of Bryozoa brood their eggs for some amount of time, after which, there is no further parental investment. Other species, however, do not exhibit brooding behavior, and simply release gametes into the water.

Lifespan/Longevity

It is difficult to judge the lifespan of individual zooids. Colonies, once established, will continue to bud and thrive indefinitely, assuming conditions are favorable.

Behavior

Bryozoans are typically sessile, colonial animals. Only one free-swimming, solitary, species is known ( Monobryozoon ambulans ). Colonies of one genus, Cristatella (class Phylactolaemata ), grow in a gelatinous strip and may move 1 to 10 cm a day. There are reports of Selenaria species (class Gymnolaemata ) moving to orient the colony towards light with a "lurching" motion, up to 3 mm at a time (0.5 to 1 m/hr).

Communication and Perception

Zooids possess tactile cells located on their tentacles, and some larvae have light sensitive ocelli; these animals are positively phototaxic as larvae and negatively phototaxic as adults. In some bryozoans , groups of zooids work together to create increased currents for feeding and waste removal, suggesting at least some form of primitive, inter-zooid communication, although the means by which this is accomplished are currently unknown.

Food Habits

Bryozoans are most commonly suspension feeders, although some species may use their tentacles to move food particles to their mouths. In some species, the ciliated tentacles of the lophophore are arranged in a horseshoe shape, while in others they are arranged circularly. In species with the first pattern, there is a food groove at the base of the lophophore, leading to the mouth. In those with a circular arrangement, each tentacle has one ciliated frontal tract and two ciliated lateral tracts. The cilia create a feeding current that flows toward the mouth, also directing larger particles toward the mouth by changing the direction of their stroking motions, if necessary. These types of zooids also have a ciliated tract leading to the mouth, located inside the tentacle area of the lophophore. In some species, groups of zooids work together to create increased currents for feeding and waste removal. Bryozoans typically feed on diatoms (phylum Bacillariophyta ) and other unicellular algae.

Predation

Predators of bryozoans include fish, nudibranchs, snails, sea spiders, and sea urchins who graze on their colonies. They are capable of withdrawing their lophophore into their zoecium by using changes in internal hydrostatic pressure, in order to avoid predation. Other anti-predator adaptations found in some species include zoecium spines, which may be re-grown rapidly if grazed (particularly in Membranipora membranacea ), and the production of toxic chemicals.

Ecosystem Roles

As filter feeders, bryozoans control planktonic populations in their environments; it has been reported that a single zooid may filter as much as 8.8 mL of water a day. The structures of bryozoan colonies may serve as habitat and shelter for juvenile fishes, as well as copepods , amphipods and polychaetes . The species Hypophorella expansa has symbiotic relationships with tube-dwelling polychaete worms, such as Lanice conchylega . The zooids of Harmeriella terebrans are known to attack Tubiporella species and take up residence in their zooecia. Bryozoan zooids may host a variety of parasites, including one species that causes proliferative kidney disease (PKD) in salmonid fishes. Some species may be parasitic on echinoderm species.

  • Ecosystem Impact
  • creates habitat
Species Used as Host
Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

As filter feeders, bryozoans filter and recirculate water. It has been estimated that a colony of Zoobotryon verticillatum approximately 1 m^2 in size has the potential to filter up to 48,600 gallons of seawater per year.

Economic Importance for Humans: Negative

A myxosporean parasite, Tetracapsuloides bryosalmonae , is carried by some species of Bryozoa and causes proliferative kidney disease (PKD) in wild and farmed populations of salmonids , one of the most serious parasitic infections of these fish, causing up to 90% loss in some populations. At one time, bryozoans caused problems for humans by building colonies within water-carrying pipes; the advent of water filtration solved this problem.

Conservation Status

As a cosmopolitan phylum, bryozoans as a whole are not in any danger. However, certain populations may be at risk due to introduced predators.

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

native range

the area in which the animal is naturally found, the region in which it is endemic.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

native range

the area in which the animal is naturally found, the region in which it is endemic.

native range

the area in which the animal is naturally found, the region in which it is endemic.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

native range

the area in which the animal is naturally found, the region in which it is endemic.

native range

the area in which the animal is naturally found, the region in which it is endemic.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

freshwater

mainly lives in water that is not salty.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

oceanic vent

Areas of the deep sea floor where continental plates are being pushed apart. Oceanic vents are places where hot sulfur-rich water is released from the ocean floor. An aquatic biome.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

colonial growth

animals that grow in groups of the same species, often refers to animals which are not mobile, such as corals.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

year-round breeding

breeding takes place throughout the year

protandrous

condition of hermaphroditic animals (and plants) in which the male organs and their products appear before the female organs and their products

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

asexual

reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

fertilization

union of egg and spermatozoan

external fertilization

fertilization takes place outside the female's body

internal fertilization

fertilization takes place within the female's body

male parental care

parental care is carried out by males

female parental care

parental care is carried out by females

diurnal
  1. active during the day, 2. lasting for one day.
nocturnal

active during the night

crepuscular

active at dawn and dusk

sessile

non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa

sedentary

remains in the same area

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

visual

uses sight to communicate

tactile

uses touch to communicate

chemical

uses smells or other chemicals to communicate

planktivore

an animal that mainly eats plankton

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To cite this page: Wright, J. 2014. "Bryozoa" (On-line), Animal Diversity Web. Accessed {%B %d, %Y} at https://animaldiversity.org/accounts/Bryozoa/

Last updated: 2014-02-26 / Generated: 2025-10-03 01:00

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