Bryozoa

Bryozoa—also called Polyzoa, Ectoprocta, or moss animals—are a phylum of simple, aquatic invertebrates that almost always live in colonies attached to a surface. Each individual, called a zooid, is roughly 0.5 millimeters long and uses a crown of tentacles known as a lophophore to filter food from the water. The phylum is divided into three groups: marine bryozoans (Stenolaemata), freshwater bryozoans (Phylactolaemata), and mostly marine bryozoans (Gymnolaemata), a few of which tolerate brackish water. Most marine species live in tropical waters, though some are found in polar regions and oceanic trenches. There are 5,869 known living species.

Originally, all bryozoans in the crown group formed colonies, but as an adaptation to life in the spaces between sand grains or in deep-sea habitats, a few secondarily solitary forms evolved. Five genera are solitary: *Aethozooides*, *Aethozoon*, *Franzenella*, and *Solella* (grouped as the family Aethozoidae), and *Monobryozoon*, which has a statocyst-like organ thought to have an excretory function.

The names Polyzoa and Bryozoa were introduced in 1830 and 1831. Shortly afterward, another group of animals with a similar filtering mechanism was discovered and included in Bryozoa until 1869, when internal differences became clear. That group was renamed Entoprocta, while the original Bryozoa were called Ectoprocta. Terminology debates continued into the 20th century, but "Bryozoa" is now the standard term.

Colonies come in many forms, such as fans, bushes, and sheets. The zooids within a colony are not fully independent and can have specialized roles. All colonies contain autozooids, which handle feeding, excretion, and distributing nutrients through various channels. Some classes also have specialist zooids, such as brood chambers for fertilized eggs, defensive structures, or root-like attachments. The order Cheilostomata is the most diverse, possibly because it has the widest range of specialist zooids. These bryozoans have mineralized exoskeletons and form single-layered sheets that encrust surfaces; some colonies can creep slowly using spiny defensive zooids as legs.

Each zooid has a cystid (the body wall that produces the exoskeleton) and a polypide (which holds the organs). Zooids lack special excretory organs; when an autozooid’s polypide becomes overloaded with waste, it is discarded, and the body wall grows a replacement. The gut is U-shaped, with the mouth inside the tentacle crown and the anus outside it. All freshwater species are simultaneous hermaphrodites. Many marine species function first as males, then as females, but colonies always contain a mix of male- and female-stage zooids. All species release sperm into the water. Some also release eggs into the water, while others capture sperm with their tentacles to fertilize eggs internally. In some species, larvae have large yolks, feed briefly, and settle quickly. Others produce larvae with little yolk that swim and feed for a few days before settling. After settling, all larvae undergo a radical metamorphosis that destroys and rebuilds nearly all internal tissues. Freshwater species also produce statoblasts that remain dormant until conditions improve, allowing a colony’s lineage to survive even if the mother colony dies.

Marine bryozoans are preyed on by sea slugs, fish, sea urchins, sea spiders, crustaceans, mites, and starfish. Freshwater bryozoans are eaten by snails, insects, and fish. In Thailand, an introduced snail species has wiped out many populations of one freshwater species. The fast-growing invasive bryozoan *Membranipora membranacea* off the northeast and northwest coasts of the United States has reduced kelp forests so severely that it has affected local fish and invertebrate populations. Bryozoans have also spread diseases to fish farms and fishermen. Chemicals extracted from a marine bryozoan have been studied for treating cancer and Alzheimer’s disease, but results have not been encouraging.

The earliest mineralized bryozoan skeletons come from the early Cambrian. Early fossils are mostly erect forms, but encrusting forms gradually became dominant. It is uncertain whether the phylum is monophyletic. Bryozoans’ evolutionary relationships with other phyla are also unclear, partly because scientists’ understanding of the animal family tree is shaped by better-known groups. Both morphological and molecular analyses disagree on bryozoans’ ties to entoprocts, whether they belong with brachiopods and phoronids in Lophophorata, and whether they are protostomes or deuterostomes.