The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals that resemble corals. They are found in marine, brackish and freshwater habitats. Marine species are common on coral reefs but a few occur in oceanic trenches, and others are found in polar waters.. Some species have been found at depths of 8,200 metres but most live in much shallower water. There are around 5,000 to 8,000 species known.
Nearly all live as colonies, individual members of a bryozoan colony are about 0.5 millimetres. Colonies range in size from 1 centimetre to over 1 metre with most under 10 centimetres across. The shapes of colonies vary widely, this depends on the pattern of budding by which they grow, the variety of zooids present and the type and amount of skeletal material they secrete. Colony lifespans range from one to about 12 years.
They are filter feeders that sieve food particles like phytoplankton, diatoms and other unicellular algae out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. The lophophore and mouth are mounted on a flexible tube which can be can be turned inside-out and withdrawn into the polypide (area that contains the internal organs).
Predators of marine bryozoans include nudibranchs, fish, sea urchins, pycnogonids, crustaceans, mites and starfish. Freshwater bryozoans are preyed on by snails, insects, and fish.
Individuals are not fully-independent animals and are called zooids. The main type of zooids are known as autozooids, which are responsible for feeding and excretion. Autozooids supply nutrients to non-feeding zooids through channels. Some types of bryoozoan colonies have various types of non-feeding specialist zooids, some of which are hatcheries for fertilized eggs, and some also have special zooids for defense of the colony.
In autozooids the gut is U-shaped, with the mouth inside the "crown" of tentacles and the anus outside it. Colonies take a variety of forms, including fans, bushes and sheets. One type of Bryozoan, the Cheilostomata, produce mineralized exoskeletons and form single-layered sheets that encrust over surfaces.
Some encrusting colonies may grow to over 50 centimetres and contain about 2,000,000 zooids. These species generally have exoskeletons reinforced with calcium carbonate, and the openings through which the lophophores protrude are on the top or outer surface. There are no respiratory organs, heart or blood vessels. Instead zooids absorb oxygen and eliminate carbon dioxide through the body wall and especially through the lophophore. Within a colony the individual zooids are not completely isolated. Each zooid is connected to its nearest neighbours by a strand of protoplasm. this enables nutrients to be transferred from one individual to another.
Zooids of all the freshwater species are both male and female at the same time. 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 release sperm into the water. Some also release eggs into the water, while others capture sperm using their tentacles to fertilize their eggs 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 quick and major metamorphosis that destroys and rebuilds almost all the internal tissues.
Each colony grows by asexual budding from a single zooid known as the ancestrula, which is round rather than shaped like a normal zooid. This occurs at the tips of "trunks" or "branches". Encrusting colonies grow around their edges.
The body wall and whatever type of exoskeleton is produced by the epidermis is called the cystid. The exoskeleton may be organic (chitin, polysaccharide or protein) or made of the mineral calcium carbonate. 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 other main part of the bryozoan body is known as the polypide. It is located almost entirely within the cystid and contains the nervous system, digestive system, some specialized muscles and the feeding apparatus.
One fast-growing bryozoan found off the northeast and northwest coasts of the USA has reduced kelp forests so much that it has affected local fish and invertebrate populations. Bryozoans have spread diseases to fish farms and fishermen. Over 125 species are known to grow on the bottoms of ships, causing drag and reducing the efficiency and maneuverability of the ships. They may also cause fouling of piers, and docks.