There are about 5,000 living species
of the Bryzoa ( "moss animals
"), and several times that number of fossil
species. They are aquatic
organisms, living for the most part in colonies
of interconnected individuals. A conlony may consist of a just a few to many millions of individuals. Some bryozoans encrust rocky surfaces, shells, or algae
. Others form lacy or fan-like colonies. Bryozoan colonies range from millimeter
s to meter
s in size, but the individuals that make up the colonies are rarely larger than a millimeter. Colonies may be mistaken for hydroids
, or seaweed
Some bryozoans are considered nuisances by humans -- over 125 species are known to grow on the bottoms of ships, causing drag and reducing the efficiency and maneuverability of the ships. Bryozoans may also foul pilings, piers, and docks. Certain freshwater species occasionally form great jellylike colonies so huge they clog public or industrial water intakes. On the other hand, bryozoans also produce a remarkable variety of chemical compounds, some of which may find uses in medicine. One compound produced by a common marine bryozoan, the drug bryostatin 1, is currently under serious testing as an anti-cancer drug.
Bryozoans can reproduce both sexually and asexually. Asexual reproduction occurs by budding off new zooids as the colony grows, and is this the main way by which a colony expands in size. If a piece of a bryozoan colony breaks off, the piece can continue to grow and will form a new colony. In some bryozoan species, colonies largely die off in the winter and regenerate the following summer. Freshwater bryzoans can also reproduce asexually by forming masses of cells surrounded by chitinous valves. These cell masses, known as statoblasts, remain dormant for some time and can withstand drying and freezing; when conditions are favorable, the statoblasts germinates and forms a new zooid.
Most bryozoans are hermaphroditic, with individuals containing both ovaries and testes, but these may not be at the same state of maturity at the same time. Some species shed both eggs and sperm directly into the water where they fuse, but the majority of species brood their eggs, within the zooecium or in special chambers known as ovicells, and capture free-swimming sperm with their tentacles to fertilize the eggs. The fertilized eggs divide and develop into free-swimming larvae, which escape from the brood chamber and swim away. These larvae eventually settle on a suitable substrate and metamorphose into a new zooid, which becomes a parent zooid (or ancestrula) for a new bryozoan colony.
The bryozoans are all aquatic animals, and the majority of them are marine. There are also freshwater forms, classified as Phylactolaemata. In aquatic habitats, bryozoans may be found on all types of hard substrates: sand grains, rocks, shells, wood, and blades of kelps and other algae may be heavily encrusted with bryozoans. Some bryozoan colonies don't grow on solid substrates, but form colonies on sediment. While some species have been found at depths of 8200 meters, most bryozoans live in much shallower
water. Most bryzoans are sessile and immobile, but a few colonies are able to creep about, and a few species of non-colonial bryozoans live and move about in the spaces between sand grains. One species makes its living while floating in the Antarctic ocean.
Bryozoans feed on small microorganisms, including diatoms and other unicellular algae. These are trapped by the protrusible ciliated feeding tentacles, or lophophore. In turn, bryozoans are preyed on by grazing organisms such as sea urchins and fish, and are also subject to competition and overgrowth from sponges, algae, and tunicates.