Bryozoans, also known as moss animals, are microscopic marine animals that live in skeleton structures made of secreted calcite that can be much larger than the size of the animal in itself. A single individual is called a zooid, measures less than a millimeter and is enclosed in a sheath of tissue (zooecium) that in most cases secretes the calcium carbonate skeleton.
The bryozoan skeletons can vary in sizes and shapes, such as mound-shape and corkscrew-shape. They possess a coelomate body with a gut in form of a loop, which opens at the mouth and ends at the anus. They also have a structure called a lophophore, a ciliated organ consisting of a crown of tentacles that surrounds the mouth, which enables them to feed. The beating of the cilia creates a force that attracts water and food particles towards the mouth. Bryozoans do not have a specific respiratory, blood and circulatory systems, but they do have a simple nervous system and can perform gas exchanges throughout the whole surface of their body. All bryozoans are hermaphrodites and can reproduce sexually or asexually.
A particularity for bryozoans is that they form colonies and one colony can encompass millions of individuals. Bryozoans will generally form a colony on hard substrates such as sand grains, rocks, pipes, shells, wood and hulls of boats. The size of the colony can vary from a couple of millimeters to several meters and a structure of roles is attributed within each colony in order to maximize of survival of this colony. For example, some individuals gather food for other members of the colony (autozooids), others assume the responsibility to defend the colony (kenozooids) and finally others are responsible of the cleaning of the colony (vibracula). If one colony gets segmented, the different parts will each form a new colony.
Almost all Bryozoans live in marine environments and can be encountered in very diverse habitats throughout the globe. There are at least 50 species that live in freshwater environments. They eat small organisms such as diatoms and unicellular algae.
A lot of what is known about bryozoans comes from the large and diverse fossil records that exist for these animals. The skeletons composed of calcium have deposited on the sediment and been conserved in rocks for millions of years and are thought to have contributed to sediment stabilization. Fossil records detected the presence of bryozoans in the early Ordovician, but it is thought that bryozoans existed in the Cambrian era but had not acquired a hard mineralized skeleton.
The phylum Bryozoa encompasses 3 different classes: Stenolaemata, Gymnolaemata and Phylactolaemata.