Bryozoans are small, often inconspicuous, sessile (living in one place) colonial animals, which may be found in most marine habitats, with a few fresh water species. The functional units of these animals or zooids lay next to each other and are in contact via pore plates. Some are specialised to be feeding zooids or Autozooids, each possesing a lophophore or crown of tentacles which capture prey and is then digested in a true gut. Nutrients are passed down through the colony by connective pores in the pore plates.

Bryozoans are also true hermaphrodites and alternate in releasing sperm and eggs to avoid any one colony being fertilized by itself. Colonies are produced by repeated budding of the zooids from one sexually produced zooid termed an ancestrula. Each zooid within a colony is thus a functional clone of the next. Because every ancestrula is produced sexually, every colony has its own unique genetic identity. Furthermore depending on the pattern of budding different types of colonies are formed. They may be encrusting or erect. The latter may be calcified or uncalcified; delicate, bushy and plant-like or forming honeycomb structures of fused plates.

The oft-quoted figure of 4000 known bryozoan species appears to be a gross underestimation of the true number that exists globally, as approximately 1000 species of bryozoans were described in the second half of the twentieth century and the rate of discovery exhibits no signs of decreasing. Bryozoans are generally classified by the structure of their skeletons which are primarily composed of chitin or calcium carbonate. They are divided into two classes, the Phylactomata which occur in freshwater and the Gymnolaemata which is almost exclusively marine. The latter is divided into three classes. The Ctenostomata are soft bodied bryozoans that form stoloniferous networks and are mostly freshwater. The Cyclostomata have simple tubuliferous zooids and have swellings which brood eggs called gonozoids. The Cheilostomata have zooids that are generally box-shaped and commonly brood larvae in ovicells. At present ~296 bryozoan species are known from South Africa.

Ecology

Bryozoans are largely predated upon by nudibranchs and flat worms, while they use a ring of tentacles called a lophophore to filter small prey items out of the surrounding water. As a group they are generally protected against predators by defensive zooids called avicularia, which have biting teeth that also prevent any organisms from settling on the colony surface. 

Bryozoans are able to form unusual interactions and associations with other organisms. They have been observed colonising the surfaces of seaweeds, mulluscs, turtles, sea snakes and even plastic debris that drifts in the current systems. The gastropod, Burnupena papyracea is easily recognisable in the field because it is covered by Alcyonidium nodossum, which forms a striking orange or purple cloak on its shell.  Associations of this nature result in bryozoans that are no longer sessile and thereby increase their distributions.