Despite the fact that there are about 5000 living species, with several times that number of fossil species, the Bryozoa remain largely unknown to most people. A few to many millions of these individuals may form one colony.
Some bryozoans encrust rocky surfaces, shells, or algae.
Others, like the fossil bryozoans shown here, form lacy or fan-like colonies that in some regions may form an abundant component of limestones. Bryozoan colonies range from millimeters to meters in size, but the individuals that make up the colonies are rarely larger than a millimeter. Colonies may be mistaken for hydroids, corals, or even seaweeds. Bryozoans may also foul pilings, piers, and docks. Yet bryozoans produce a remarkable variety of chemical compounds, some of which may find uses in medicine.