Bryozoa: Life History and Ecology

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; however, 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 the parent zooid, or ancestrula, of a new bryozoan colony.

Bryozoans are all aquatic animals, and most are marine, except for the freshwater forms classified in the 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, however, do not grow on solid substrates, but form colonies on sediment. While some species have been found at depths of 8200 meters, most bryozoans inhabit 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 remarkable species makes its living while floating in the Antarctic ocean. Curious for more information?

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.

Source: Buchsbaum, R., Buchsbaum, M., Pearse, J., and Pearse, V. 1987. Animals Without Backbones. 3rd edition. University of Chicago Press, Chicago.