Bryozoa are aquatic, colonial invertebrate animals. They mainly inhabit marine waters but some species are found in freshwater environments (Phylactolaemata class). Although they are very common, bryozoans (literally moss animals) often pass unnoticed and are mistaken for hydrates, coral, or algae. There are more than 6,000 living species but over 15,000 species have been recorded in fossil history, most of which are found in sedimentary rock from the Paleozoic.

The colonies are composed of individuals called zooids. Each colony grows by budding off new zooids from a primary or ancestral zooid following the metamorphosis of a larva of sexual origin (or from a resistant gemma or statoblast in the case of freshwater bryozoa). The colonies, but not all zooids, are generally hermaphroditic. Most zooids are autozooids as they feed themselves and reproduce asexually. Zooids in some groups, however, exhibit polymorphism and have a particular function, such as to provide support (stolons, rhizoids), fill spaces (heterozooids), act as sexual structures (gonozooids) or as organs of defense (avicularia), or clean the colony (vibracula).

In most species, each zooid is protected by a rigid exoskeleton of calcium carbonate. This skeleton has a porous network structure (the funicle) connecting all the zooids of the colony. The soft gut inside the skeleton is U-shaped so that the mouth and the anus are close to each other. The mouth has a “crown” (which may be horseshoe-shaped in fresh-water phylactolaemates) of ciliated tentacles known as a lophophore. The lophophore is retractable and its function is to catch particles of food and exchange gases.

Bryozoa are benthic organisms that usually live attached to a substrate (sessile), but some rare species live freely and can even move a little. Colonies encrust on rocky surfaces, shells, or algae, and other hard or soft substrates of many types. They may grow along a surface, or grow outward or upward. Zooids average about 0.5 mm in length, but the colonies they form vary in size. Although they are not usually more than a few centimetres in diameter, some may build structures of up to one metre thick. Bryozoans are suspension filter feeders whose diet consists of small microorganisms, such as diatoms and other unicellular algae. In turn they are prey for fish and other animals, such as some nudibranch molluscs, pycnogonid crustaceans, and other more generalist grazing animals such as sea urchins and fish.

In the sea and habitats where they are quantitatively abundant (such as in the Antarctica), bryozoa provide a substrate or habitat for many organisms, such as fish and other species of commercial interest. Human interest in bryozoans is limited, however, because they are not a profitable food source or an important link within trophic networks. Nevertheless, thanks to their calcified skeletons, bryozoans have excellent fossil records that date back to the lower Ordovician. They play a valuable role in several areas of research, such as the history of Earth and theories of ecological and economic impact. They are of great use to reconstruct geological records, to support evolutionary hypotheses, and to study the effects of climate change. Bryozoans also generate biochemical products that have applications in pharmacology. From a negative point of view, a significant number have become invasive species. They are also harmful for human interests as components of marine biofouling (the settling and subsequent growth of organisms on submerged surfaces with negative economic, environmental, or safety-related effects).

Mikel Zabala i Teresa Madurell

Cover photo: Enric Ballesteros (Cova d'Anciola, Parc Nacional de l'Arxipèlag de Cabrera, a 25 metres de fondària)

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Tiles

  • Mollia patellaria (Moll, 1803)

    Zabala, M., & Maluquer, P., 1988. Illustrated keys for the classification of Mediterranean Bryozoa. Treballs del Museu de Zoologia, Barcelona, 4: 1-294.

  • Smittoidea reticulata (MacGillivray, 1842) 

    Hayward, P.J., & Ryland, J.S., 1999. Cheilostomatous Bryozoa. Part 2. Hippothoidea - Celleporoidea. Synopses of the British Fauna (New Series), 14: 1-416, (Barnes, R.S.K., & Crothers, J.H., editors). Field Studies Council, Shrewsbury

  • Prenantia inerma (Calvet, 1906)

    Zabala, M., & Maluquer, P., 1988. Illustrated keys for the classification of Mediterranean Bryozoa. Treballs del Museu de Zoologia, Barcelona, 4: 1-294.

  • Glabrilaria pedunculata (Gautier, 1956)

    Zabala, M., & Maluquer, P., 1988. Illustrated keys for the classification of Mediterranean Bryozoa. Treballs del Museu de Zoologia, Barcelona, 4: 1-294.