Bryozoans are a phylum of aquatic invertebrates, mostly marine but with some species inhabiting fresh or brackish waters. They are the only phylum in which all species form colonies each colony comprises tens to many thousands of individuals called zooids. At least some of these zooids possess a lophophore, a feeding apparatus comprising an inverted cone-shaped crown of tentacles. Rhythmic beating of tiny cilia on the tentacles drives a current of water containing suspended food particles towards the mouth. Bryozoans feed on phytoplankton, like many other organisms living on the seabed. In a few instances, colonies are mobile, but most species are sessile and rely on ambient water currents to bring planktonic food into their vicinity.
Although all of the zooids in a bryozoan colony are genetically identical (clonal), they may differ in morphology, a phenomenon called polymorphism. Differences in morphology reflects differences in function. For example, avicularia, named for their resemblance to a birds head, are defensive zooids with beak-like mandibles used to capture would-be predators. Specialized zooids gonozooids are also present in some bryozoans, their purpose being to brood the embryos before they are released as larvae. A consequence of polymorphism is that the feeding zooids must provide nutrition for the non-feeding zooids. This is accomplished using the funicular system, blood vessel-like strands that pass from zooid to zooid through pores in the walls.
Bryozoan colonies are able to propagate in two ways. All species produce larvae which are the products of sexual reproduction when an egg from one zooid is fertilized by a sperm usually from a separate colony. Most bryozoan larvae are short-lived, settling on a solid surface, such as a rock, shell or seaweed, within a matter of hours and undergoing metamorphosis to give the founding zooid (ancestrula) of a new colony. In a few bryozoans, the larva is of longer duration and is able to feed while swimming in the plankton. Larvae are the main means of dispersal in bryozoans (rafting on floating objects is another), and species having long-lived feeding larvae can potentially disperse further and more quickly than those with short-lived larvae. The second mode of colony propagation is by breakage. New colonies resulting from breakage, be it accidental, physical or biological in cause, have the same genotype as the parent colony.
Most bryozoans secrete calcareous skeletons. Features of the skeleton are of great importance in taxonomy, which means that it is usually possible to identify species using dried specimens. Indeed it is common practice to bleach bryozoans to remove all of the soft organic parts and expose the bare skeleton. The resistant skeletons of bryozoans fossilize very well: bryozoan fossils are common in sedimentary rocks deposited as long ago as the Early Ordovician, some 450 million years ago. The calcareous skeletons of fossil bryozoans can be so abundant that they form bryozoan limestones.
Despite their high diversity there are probably more than 10,000 living bryozoan species and great abundance, bryozoans are unknown to many people. This is reflected in that fact that none of the three English vernacular names for bryozoans - moss animals, sea mats and lace corals are particularly familiar. Historically, bryozoans have had no economic value, although a marine species called Bugula neritina is the host for a bacterium that manufactures bryostatin, a natural product with tumour reducing properties which is currently in clinical trials.
Modern bryozoans are classified into four major groups: class Phylactolaemata and orders Ctenostomata, Cheilostomata and Cyclostomata. Phylactolaemates, which are exclusively freshwater, will not be considered further. Ctenostomes are soft-bodied marine and freshwater bryozoans that form stoloniferous or gelatinous colonies, some species of the former type boring into calcareous substrates such as shells. Their feeding zooids tend to be tubular in shape, unlike those of the related cheilostomes which typically have box-shaped zooids and also differ in secreting a calcareous skeleton. The morphology of both cheilostome zooids and colonies is remarkably varied, as reflected by the enormous taxonomic diversity of this marine order. Polymorphism is highly developed in cheilostomes, with avicularia, spinozooids and rhizozooids being present in many species. The majority of cheilostomes brood their embryos, one at a time, in distinctive hood-like structures called ovicells. In contrast, the final bryozoan group, the cyclostomes, have distinct polymorphic zooids called gonozooids, each of which broods a clutch of clonal embryos produced by polyembyonic fission. Cyclostomes are less diverse than cheilostomes and tend to have simple, tubular skeletons.