The phylum Bryozoa (also known as Ectoprocta) is a group of tiny, colonial, primarily marine invertebrates that are termed 'sclerobionts', given that most are found living on hard substrates such as rock, grainy sediment, shell or kelp leaves (although remarkably some colonies are motile, moving around on stilt-like structures). A bryozoan colony is made up of many individual zooids, each contained within a body wall (cystid) and typically secreting a calcareous (calcite or aragonite) skeleton, or zooecium. Branches or 'stolons' of zooids grow in various patterns from a single initial zooid, the ancestrula, resulting in a mature calcified colony, termed a zoarium. The zooid is polyp-like, with a simple coelomate body, U-shaped gut, and tentacular feeding organ termed a lophophore. The bryozoan lophophore has ten ciliated tentacles that filter phytoplankton (e.g. diatoms, algae) from the water column and into the mouth. Nutrients can move between all the zooids of the colony via a funicular thread that joins with each connection (funiculus) to an individual's stomach. Each zooid is hermaphrodite (containing both female and male reproductive cells), and although bryozoan colonies grow by clonal reproduction, new colonies are formed by sexual reproduction. Oocytes (eggs) mature first and may be fertilised through broadcast sperm mating, resulting in planktonic, feeding 'trochophore' larvae with ciliary bands for locomotion. In other, more advanced bryozoans, fertilised eggs are retained in specialised brood chambers, may receive placental nutrition (matrotrophy) and are released as non-feeding larvae.Bryozoan colony form is largely dependent ecological factors such as water depth, temperature, salinity, wave energy (and correspondingly destructive power) and hard substrate availability. Most bryozoan species inhabit the sublittoral zone between 20-80m depth (just below the intertidal zone), living as (i) encrusting runners, sheets or mounds, (ii) erect sheets or tree-like structures, or (iii) free-living disc- or cap-shaped colonies. At >1000m depth, deep sea oozes and muds cover the sea floor and rooted bryozoans may be found, an erect colony supported by a tubular attachment buried within the sediment.

Relationships and classification

The possession of a lophophore, as well as a trochophore larval stage unites bryozoans with rotifers, platyhelminths, brachipods, phoronid worms, annelids and molluscs in a large clade called the Lophotrochozoa. Exact relationship between these groups are not completely clear, but within the bryozoans themselves three Classes are recognised: Stenolaemata, Gymnolaemata and Phylactolaemata. Dominant Palaeozoic stenolaematans included the branched or globular Trepostomata and Cryptostomata in the Ordovician, less diverse Cystoporata in the Devonian and bifurcating, net-like Fenestrata in the Carboniferous. The efficient, well-integrated Cyclostomata diversified enormously in the Jurassic and Cretaceous, and from the Cretaceous onwards a radiation occurred in the main gymnolaematan order, Cheliostomata (having evolved from the organic-walled, and rarely preserved Ctenostomata). Cheliostomatans are the only major living bryozoan group, accompanied by a few cyclostomatans and the organic-walled phylactolaematan family Plumatellida. The extreme success of the cheilstomatans has been attributed to the evolution of complex wall structure and specialised non-feeding zooids (heterozooids), allowing more efficient colony integration, defence, feeding and reproductive strategies. Typical feeding zooids (autozooids) were modified into strength-enhancing kenozooids, single-tentacled nanozooids, whip-like seta-bearing vibraculae and snapping mandible-bearing avicularia.

Homeomorphy

Bryozoans are famous for showing convergence in form, or homeomorphy, driven by their need to maintain colony efficiency within distinct ecological niches. Key examples include fenestrate, spiral and lyriform colonies.

Fenestrate colonies

Many bryozoans adopted a 'fenestrate' or reticulate colony form, in which narrow, erect branches bifurcate or are joined by lateral connections (dissepiments) to create a fan- or cup-shaped net. The feeding surface is one-sided, with zooid tentacles projecting into the holes or 'fenestrules' in the net, generating a unidirectional filtering current across them. Fenestrata (e.g. Fenestella, Polypora) were highly diverse during the Palaeozoic, and yet similar colonies also evolved by convergence in several Mesozoic-Recent cyclostomes (e.g. Hornera, Fenestulipora cassiformis) and cheilostomes (e.g. Sertella, Reteporidae). An almost identical strategy to the cup-shaped fenestellid filtration fan was adopted by so-called 'fenestrate' or dendroid graptolites such as Dictyonema. Graptolites are an extinct group of colonial marine hemichordates, more closely related to sea urchins and vertebrates than any bryozoans! Dictyonema and its allies lived attached to the sea floor with a cone- or cup-shaped array of connected, zooid-bearing branches or 'stipes' reaching upwards, with water currents flowing through the mesh and out at the top of the structure.

Spiral colonies

Several lophotrochozoans (e.g. phoronids, brachiopods, polychetes), as well as hydroids and ciliate protists possess spiral-shaped structures for efficient filter feeding. On at least six independent occasions, fenestrate bryozoans have also evolved spiral geometry, with feeding zooids arranged on a single (unilaminar) filtering surface within an erect, helical colony. Within the Fenestrata, Helicopora ulrichi (Devonian) and Archimedes (Carboniferous) adopted spiral geometry independently, with the height and size of the whorls varying in sheltered shallow water and deep offshore species. Among the cyclostomatans, the Cretaceous Spiridmonea lundgreni and Eocene Crisidmonea archimediformis were also spiral shaped. A few living cheilostomes form helical filtration sheets, including Bugula, Spiralaria and Retiflustra. Bugula inhabits high-energy shallow waters and has a flexible morphology, whereas mature Retiflustra colonies form a downward planispiral, supported on the substratum to allow water to be filtered in at the sides and up towards the colony apex.

Lyriform colonies

A few Palaeozoic fenestrate bryozoans and one Eocene cyclostomatan evolved a lyre-like, or lyriform geometry. They form reticulate colonies that are fan-shaped and low lying on the sediment, with a transverse arch creating empty space beneath the fan. The colony is weighed down by strong calcification at the base (upstream end) and edges, forming a heavy U or V-shaped margin and hence the overall 'lyre' shape. Feeding zooids are located on the outer surface of the lyre, and as the colony is aligned with the open end facing downstream, inhalant water currents flow passively through the zooids' tentacles before exiting as an exhalant current through the open, distal end. Bowed, lyriform colonies are superbly well adapted to higher energy environments with a dominant current direction (e.g. marine shoals, shallow shelf) and large-grained sediments (e.g. calcareous skeletal fragments, quartz grains) for anchorage. In the Fenestrata, four genera exploited this colony form and it is clear that at least Lyropora and Lyroporella did so independently. Much more distantly related is Hornera reteramae, a cyclostomatan (Cancellata) that is also lyriform and may have inhabited a similar niche to the lyriform fenestrates.

Matrotrophy

Other than homeomorphy, perhaps the most striking example of convergence is between those bryozoans that display advanced forms of matrotrophy (e.g. placentation) and other members of the animal kingdom that have also evolved this mode of embryonic nutrition. All living bryozoans (mainly Cheilostomata) are matrotrophic to some degree, as are all members of the Pseudoscorpionida (scorpions), Salpida (tunicates) and placental mammals, many cartilaginous fish (e.g. sharks), bony fish (e.g. pipe fish), some amphibians, reptiles (e.g. skinks) and an assortment of invertebrate species, from brooding echinoderms and molluscs to viviparous velvet worms.