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Ectoprocta (bryozoans)

© 2000 by Image Quest 3-D
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© 2000 by Image Quest 3-D
Read our copyright notice

Bryozoan cyphonautes larvae

By Peter Parks

Lace Bryozoan Colony - Triphyllozoon sp.

By Roger Steene

Illustrated here are the larval and adult stages of different species of bryozoa, Phylum Ectoprocta.

Bryozoans, along with phoronid worms and brachiopods, are lophophorates. Although superficially different in appearance, they all share a common feeding structure, the lophophore. This structure is a ciliated tentacular crown surrounding the mouth. Other body plan similarities between these organisms include a U-shaped gut, transient reproductive system and outer casings such as tubes, compartments or shells.

Bryozoans, traditional phylum name Bryozoa, new phylum name Ectoprocta, are often commonly known as moss animals. They are sessile, colonial animals that can form erect sturdy colonies or can encrust rocks and other organisms' carapaces forming a uniform 'moss-like' covering. The colonies are composed of individual zooids. These zooids are often polymorphic, with individuals performing different functions and taking forms. The autozooids are adapted for feeding, with a U-shaped gut and mouth located in the centre of the lophophore, with the anus opening outside the lophophore. When the lophophore is not in use it is retracted within the horny calcified exoskeleton and covered with an operculum 'lid'. Other zooids include the avicularia, where the operculum has been modified into a jaw, and the vibracula where the operculum has been modified into a bristle.

Bryozoan zooids are mostly hermaphroditic. Gametes are released into the water through minute pores around the tentacles. Fertilization occurs during discharge of the ova, when another zooid's spermatozoa are wafted by water currents to the first zooid's lophophore. The bryozoan zygote develops rapidly into a minute, ciliated larva. In most species these larvae are brooded in ovicells within the parent colony before being released into the water to then fall, after approximately one day, to the benthos. However, many non-brooding species have a triangular laterally compressed feeding larva (cyphonautes) (see above left) that is immediately released and swims freely for several months before settling to the substrate where it metamorphoses into a typical polyp-like first zooid (ancestrula). Repeated asexual budding then produces many polyp-like zooids that form a single sessile colony.

Individual zooids are usually less than 1 mm in size but colonies may be 0.5 m or more in diameter. These animals are rapid colonziers and colourful additions to reef habitats, forming calcareous encrustations to continental shelf frameworks and inshore rocks. The phylum Ectoprocta has both freshwater and marine species. The marine representatives, one of the most poorly known groups of marine invertebrates, can be found in great abundance and diversity in the correct environmental conditions. They are thought to have a wide global distribution within the topical and sub-tropical belts.

Bryozoans are moderately tolerant of variations in environmental conditions. They prefer well-oxygenated, clear, moderately agitated waters. As suspension feeders they filter small organic particles from the water currents with their lophophore (see above right). They are themselves predated by echinoids and nudibranchs, and are ocasionally parasitized by animals such as nematodes.


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