Bryozoa (moss animals): colony life, structure, and fossil history

Colony structure and body plan

Each colonial unit is composed of numerous tiny zooids, typically a fraction of a millimetre in size. A single zooid is a complete animal that may zooids feed, grow and reproduce, and most live joined together by shared tissues or a common skeleton. Zooids often secrete protective tubes or chambers; many build skeletons of lime (calcium carbonate) while some produce organic coverings of chitin or other organic materials. The members of a colony are genetically identical clones produced by asexual budding, but colonies frequently contain differentiated zooid types specialized for feeding, defense or attachment.

Feeding — the lophophore

All bryozoans possess a distinctive feeding organ called a lophophore: a crown of tentacles that surrounds the mouth. Each tentacle is lined with beating cilia that generate currents and trap suspended particles. Typical bryozoan lophophores have a ring of tentacles—sometimes described as ten or more in number—and the tentacles themselves are often called tentacles in anatomical descriptions. In habit and function the lophophore resembles the feeding structures seen in other lophophorate animals such as the brachiopods, and bryozoans are sometimes compared to the polyps that form coral colonies, though they differ in many anatomical details.

Reproduction, life cycle and diversity

Bryozoans reproduce both asexually and sexually. Asexual budding builds the colony framework while sexual reproduction produces larvae that disperse and found new colonies. Many species produce different functional forms within a colony—worker-like feeding zooids, defensive avicularia and enlarged protective zooids—so that a single colony contains several morphs. There are roughly a few thousand living species described, and more than a few times that number known from fossils; estimates commonly cite about 5,000 living species, though taxonomic revisions are ongoing.

Fossil history and scientific importance

Bryozoans have an extensive fossil record beginning in the early Paleozoic, with reliable occurrences dating back to the Ordovician. Fossil bryozoans are abundant in many marine strata and contribute to the study of past environments and biostratigraphy; their skeletal remains are often preserved and provide information on past biodiversity. Paleontologists and stratigraphers make frequent use of bryozoan fossils in reconstructing marine communities and in recognising changes through geological time. See further references on their fossil distribution and evolutionary trends.

Ecology, roles and notable distinctions

• Ecological roles: Bryozoan colonies filter suspended particles, influence sediment dynamics, and provide habitat for other small organisms.
• Habitats: They occupy a wide range of substrates from rock, shells and algae to man-made structures in both marine and freshwater systems (aquatic environments).
• Practical significance: Their encrusting growth can foul ship hulls and aquaculture equipment; conversely, bryozoans are used as indicators of water quality and past environmental conditions.
• Taxonomic notes: The group is commonly recognized as a distinct phylum, and its colonies are true integrated animal units rather than simple aggregates; they differ from superficially similar groups and were historically subject to various names and classifications.

Bryozoans remain a subject of active research in systematics, developmental biology and paleoecology. For general overviews and taxonomic treatments consult introductory sources and specialist literature; for quick access to names and descriptions see curated databases and reviews that summarise current understanding and species lists. Additional reading may be found in standard texts and online resources covering marine invertebrates and fossil record summaries (colonies, skeleton, calcium carbonate, fossil, Ordovician, polyps, coral, species, zooids, secrete, lime, chitin, organic, clones, asexual, morphs, tentacles, cilia).