Key to Australian Freshwater and Terrestrial Invertebrates

Class Gymnolaemata (Eurystomata)
Order Ctenostomata

Common names: freshwater bryozoans, moss animals

Overview

Ctenostomata are colonial, sessile, moss- or algal-like animals that form branching structures without calcified skeletons made up of joined identical, microscopic zooids (typically only 2-3 mm in length), generally cylindrical to flattened in shape. Each zooid is encased in a gelatinous or membranous covering (zooecium) secreted by the body wall. Colonies are often made up of feeding zooids (autozooecia) plus spine- or stolon-shaped zooids that may be adapted to perform specialised functions, such as protection, cleaning the surface, anchoring the colony or housing the embryo. Feeding zooids possess tentacles that collectively form a circular, feeding, bell-shaped retractable, circular crown of tentacles (lophophore) centred on the mouth. The lopophore opening is often enclosed by a pleated collar. Most ctenostomes grow as encrusting or erect, ramifying, thin branches that consist entirely of zooids or of individual zooids or zooid groups connected by stolons. Others grow as densely packed zooids that form flexible encrustations or thick, erect, branched growths. Two freshwater species occur in the region: Victoriella pavida has chains or upright bunches of zooids with new branches arising from the base of an existing zooid while Paludicella articulata has branch-like colonies in which each zooid arises as a bud on the end of the zooid next to it. New branches begin as lateral buds, often paired, so that the branching pattern generally is cross-like.

Distribution and diversity

The vast majority of freshwater Bryozoa belong to the class Phylactolaemata, although a handful of species of the largely marine Ctenostomata occur in freshwater. The cosmopolitan, brackish- and freshwater species Victorella pavida (Victorellidae) has been recorded from Australia. Another cosmopolitan species, Paludicella articulata (Paludicellidae), which has been reported from every continent except Australia, occurs in a few locations in New Zealand.

Life cycle

Ctenostomata colonies are hermaphroditic and the life cycle includes both sexual and asexual reproduction. During the sexual reproduction phase, sperm are formed and released to circulate freely within the colony. A few species release fertilised eggs directly into the water but in most the fertilised eggs are usually brooded as embryos in the space normally occupied by the retracted tentacles of feeding zooids or in specialised embryo sacs. Once released from the colony, the larvae can swim actively for several hours before settling on a substrate for metamorphosis and then budding asexually to form a new colony. With the onset of unfavourable water conditions, such as winter or drought, ctenostomates do not produce statoblasts like Phylactolaemata, however many species do produce �resting zooids� which can resist desiccation and freezing and may be transported to other water bodies by aquatic plants or animals.

Feeding

Ctenostomata are filter-feeders, extracting protozoans, bacteria and organic matter from the water. Numerous cilia on the tentacles of the lophophore of the feeding zooids create currents that draw water through the tentacles and transport food down and around the inner edge of the lophophore towards the centre where the mouth is situated.

Ecology

Freshwater Bryozoa are typically found in lakes, rivers and wetlands with clear, still or slowly moving water and high densities of phytoplankton and suspended organic matter. They contribute to nutrient cycling and are grazed upon by fish and invertebrates. Colonies may grow on any submerged object, such as rocks, roots and branches. Paludicella articulata occurs in reservoirs in public water systems in New Zealand.