Key to Australian Freshwater and Terrestrial Invertebrates

Class Phylactolaemata
Order Plumatellida

Common names: freshwater bryozoans, moss animals

Overview

Phylactolaemata are an exclusively freshwater class of Bryozoa (Ectoprocta). They are colonial, largely sessile, moss- or algal-like animals that form branched structures without mineralised skeletons made up of joined identical, microscopic zooids (typically only 2-3 mm in length). Each zooid is capable of independent feeding, digestion and reproduction and is encased in a gelatinous or membranous covering (zooecium) secreted by the body wall. At the top of each zooid is a retractable, circular crown of tentacles (lophophore) that extends into the water to catch food. When disturbed, the tentacles are retracted and the body is drawn down into the zooecium. The three families that occur in Australia have tentacles in a horseshoe- or elliptical-shaped pattern around the mouth. Colonies of Fredericellidae are sparingly and dichotomously branched, the zooids distinct and circular, and the lophopore is more or less elliptic with 20-30 tentacles. Plumatellidae have antler-like or massed branches, with distinct and bilaterally symmetric zooids and a horseshoe-shaped lophopore with 20-80 tentacles. Lophopodidae are characterised by a gelatinous outer body covering with a radial branching pattern and each lophopore containing upward of 100 tentacles.

Distribution and diversity

Phylactolaemata includes approximately 70 known species and five families worldwide. All extant species are placed within the Plumatellida. Three families occur in Australia across a range of clear, slow-flowing freshwater habitats: Fredericellidae (two species of Fredericella), Plumatellidae (one species of Hyalinella and six species of Plumatella) and Lophopodidae (Lophopodella carteri). A single species of Fredericella and three of Plumatella have been recorded from New Zealand.

Life cycle

The life cycle of freshwater Bryozoa includes both sexual and asexual reproduction. During the brief period of sexual reproduction, sperm are formed and released to circulate freely within the colony. Clusters of eggs appear on the inner colony wall near the tip of certain zooids. The fertilised egg develops within a special embryo sac, becoming a free-swimming stage composed of one or two fully formed zooids surrounded by a ciliated mantle. Once released from the colony, it 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 adverse water conditions, such as cold winters or drought, Phylactolaemata produce statoblasts that are released when the colony collapses. The shape and ornamentation of the disk-like statoblasts are species specific and important for species identi?cation. They may either float or sink to attach to the substrate. They are extremely resistant to desiccation and freezing and can be transported to other water bodies by aquatic plants or animals. When favourable conditions return, the statoblast splits and a new individual zooid emerges and attaches to an appropriate surface to eventually grow into a new colony.

Feeding

Phylactolaemata are filter-feeders, extracting protozoans, bacteria and organic matter from the water. Numerous cilia on the tentacles of the lophophore 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. Colonies may grow on any submerged object, such as rocks, roots and branches. Branching species are fully immobile while some gelatinous species can creep slowly on the vegetation or on rocky or sandy substrates. They contribute to nutrient cycling and are grazed upon by fish and invertebrates. In contrast to many other aquatic invertebrates, Phylactolaemata display considerable tolerance of pollution and low oxygen levels and can live downstream of sewage contamination and agricultural fertiliser run-off, thriving on algal blooms. Pipelines that carry untreated water from a lake or river is often at risk of becoming fouled with ectoprocts colonies where they can block or seriously disrupt the flow of water in irrigation, wastewater and cooling water systems.