Moss Animals

Moss animals, polyzoa, or ectoprocts (phylum Bryozoa) are some of the most abundant and widespread organisms in the animal kingdom. A considerable amount of confusion has surrounded the taxonomic arrangement of the moss animals. A similar, although not so diverse, group of animals known as entoprocts have now been distinguished in their own phylum, Entoprocta. The vast majority of moss animals are exclusively marine-dwelling species (class Gymnolaemata), with just a few of the more than 4,000 species described to date found in freshwater ecosystems (class Phylactolaemata). A third class, Stenolaemata, is also recognized, but contains relatively few living marine species. More than 500 species, however, have been recorded in fossil history.

This phylum is one of the largest of all animal taxonomic groups. Individually, moss animals are all tiny, often microscopic, animals that rarely measure more than 0.04 inch (1 mm) in length. However, individual animals (or zooids, as they are known) congregate to form large encrusting colonies, which may reach more than 3.3 feet (1 m) in diameter. Most species live in shallow waters, although some have been recorded from a depth of 18,045 feet (5,500 m). Colonies may form on any hard surface that is continuously under water—rocks, shells, submerged timbers, seaweeds, etc.

Each animal is contained within its own body wall, which has just a single opening. A thin outer covering is secreted from the body wall to provide some support; there are no muscles within the body wall. The bulk of the body is taken up with the feeding and digestive system. There are no respiratory, circulatory, or excretory organs.

Moss animals vary widely in appearance: some are oval, while others are almost square or vase-shaped. The top of each animal is dominated by a special feeding structure known as the lophophore. Basically this consists of a crown of 8-100 ciliated tentacles which are used to create watercurrents for filter-feeding. The mouth opening lies within the circle of tentacles, but the anus is outside—an adaptation to prevent fouling the tentacles with body wastes. When the animal is feeding, the lophophore is pushed out through the opening and the tentacles extended. The current created by the beating of tiny cilia sweeps a small water current down toward the base of the lophophore. Tiny food particles, mainly plankton, then enter the pharynx and pass through the digestive system to the stomach. Some species have a special lid that closes over the opening of the lophophore when the animal is not feeding.

Most bryozoans (including all freshwater species) are hermaphroditic—that is, each individual animal contains male and female reproductive organs. Reproductive patterns vary considerably across the phylum: some species release eggs and sperm simultaneously to the sea, with fertilization taking place outside the body. Others retain their eggs, which are fertilized by sperm borne in with the water column. Some species may be self-fertilizing. The resulting trocophore larvae are released from the body wall and disperse with the water currents. Most larvae are non-feeding and settle within a few hours of having been released from the parent animal. When an individual larva settles, it may reproduce and spread by asexual means, forming identical zooids to the parent cell.