Moss animals are tiny architects of underwater colonies
Bryozoans—also Polyzoa or Ectoprocta—are mostly sedentary colonial aquatic invertebrates nicknamed moss animals. Names coined around 1830–1831 nearly collided with look-alike filter feeders before anatomy split the groups. Fans, bushes and sheets of connected zooids build the living architecture.
Each member of a colony, called a zooid, is typically only about half a millimetre long. It has two parts: the cystid, a body wall that secretes the skeleton, and the polypide, which holds the organs. Skeletons can be organic, made of chitin or protein, or of calcium carbonate, though mineral skeletons never occur in freshwater species. Every colony has feeding zooids, and some groups add non-feeding specialists that serve other roles. Only one genus and one family break the rule of colonial life.
Feeding relies on a lophophore, a ring of hollow tentacles whose beating hairs drive water from their tips to their bases, straining out food. The crown sits on a flexible tube that can be pulled inside the body like a glove finger turned in, sometimes within 60 milliseconds. The gut bends into a U, with the mouth in the centre of the crown and the anus just outside it. There are no dedicated excretory organs: when a polypide clogs with waste, it is discarded and the body wall usually grows a new one.
Similar-looking animals caused confusion. A group with a comparable filter was lumped in until 1869, when anatomists found it was very different inside; it became Entoprocta, with solid tentacles, the anus inside the crown and no body cavity. Some crusty colonies resemble small corals, but a bryozoan colony starts from a round founding zooid, whereas a coral's first polyp looks like its offspring.
Mineralised bryozoan skeletons go back to the early Cambrian, when upright forms dominated before encrusting ones took over. Whether the phylum forms a single lineage remains unclear. Sea slugs, fish, urchins, sea spiders and starfish eat marine species, and one fast-growing invader, Membranipora membranacea, has spread along North American coasts.
Source: Bryozoa
