Bryozoa

Bryozoans, also called moss animals, are tiny water animals that live together in colonies. Each colony is made of hundreds or thousands of little individuals called zooids, and the whole group can look like moss, lace, seaweed or a crusty patch on a rock.

Scientific Classification & Quick Facts

Common name	Bryozoan
Scientific name	Bryozoa
Kingdom	Animalia
Type	Colonial aquatic invertebrate (moss animal)
Diet	Filter feeder: plankton, algae and bacteria
Habitat	Mostly sea and brackish water; some in fresh water
Size	Each zooid about 0.5 mm long
Special feature	Catches food with a crown of tentacles

What Are Bryozoans?

Bryozoa is a whole phylum (a major group) of invertebrate animals. Scientists have described around 6,000 living species. Most live in the sea, but a smaller group lives in fresh water such as ponds, lakes and slow rivers.

Each zooid is usually only about half a millimeter long, so you need a magnifying glass or microscope to see one clearly. Colonies, however, can grow to many centimeters across, and some freshwater colonies form jelly-like blobs bigger than a fist.

The name Bryozoa means “moss animals” in Greek. In the past they were also called Polyzoa (“many animals”). Scientists once grouped them with some similar-looking creatures now called Entoprocta, so true bryozoans are sometimes known as Ectoprocta.

Anatomy

Photo by: Bryan Wasson

Every feeding zooid lives inside a tiny box or tube that it builds around itself, often made of chalky calcium carbonate. From an opening in this box, it pushes out a crown of thin, hollow tentacles called a lophophore. Bryozoans share this kind of feeding crown with a few other animals, such as phoronid worms and brachiopods.

Inside, each zooid has a U-shaped gut. Food goes into a mouth in the middle of the crown, passes down to the stomach and loops back up to an anus just outside the ring of tentacles. A small nerve center near the throat acts as a simple brain. When danger appears, special muscles pull the whole crown back into the box in a split second.

Feeding

Most bryozoans are filter feeders. Tiny hairs called cilia on their tentacles beat to create a water current that pulls food toward the mouth. They eat mostly phytoplankton (microscopic floating algae), plus bacteria, single-celled creatures and other tiny particles. Freshwater species also feed on green algae and diatoms.

Different Kinds of Zooids

A bryozoan colony works like a team. Ordinary feeding zooids, called autozooids, do most of the eating and share food with the rest of the colony. In many species, some zooids have changed shape to do special jobs:

• Avicularia look like tiny birds’ beaks. They snap at small animals that crawl over the colony and help keep it clean.
• Vibracula have long bristles that sweep across the colony surface to brush away dirt and intruders.
• Kenozooids have no feeding parts and act as stalks, roots or spacers that help the colony grow and hold on.
• Spinozooids grow defensive spines.
• Gonozooids and ovicells work as brood chambers, where eggs and young develop safely.

Breathing and Circulation

Bryozoans have no heart, blood vessels or gills. Because each zooid is so small and its tentacles are so thin, oxygen can pass straight in from the water and carbon dioxide can pass straight out. This simple process is called diffusion.

Reproduction and Life Cycle

Most bryozoans are hermaphrodites, which means a zooid can make both eggs and sperm, though often at different times. Sperm are released into the water through the tips of the tentacles and caught by other zooids. The fertilized eggs grow into tiny swimming larvae, which settle on a surface and start a new colony. The first zooid then buds copies of itself again and again to build the colony.

Freshwater bryozoans have an extra trick. They make tough little capsules called statoblasts that can survive freezing winters or dry spells. When conditions improve, each statoblast opens and grows into a new colony.

Predators

In the sea, bryozoans are eaten by nudibranchs (sea slugs), fish, sea urchins, sea spiders, starfish and tiny mites. In fresh water, snails, insect larvae and fish feed on them.

Bryozoans can also cause problems. One species from Europe, the lacy “coffin box” bryozoan, spread to the northeastern coast of North America. It grows over the blades of kelp, making them brittle so they break off in storms, which has damaged some kelp forests.

Fossil Record

Bryozoans have a long history. Their oldest widely accepted fossils are about 480 million years old, from the Ordovician period. Because many species build hard, chalky skeletons, they fossilize well, and scientists have named roughly 15,000 fossil species. In some places, whole layers of rock are made mostly of old bryozoan colonies.

Interesting Facts

• Some bryozoans grow on boat hulls and pipes, where they can be a nuisance.
• A chemical first found in a marine bryozoan, called bryostatin, has been studied by medical researchers.
• Freshwater colonies can sometimes be found on sunken branches in ponds, looking like a lump of jelly.

Bryozoan FAQs