Bryozoans

In the kingdom Animalia, there is a phylum of animals called Bryozoa. Within this phylum, there are about 6,000 species. However, scientists can tell from fossils that there used to be as many as 20,000 different species!

If you saw a bryozoan, you might be surprised to find that they are animals, as many of them actually look a lot like a plant or even a rock! However, look very closely and you will see that bryozoan colonies are actually made of lots of tiny individuals called zooids. These zooids look like tiny worms with a nose that has tentacles around the outside.

These individual animals are so tiny (up to 1 mm across) that they band together to form a colony so they can find food more easily. Colonies are like an apartment building where each individual has their own flat. They can open their doors and interact. They can communicate with each other, and groups within the colony play different roles. For example, the autozooids gather the food for the colony by capturing tiny planktonic particles.

Bryozoan colonies have an enormous range of shapes and sizes. Some colonies look like a lump of rock, some grow in spirals, and some look like underwater trees. A group of bryozoan colonies is called a thicket and sometimes looks like a smaller version of a coral reef.

Bryozoan habitats

The majority of bryozoans live in marine environments, with only about 50 species living in freshwater.

Bryozoans can form colonies on a variety of different surfaces, from rocks to sandy sediments to the hulls of ships! Scientists have found bryozoans at depths of up to 8,200 metres but the majority live in much shallower waters.

Most of the species that live off the coast of New Zealand are found on the mid-continental shelf, between 60–90 metres below the surface. In these temperate waters, bryozoans are an important phylum, growing in great numbers and producing large thickets.

The role of bryozoans in the ecosystem

The bryozoans found off the coast of New Zealand play a variety of roles in the ecosystem. For example, the thickets provide important pieces of structure in areas that are often very flat and sandy. These structures provide shelter and a place for animals, such as fish and sea stars, to live and reproduce. In the tropics, reefs of corals and algae fill the role of these structures.

Bryozoan shell mineralogy

Bryozoans are calcifying animals. This means that they make their structure (in the form of a shell or skeleton) out of calcium carbonate. Many other marine organisms also produce calcium carbonate shells but these are not as diverse in mineral composition as bryozoans. For example, barnacle shells are almost always composed of calcite, and bivalve shells are composed of aragonite, both types of calcium carbonate.

Byrozoans are among the most unusual and diverse group of organisms on the planet in terms of their shells. They make very different kinds of shells with huge variance in composition, in particular, the calcite:aragonite ratio and the magnesium content. As a result, bryozoans are an important phylum to research in terms of how potential changes in water chemistry due to ocean acidification could affect species with different shell compositions.