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What are Bryozoans?

Michael Anissimov
Updated: May 21, 2024

Bryozoans, also known as moss animals or sea mats, are encrusting colonial animals found throughout the world's oceans. They prefer warm, tropical water. Bryozoans have their own phylum, Bryozoa, which is a member of the superphylum Lophotrochozoa, the lophophorates. What all members of this group have in common is that they use a characteristic horseshoe-shaped, cylinder-shaped, or coiled ciliated feeding organ called a lophophore. Lophophores are used for filter feeding and evolved from a simple ring of cilia around the mouth. Most bryozoans are stationary, though some colonies can creep around, and at least one species is free-floating.

To protect themselves, most bryozoans surround their soft parts with a stony calcium carbonate exoskeleton, like coral does. This skeleton can often be found encrusting mollusk shells found on a beach, and can be scraped off by rubbing the shell with a finger under running water. Some species of bryozoan don't build skeletons, and are instead held together by mucus. Bryozoans are highly colonial, again like coral, to which they are only distantly related, and form colonies up to a few meters across, though a few centimeters across is more typical. The individual members in a bryozoan colony are tiny, usually between 0.5 and 5 mm in size.

Bryozoans are coelomate animals, meaning they have a body cavity, and a simple gut with a mouth and anus. Aside from the lophophore, that's about it. Bryozoans lack circulatory, motor, or respiratory systems, due to their small size and stationary lifestyle. Oxygen diffuses directly into the cells of the animal, because it is so small. They also have an extremely simple nervous system. Bryozoans are found in the fossil record beginning the in Early Ordovician (488 million years ago), but they may have existed earlier, in the Cambrian, but lacked a hard skeleton at this time. They probably evolved from a phoronid-like ancestors. Phoronids are another simple group of lophophorate.

There are about 8,000 living species of bryozoan, with 50 freshwater species and the remainder marine. They are sometimes colorful - blue, brown, purple, or red -- and their colonies can be seen while snorkeling. Bryozoans serve as first-level consumers in the aquatic food pyramid, consuming small bacteria and unicellular organisms and providing food to grazing animals such as sea urchins and fish. They form a prominent part of the post-Cambrian Paleozoic fossil record. Sometimes their skeletons are found in thick sheets in Paleozoic strata, making it difficult to locate fossils of anything else.

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Michael Anissimov
By Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology, astronomy, chemistry, and futurism to his articles. An avid blogger, Michael is deeply passionate about stem cell research, regenerative medicine, and life extension therapies. His professional experience includes work with the Methuselah Foundation, Singularity Institute for Artificial Intelligence, and Lifeboat Foundation, further showcasing his commitment to scientific advancement.

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Discussion Comments
By TreeMan — On Dec 07, 2011

@Emilski - Luckily, it sounds like both of the things you are mentioning are plants. The round things on your pound sound like a plant called duckweed. Like I mentioned, it's a member of the plant kingdom and can form dense floating mats. They don't really clump together like moss, though.

As for bryophytes, they are also plants. It is the same group that true mosses belong to. That being said, I would assume that is where the similarity in names come from, since I would assume bryozoans were named after mosses. I couldn't tell you what the prefix bryo- meant, though.

I know when I was in Australia a few years ago we went on a snorkeling trip, and the guide pointed out the differences between the corals and bryozoans. I think even when they are found in freshwater, bryozoans are usually in warm, tropical areas. I'm sure there are some that don't fit that mold, but I think that's the general rule.

By Emilski — On Dec 07, 2011

@jcraig - Since the bryozoans are so small, I would assume they are eaten by the next group of creatures like small fish and starfishes and things like that.

This says that bryozoans are often called moss animals. Every summer, I get these small things on my pond at home that are really a pain to deal with. They are little green round looking things that float on the water in groups. I never bother to get rid of them, they are just a pain to deal with if you are trying to fish or something. Does anyone have any ideas whether those are the same thing?

Also, I have heard the term bryophyte used before. Is that just another name for bryozoans? If not, how are they different?

By jcraig — On Dec 06, 2011

@jmc88 - I was kind of curious about a lot of the same things you mentioned. Out of interest, I looked up some pictures of bryozoan pictures. They are pretty odd looking. The individual animals kind of remind me of a weird flower or something like that. I also searched for some pictures of the groups of bryozoans. A lot of the pictures I found just looked like groups of coral. I don't know if they just look the same or if the pictures that came up were improperly labeled.

I am far from being a biologist, but one of the pieces of trivia I still remember from biology class for some reason is that corals don't have a true body cavity. Seeing as how bryozoans do, I would say that they rank somewhere above corals but below bivalves or whatever comes next.

The article talks about what bryozoans eat. I am curious what their predators are.

By jmc88 — On Dec 05, 2011

So, where do bryozoans fall in the animal hierarchy in terms of complexity? I know I usually see sponges listed as been the most simple, at least that is what we always talked about first in all of my biology classes. Next I think was usually coral and then mollusks. It sounds like bryozoans are more complex than sponges, but they could either be above or below coral. Does anyone have any ideas?

The thing I think is really interesting is that the article says there are 8000 species but only 50 of them live in freshwater. I would have expected a lot more. I am curious where in freshwater they live, like what countries and climates and things.

By DentalFloss — On Dec 05, 2011

@FernValley- I was look ating bryozoan species in pictures in a friend's biology book the other day. There is a lot that people still don't know about them, which I think is interesting too. There are other types of organisms that are mysterious, too, like the water bears, or tardigrades. I would have thought all these organism were closely related, too, which they aren't.

By FernValley — On Dec 04, 2011

Awhile ago I was watching something on television about bryozoans. They're so beautiful, many of them, and most people don't even know they exist. Things like that make me think people really aren't the center of everything, when there is so much detail in parts of the world we don't even see.

Michael Anissimov
Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology...
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