Fossil Friday #10 – Bryozoans!

Bryozoans are the coolest little animals you’ve never heard of.  And when I say little, I mean really little.  As I tell my students, if you aren’t using a microscope, you’re missing the point.  You can’t really see anything without a scope.

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Each of these contains hundreds to thousands of individuals.  Image credit:
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There, that’s better.  Notice the scale bar.  Image credit: https:/

Otherwise know as “moss animals”, these tiny little critters are mostly colonial, and most commonly found in marine environments.  They are generally thought to be related to brachiopods and another group called phoronids, all of which have a specialized feeding organ called a lophophore.  The lophophore is a cilliated (tentacled) structure which actively pumps water/food to the mouth (located at the base of the lophophore).

Most bryozoans secrete a hard skeleton in which they live, much like a coral.  Individuals within the colony are called zooids.  Some types of bryozoans have specialized zooids that only perform one function for the colony, such as providing food, defense, or reproduction.  Because they are colonial, bryozoans are capable of both sexual and asexual (budding) reproduction.  To reproduce sexually, sperm, and sometimes ova, are released into the water column (some colonies with specialized reproductive zooids will keep ova in brooding chambers).

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Fossil bryozoan zooids. Image credit:
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Reproductive zooids (ovicells).  Image credit:

I won’t get into the systematics (grouping) of bryozoans, because it is complicated, and generally unhelpful to the non-specialist (if any specialists are reading this, I apologize.  Just know that I love bryozoans!).  However, there are two basic ways that bryozoan colonies can grow: erect, or encrusting.

Erect colonies grow upright into all sorts of beautiful shapes.  Some grow into simple branching structures that look like trees , and some, like the fenestrates, grow to look like screen doors (but pretty).  Others, like the fossil Archimedes, grow into corkscrew spirals.  A lot of modern erect bryozoans look leafy, like a head of lettuce, or bushy.

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Fossil branching bryozoans.  Image credit: http://www/
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Archimedes close up.  Image credit: 
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Fenestrate bryozoan colony.  Image credit: http://www/

Encrusting forms are usually flattened, but may grow into large bulbous structures.  They can grow into sheets that encrust other organisms, or form large, dense colonies.  Interestingly, during much of the fossil record, the majority of bryozoans grow into erect forms, but in the modern, most bryozoans are flat and encrusting.  These modern encrusting forms are sometimes called “foulers” because they clog up pipes and foul up the sides of ships.

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Encrusting bryozoans. Image credit:

Bryozoans can form colonies of millions of individuals, but still never come close to reaching the size of other colonial animals, like coral reefs.  The result is that bryozoans just never reach the same biomass as other marine fossils, which might make them seem like a somewhat unimportant group, despite being common marine organisms since the Cambrian/Ordovician.  Bryozoans are, however, an important group, and totally rad (in my humble opinion).

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Modern encrusting bryozoans are sometimes really red!  Image credit:

For example, bryozoans are often used to examine how sessile (non-moving) animals interact with one another, or compete for space.  Imagine you are a little larvae that has settled on a susbtrate that is now going to be your home for the rest of your life, and some other larvae settles too close to you, or perhaps on top of you.  If you can’t move, how do you deal with this crowding or lack of space?  As bryozoans are colonial, they are able to respond and grow the colony in all sorts of interesting ways.  Some even have claw-like zooids that can pinch  predators or anyone that gets to close.  Many bryozoans will compete for “superiority” by trying to overgrow the other colony, allowing us to directly examine competitive relationships, something that is usually not possible in the fossil record (e.g. McKinney 1995).

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Overgrowth and competition in bryozoans.  Image credit:

Bryozoans also commonly encrust other organisms, such as brachiopods, allowing us to examine these relationships “in place”, which again, is a rare occurrence in the fossil record  For example, are bryozoans beneficial to their hosts?  Do they provide protection/camouflage from the host’s predators?  Or are they parasitic, and prevent the host from feeding or moving properly?  This is the area of study that I specialized in for my undergrad and M.Sc (check out my publications).

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Brachiopod with encrusters (sheet in the middle is a bryozoan – zoom in!).  Image credit:

I think bryozoans are fascinating, simply because they operate on a fundamentally different size scale compared to most other animals.  They are also very aesthecially pleasing.

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Look at them smiling for you.  What’s not to like?  Image credit: http://www/

Fun tidbit for those of you that read this far: apparently the bryozoan Bugula neritina creates a chemical (bryostatin) which is being tested for use in the treatment of cancer and Alzheimer’s disease.  Can bryozoans get any more awesome?

Want to learn more about bryozoans?  Check out these resources:

McKinney, F. K. 1995. Taphonomic effects and preserved overgrowth relationships among encrusting marine organisms. Palaios. 10:279-282.

Coming soon: Bryozoan Paleobiology by Paul D. Taylor


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