The Blob in Your Back Yard – Bryozoans in Backyard Ponds

A few weeks ago, a co-worker presented me with a photo, and asked me if I knew what the thing represented in the pic was. I’ve seen a lot of things in my 15 years with That Fish Place (many of which spent in the fish room), but I had never seen the object I was looking at in the pic. My first guess was a bizarre form of algae, as ponds can be home to some strange sludge. Then I contemplated that the mass might be an egg mass of some type, the gel of which possibly being coated or having incorporated algae and muck into it as it floated along. But something still wasn’t right about it. I turned to Google, and with a few quick clicks I found similar photos helping me to identify the blob with relative certainty. It was a freshwater species of Bryozoan! I was familiar with marine forms of bryozoans, but this was quite a different and interesting specimen from others I’d seen, and certainly worth a little more research.

What is a Bryozoan?

The word Bryozoa translates literally to “moss animal”. What you see in this pic is not a moss or a single animal, but a large colony of these aquatic invertebrates, bound together by their jelly-like protective sheaths. They may also be found on submerged surfaces attached with their tubes as small colonies. Bryozoans found in freshwater are all species in a single class of Bryozoa, Phylactolaemata.  All other classes live in marine, or in a few cases, brackish environments.

Bryozoans are may appear similar to coral polyps in many ways, but they are not related. They are composed of relatively simple anatomical structures, including a mouth, gut, and lophophore, the tentacled feeding apparutus that emerges from the sheath to catch passing microorganisms to eat. The clear jelly that protects the little guys often picks up silt and free-floating algae, giving it the deceptive algae or mossy appearance.

Where do they come from?

Large colonies of Byrozoans may seem to magically appear in your backyard pond overnight. These sometimes soccerball-sized masses typically become more visible in the fall, when temperatures drop. Odds are these little guys have been your pond for months, but you may not even notice them when they are anchored below the water’s surface. At the closing of the summer season, the colony begins to decline and individuals die off. The mass becomes visible at the surface as the organisms decay and gasses collect in the membranes causing the colony to float. The colonies don’t dissappear entirely though…they ensure their survival by developing and releasing structures called stratoblasts, cysts which are much like eggs or spores in nature. As the parent colony decays, these delicate little structures are freed into open water. As they float around waiting for warm temps to spring back into existance, they may become stuck to birds and other animals that drink from or swim in the pond. The visiting fauna carries the hitchhiking stratoblasts from one pond to another, and the result is a squishy new bryozoan colony in your pond the following season.

What do you do about Bryozoans?

They’re weird, and not the prettiest of things, but do these bryozoans mean any harm? The simple answer is no. They aren’t here to take over the world, they are just doing what bryozoans do. That being said, they can cause some minor issues in ornamental ponds. Loose balls may clog filters, nets and intakes. Some species (there are about 50 freshwater species) may be toxic to fish, though the odds are small. They also look like floating poo, and despite being very cool if you’re science-minded, they may be frightening to guests that you show off your pond to. The good news is that if these guys thrive in your pond, it’s a good indication that you have a healthy, organic pond environment. Bryozoans also feed on microscopic organisms, both good and bad, so they may be advantageous to have around.

People like me would generally leave the colony to do it’s thing. If you just can’t stand to look at it, or if these critters become a problem in your closed environment, manual removal is probably the best solution. Keep in mind that removal of the masses will not eradicate them, and chances are you’ll have new colonies next year. Some snails, fish, and insects may feed on bryozoans, but again, chances are the population will not be consumed entirely. Herbicides and medications are not typically effective at killing bryozoans either, unless the pond is treated at lethally high dosages that would probably eradicate your biological bed and anything else alive in the pond. The simple solution is to allow these colonies to make their home in your backyard if they find their way there. Once the temperatures drom below 60F in the fall, the unsightly masses will dissolve, springing back to existence only once conditions are favorable again, and doing your pond a big favor while they thrive.

We’ll discuss the blob’s marine cousins in a future entry…you may be surprised at how diverse these simple creatures are!