Bryozoans or Frog Eggs?!

by R.D. Gainar, CEBS – Lake Management Committee

Several residents have asked me “What were those strange, jelly-like, blobs stuck to my dock this summer that look sort of like frog eggs?”  They are actually aquatic animals known as bryozones, a name that literally means “moss animal”.  Bryozoans are fairly common in lakes and streams and form colonies of gelatinous mass attached to submerged tree branches, docks, pilings, etc.  Each colony, sometimes growing to the size of a soccer ball, is made of many individual creatures called “zooids” which are microscopic creatures with a mouth, digestive tract, muscles, and nerve centers. 

Freshwater bryozoans are harmless, though they occasionally clog water pipes and sewage treatment equipment.  Bryozoans eat microscopic organisms and are eaten by several larger aquatic predators, including fish and insects.  Snails graze on them, too. Like mussels and other filter feeders, bryozoans gradually cleanse the water as they feed. The good news is that their presence usually indicates good water quality.

Bryozoans are filter feeders, sucking algae, bacteria (both good and bad), and decaying organic material out of the water, which benefits water quality.  The bryozoans that are so visible in summer will disappear as fall progresses.  At that point, they produce survival pods that contain a single zooid.  Zooids in the pods can survive long periods of dormancy, including drying out and freezing.  They start reproducing new colonies if and when the conditions are right.

They’re weird, and not the prettiest of things, but do these bryozoans mean any harm?  The simple answer is no.  Bryozoans are beneficially removing unwanted organisms from the water, so elimination of them would likely be detrimental to the aquatic environment.  I generally leave the colony to do its thing.  However, if you just can’t stand to look at it or if they frighten your guests when showing off our lake, manual removal is probably the best solution. 

The good news is that if these guys thrive in our lake, it’s a good indication that we have a healthy, organic lake environment.  For more information about bryozoans see http://mdc.mo.gov/discover-nature/field-guide/bryozoans-moss-animals.  Love the Lake!