It's a beautiful late summer day at the lakeshore. You stand on the dock, relaxing in the sunshine, looking down into the water and . . . what is that thing? A large, gel-like mass is attached to the dock support. Is it an alien life form, an invasive animal, or what?
What you are looking at is a colony of tiny animals called bryozoans, or moss animals. The gel-like blob is created by thousands of these tiny animals attached into one large mass. These colonies are like firm gelatin, and they hold their shape if you pull them out of the water. They are translucent or opaque, and often may have a brownish or greenish color on the surface. While colonies can grow in a ball-like shape to the size of a volleyball, they may also be irregular in form. These colonies are attached to objects in the water—branches, aquatic plants, dock supports. One form of bryozoan doesn't have the gel-like appearance, but looks like a branching growth on hard objects like boat hulls.
Bryozoans are a primitive, ancient group of creatures whose fossil remains are found in rocks from long ago. While they are more abundant and common in marine environments, they have also adapted to freshwater habitats. Some estimates suggest that there are 20-25 species in North American freshwaters, but this is a bit unclear as this group is not well studied. They can be found throughout much of North America, and are common in our lakes, ponds, wetlands, river margins, and other waters in Minnesota. While they may be found in deeper water, the vast majority will be found in water less than 5 or 6 feet deep.
The individual bryozoan is called a zooid—the animal and the tube that it lives inside. The microscopic animal extends structures filled with tentacles into the water to capture microscopic food such as algae, bacteria, and other particles. During the summer, zooids bud out new animals, which grow the colony from a few individuals in spring into a mass that may contain thousands of living and dead zooids. In late summer, the bryozoans begin to produce structures called statoblasts that will overwinter and create new zooids the next spring. These are dark, flattened oval discs that can survive harsh winter conditions. Statoblasts can be caught on feathers of waterfowl and transported to different waters.
Bryozoan feeding does not have any significant impact on the ecology and food chain. Bryozoan colonies may be fed on by aquatic animals including snails, scuds, caddisflies, midges, and small fish. However, they are not a major food item for any aquatic life in our state. Seldom do bryozoans create problems for people, although occasionally they can clog screens on water intakes. They simply are another unique, little-known animal living in Minnesota waters.
Gary Montz , DNR aquatic invertebrate biologist