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What Are They?

Have you ever seen strange jelly-like blobs in your lake and wondered what they were? Most likely they were Bryozoa. Bryozoans are aquatic animals with a name that literally means “moss animals”.

Bryozoa are colonies of tiny colonial animals called zoids and appear as gelatinous globs up to the size of a football. Most species are marine animals, however there is one class, Phylactolaemata, that live exclusively in freshwater. They are often attached to submerged surfaces such as tree branches, roots, rocks, pilings, docks, etc. Sometimes, a clump that has broken loose can be found free-floating or washed up near the shoreline.

Water Quality Indicator

Bryozoans filter water for their food like sponges and feed on small micro-organisms such as diatoms and other unicellular algae. Each zoid in the colony has whorls of delicate feeding tentacles swaying slowly in the water catching food. Some freshwater varieties are thought to be useful indicators of water quality. It is said that they like water that is eutrophic, which means very productive lakes with lots of food, and are generally indicative of moderate to good water quality.

Bryozoa Reproduction

Bryozoans reproduce by more than one method. When you look at them, there is a large number of tiny black dots visible imbedded in the jelly. Each of these tiny black dots becomes a statoblast. A statoblast is a cyst, similar to egg or spore. At the end of summer, as the lake water cools, the bryozoan dies. The jelly dissolves and releases the statoblasts; when released they look like small, dark brown disks with radiating barbed spikes.

These statoblasts can remain dormant for some time, withstand drying and freezing, and survive the winter to start new colonies next year. Reproduction can also occur by budding off new zoids as the colony grows. If a piece of a bryozoan colony breaks off, the piece can continue to grow and form a new colony.

Resources

• Moss Animals Invade Lake Cochituate — Miller, Richard A.
• Index of bryozoa pictures and articles — University of California, Berkeley Museum of Paleontology

Acknowledgements: Photos by Tim Vechter - Statoblast drawing by Jill Miller