Environmental Health
Surface Water (Lakes, Rivers & Streams)
Swimming in Thurston County

What Are They?

Photo of bryozoaWhile these strange, jelly-like, blobs pose no threat to lake users, they may cause a fright if you are unfamiliar with them. 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

Photo of bryozoaBryozoans 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.


Bryozoa Reproduction

Drawing of a statoblastBryozoans 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.

Photo of statoblastsThese 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 & Other Helpful Information

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

  • Report algae blooms or swimming-related illness
  • General Questions
This page last updated: 04/05/18