More than a few people have come across bryozoa in the St. Croix River and other waters and said, “What is that?”
Laurie MacGregor and Guillermo Cuellar saw a bryozoan back in July and asked that question. This post is the longer answer I promised these two St. Croix 360 readers, supporters, and friends. (Guillermo is a respected potter who lives and works near the river, a host for the annual St. Croix Valley Pottery Tour, and creator of the perfect mug I drank coffee from this morning.)
Bryozoans are ancient organisms also called “moss animals,” representing the blurry line between plant and animal they seem to represent. They are often found as fossils, with the first bryozoa appearing at least 450 million years ago.
While their colonies can look like something from another planet, these large gelatinous blobs are indeed alive. They are comprised of thousands of tiny creatures, all cloned from a founding member, gathered together into one interconnected organism, feeding on microscopic particles in the water.
As Laurie noted, she and Guillermo have seen similar invertebrates when swimming in Caribbean waters. There are more than 8,000 species of bryozoa in the world, and most live in saltwater. While they are most common in warm shallow waters around the tropics, some species have been seen in deep ocean trenches, and others in frigid polar oceans. About 70 species, in the class Phylactolaemata, live in freshwater.
This summer, the organisms were featured in the Minnesota Department of Natural Resources magazine, the Conservation Volunteer. Gary Montz, a DNR aquatic invertebrate biologist, shared some pretty interesting information:
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. Continue reading…
Bryozoa are usually seen by people on the St. Croix during low water conditions, like when Laurie and Guillermo saw these in early July. They are harmless, except when they occasionally clog water intakes or dock mechanisms.
They are certainly bizarre to behold, but are simply another part of the complicated and amazing St. Croix River ecosystem.
Thank you to Laurie and Guillermo for the photos and the question!