Delta Blob

August 26, 2021

Question: I enjoy recreating in the Delta, and occasionally on my walks I see these gelatinous brown blobs in the water that look a little like jellyfish. They seem to appear in summer. What are those?

Answer: The critters you see on your walks are freshwater bryozoan. The scientific name is Pectinatella magnifica. They were originally found on the east coast and made their way west. Scientists wrote in 2005 about discovering freshwater bryozoan in the Sacramento-San Joaquin Delta. However, it is likely that bryozoan arrived in the Delta years earlier. Bryozoan are very common seasonally in the Delta and probably in all lower elevation freshwater reservoirs in California. We often hear them referred to as “blobs,” but really they are a colony of small individual animals in a gelatinous matrix. As you mentioned, they are usually found in warmer waters from June through September. They are not poisonous and do not sting, but we’d suggest washing your hands if you come into contact with one. See the U.S. Geological Survey website(opens in new tab) for more information and links.