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According to the Virginia Institute of Marine Science, Bryozoans are “inconspicuous filter feeders that grow in thin, encrusting colonies atop rocks, kelp blades, shellfish, and other hard objects.”
More than 125 species are known to grow on ship keels, according to UCMP data. They cause drag and reduce maneuverability — or foul intakes, as was the case for the carrier George H.W. Bush in 2014 and now appears to be the case for the Theodore Roosevelt. This is especially true with jellylike colonies that form in fresh water.