They can cause serious damage to such things as the bottoms of wooden boats and pilings. Shipworms invade new wood as free-swimming larvae. Because their entrance holes are so tiny, their invasions are hard to detect. By the time harbormasters or boat owners realize there is damage, it's often too late. To protect against shipworms and other fouling organisms, the bottoms of wooden boats today are often treated with copper paint.
Pilings are another story.
Last year in Maine, shipworms chewed through more wood than in any previous years, destroying hundreds of pilings and causing untold damage to docks. Although constructing pilings of new materials may be the answer in the long run, in the meantime, researchers at the University of Maine are studying ways to repair and protect the damaged structures using fiber-reinforced compounds. But sometimes a fish needs to recognize one particular other fish, much as people need to recognize each other. The anemone fish has only one mate, which it recognizes by its head markings. When the fish was put back in its tank, its mate was no longer able to recognize it and attacked its partner immediately.