Good or Bad?
The term Bristle worm is commonly applied to a wide variety of species of worms which complicated any discussion about Bristle Worms, since different species can look and behave differently. The main topic here is the common smaller pink Bristle worms that are typically seen in reef aquaria.
Bristle worms are probably the most common, yet misunderstood hitchhikers in the reef tank. One reason for this is that they are scavengers of meaty foods. If something dies in the tank such as a clam or fish for whatever reason, the Bristle worms will move in and start to consume the body. The hobbyist sees the dead animal and the Bristle worms eating it and assumes they are the reason for it’s demise which wasn’t the case at all. Many years ago, I had a small Tridacna clam that was clearly on its way out. It was gapping and I knew that it would soon die. The next day, I find the clam shell mostly empty and crawling with bristle worms which are consuming the remains. My wife saw this scene and to this day she hates Bristle worms because they ‘killed’ that clam and I have never been able to convince her otherwise.
Bristle worms will reproduce readily in the reef tank. They can experience population explosions and become somewhat unsightly, but their numbers are controlled by the amount of left-overs in the tank. If you have a population explosion, it is a pretty sure sign that you are overfeeding the tank and in this regard, they are a pretty good barometer of your feeding regimen.
The last concern with the common Bristle worm is that they have bristles. These are very sharp and can easily pierce the skin much like a miniature porcupine quill. The bigger the worm, the bigger the bristles, the bigger the concern this is. Large ones can also have fairly large jaws which can potentially bite. The main lesson here is to not touch them with your bare hands. If you do get bristles stuck in your skin, you can try using the sticky side of tape to pull them out.
Bottom line for the common Bristle worms typically seen in a tank are that they are excellent scavengers and are good to have in a tank as part of the left-over food clean-up-crew. They are typically less destructive than many of the hermit crabs that are sold in the hobby.
Having said all that, there are larger predatory versions of Bristle worms that can be destructive. These tend to be fairly uncommon in reef tanks, so it is usually safe to assume you have the more harmless variety unless the worms start to grow very large or you catch them in the act of attacking other live animals in the tank. In the pictures below are some poor quality shots of a couple of large bristle worms. The large wide brown colored one lived in one of my reef tanks for years next to a couple of clams. When the tank was taken down, he was close to 2′ long and about 1/2″ wide. To my knowledge he never bothered anything, though in larger reef tanks some amount of predation can go on unnoticed.
