Common Names: Bristle Worm
Scientific Name: Various Species
Bristle worms are free-living segmented worms with an elongated body which bear a pair of appendages as well as tufts of bristles (setae) on each segment of the body which can clearly be seen in the picture here.
They can range in size from under an inch to being at least 2 feet long. Smaller specimens commonly seen in aquaria which usually range from 1″-6″ in length are usually pink in color while larger specimens sometimes encountered are frequently gray or brown in color.
They generally live on or in the substrate and live rock. They are primarily nocturnal, so a look at the tank substrate and rocks after dark with a flashlight will usually spot these critters if they are present in the tank. They may also come out when the tank is being feed. Large ones are often only noticed when a rock is moved that they are living underneath of.
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.
If for some reason you still want to get rid of them because they creep you out or you have some that are getting too large for your comfort, or you think you may have a predatory version in your tank, there are a couple of techniques that can be used. Removal can be difficult as they tend to be secretive and nocturnal, hiding within the rock work during the day. When they are out and about, they usually have their posterior anchored in a hole into which they can quickly retreat if they perceive any danger.
One strategy is to place a rock with a hollow in it in the bottom of it in the tank in the evening on the sand. A piece of shrimp or similar can be placed into the hollow to act as bait and with the hollow side down so that only worms can get to the bait. The next day, the rock can be removed and the worms will either come out with the rock since they collected in the hollow or they will be on the sand bed under where the rock was where they can be netted out. There are also Bristle worm traps that you can buy, but I am not sure of their effectiveness.
Biological control is another possible option. Coral Banded Shrimp and Arrow Crabs are reputed to eat Bristle worms, though I have not seen this activity in the specimens I have kept. Some fish such as Wrasses may also eat them. In any case, they would probably only eat the smaller worms and not touch larger worms which are the only ones that you might have a reason to be concerned about.
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