Will Bristleworm Hurt My Fish?

Bristle Fire Worm
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Bristleworms (or bristle worms) are segmented worms with bristly tufts extending from each of their segments. They can grow very large—up to 24" in a tank—but most are between 1" and 6" long. They are nocturnal and tend to stay in or under a live rock or in the tank substrate. You may never see a bristleworm in your tank unless you look for them at night, using a flashlight, or expose them by moving a rock or displacing the substrate.

Identifying Bristleworms

If you suspect you have bristleworms, the first thing to do is positively identify them. Some types are beneficial, but those that are not can, if left alone, overrun your tank and cause irritation and other problems for your tank inhabitants. They multiply rapidly, and some are carnivorous, so they may need to come out.

Warning: Do not touch bristleworms with bare hands! Their bristles are very thin and will embed in your skin, causing a severe itch. 

Beneficial Bristleworms

Bristleworms may look ugly and a little creepy, but most are actually good for your tank — if they are not the poisonous type. They consume materials in your tank that would otherwise decompose and produce ammonia, adding to the load that must be processed by your biological filter. Bristleworms primarily are scavengers and consume uneaten food, detritus, and carrion in a saltwater aquarium. Some people believe that a bristleworm in their tank has killed a fish when they find the bristleworm chowing down on a carcass. But in most cases, the fish was already dead or near death when the bristleworm decided to make a meal of it. The stinging types of bristleworms, such as the fireworms, are an exception. Fireworms have been known to attack perfectly healthy fish (usually small ones) at night when the fish is sleeping in a crack or crevice of a live rock.

Getting Rid of Bristleworms

Getting rid of bristleworms in a reef tank with a lot of live rock can be difficult and time-consuming. There are natural predators of bristleworms that can work quite well in a tank. These include:

  • Dottyback
  • Wrasse of the Halichoeres family
  • Bird Wrasse (Gomphosus varius)
  • Maori Wrasse (Cheilinus oxycephalus)
  • Sunset Wrasse (Thalassoma lutescens)
  • Coral Banded Shrimp (Stenopus hispidus)
  • Arrow Crab (Stenorhynchus setrcornis

While this option is very popular, caution is advised. An introduced predator will eat the bad worms, but species such as these also will eat desirable inverts and crustaceans. Once the bristleworms have been consumed, the new predators in your tank will have to be dealt with in order to preserve the desirable invertebrates in your tank.

If you have bristleworms lodged in cracks and holes of your live rock, simply removing each piece of rock from the tank and dipping it in a bucket of dechlorinated freshwater for just a few seconds usually result in the worms pouring out of the rock and into the bottom of the bucket.

If the offending Bristleworms are found underneath your live rock, they can usually be picked up with a pair of tweezers or tongs and disposed of.