Bristleworms (or bristle worms) are segmented worms with bristly tufts extending from each of their segments. They can grow very large—up to 24 inches in a tank—but most are between one and six inches long. They are nocturnal and tend to stay in or under a live rock or in the tank substrate. You may never see a bristle worm in your tank unless you look for them at night with a flashlight or expose them by moving a rock or displacing the substrate.
What is a bristle worm?
Members of the annelid family, the term bristle worm encompasses more than 10,000 species. Generally found in the ocean (but also occasionally fresh water), they're characterized by segmented bodies with bristle-like protrusions—called chaetae—along their sides.
If you suspect you have bristle worms, the first thing to do is positively identify what type they are. 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.
Do not touch bristleworms with bare hands. Their bristles are very thin and will embed in your skin, causing a severe itch.
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 bristle worm in their tank has killed a fish when they find the bristle worm chowing down on a carcass. But in most cases, the fish was already dead or near death when the bristle worm decided to make a meal of it. The stinging types of bristle worms, such as 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 live rock.
Getting Rid of Bristleworms
Getting rid of bristle worms in a reef tank with a lot of live rock can be difficult and time-consuming. There are natural predators of bristle worms that can work quite well in a tank. These include:
- 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 bristle worms 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 bristle worms 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 bristle worms are found underneath your live rock, they can usually be picked up with a pair of tweezers or tongs and disposed of.