The bristle worm, also known as the Polychaeta, is one of the most controversial critters in the saltwater aquaria hobby.
Some reef keepers are adamant that they should be avoided at all costs. While others suggest that they can be beneficial to the health of a saltwater fish tank.
So, should they stay or should they go? Do bristle worms cause problems? In this guide, we look at the pros and cons of leaving bristle worms in your aquarium.
| Common Name | Bristle Worm |
| Scientific Name | Polychaete |
| Size | Around 1″ – 6″, but some grow up to two feet |
| Diet | Mostly algae, uneaten fish food, and waste |
| Color | Pink or gray in color |
What is a Bristle Worm?
The term “bristle worm” refers to several thousand different segmented worms, each with separate behaviors and anatomy. They are a type of annelid worm and generally present with bristles along their sides, many of which are toxic and poisonous.
This presents a major problem for both you and some of your inhabitants because if you happen to brush against a rock hiding a bristle worm, you will end up with these bristles in your hand or arm.
As you may imagine, this is an issue for both you and the other inhabitants of your tank.
However, most bristle worms are peaceful and generally non-harmful, unless a fish runs into them or is overly curious.
Even then, fish normally survive a few bristles.
In addition, bristle worms are not capable of shooting thier bristles at fish or using them aggressively; they only come off once something touches them.
Most of these segmented worms are either detritivores or omnivorous, eating the stuff that nothing else in the tank will eat.
They will take care of food other fish and invertebrates missed, carcasses, and reprocess the waste of other creatures. In this manner, they can be helpful in the tank, similar to detritus worms in freshwater tanks.
Benefits of Keeping a Bristle Worm in Your Aquarium
Bristle worms eat many kinds of decaying matter, which is very helpful.
Most of the time you won’t realize that you have bristle worms. Usually, they often come out at night to clean up the tank of anything the other inhabitants missed.
If you suddenly remove one, you’ll start seeing more decaying matter than you previously did. This can sometimes cause an ammonia and nitrite spike, which is very harmful. It may be better for overall aquarium health to simply leave bristle worms where they are.
There is a low chance of a harmless bristle worm reproducing by feeding on just leftover matter. So it is unlikely that they will take over your tank.
Bristle worms will even feed on dead fish carcasses. As you know, a decaying fish will quickly raise your ammonia to unsafe levels. And if you can’t find the body fast enough, it might even crash your tank.
Even just one bristle worm will be able to help you cut down on the amount of ammonia released by a dead fish. And if you don’t have a saltwater clean-up crew, a bristle worm can help do the job that these animals normally would.
Negatives of Keeping Bristle Worms in Your Aquarium
While they can be beneficial by ridding your tank of decaying matter, they do pose some danger.
As previously mentioned, even a peaceful bristle worm can have toxic spines. Luckily, they only cause mild irritation, which feels at worst like a bee sting. But it is up to you whether or not keeping that risk around is worth it.
Simply moving some rocks can result in painful bristles ending up in your skin.
While this is not a common occurrence, as most peaceful bristle worms are well hidden in rocks, it is a possibility to be aware of.
Normal bristle worms will also leave your flora, fauna, coral, and fish alone. Animals are only at risk of being stung when they try and eat one. This is rare since most marine life knows better than to try eating spiky bristle worms.
Fireworms and bobbit worms, on the other hand, are harmful to your aquarium inhabitants. They will be discussed in more detail below.
Grubs and Worms
One of the primary reasons that bristle worms have such a bad connotation is not due to their toxic bristles, but rather to a common misunderstanding.
Back when people were first keeping marine fish, there were bristle worms carried along with the live rock.
Most of the time, people weren’t aware that they had bristle worms in their reef aquarium. Aquarists often only see them after a fish dies, with a worm feasting on the body.
This led many to conclude that the bristle worms killed the fish when poor husbandry was a more likely culprit.
Think of a bristle worm as a scavenger like maggots and grubs. If you see maggots feeding on a deer corpse you’re not going to assume that the maggots killed the deer.
Likewise, a worm feeding on a dead fish doesn’t mean it killed the fish. It’s likely just cleaning up the tank by eating a free meal.
Other Worms Related to Bristle Worms
One reason why the bristle worm has such a bad reputation is that they’re often confused for more dangerous types of marine worms. When this happens people often think bristle worms kill fish when in reality it was a different annelid altogether.
Here are a few that you should look out for:
Fireworms
Fireworms are true bristle worms. But unlike most other bristle worms, these worms are carnivorous.
They will still feed on decaying matter left in a saltwater fish tank. But they will also feed on living invertebrates, including coral, and vertebrates, such as your fish.
In addition, their bristles are much more dangerous and painful, and can even result in an emergency room visit. While most bristle worms only cause mild irritation with their bristles, fireworm bristles are very toxic.
If you accidentally touch fireworms, you will be in a lot of pain. Do not remove the bristles and refer to the below section on how to treat the stings.
As you may imagine, if they can cause severe pain to a large human, they can easily take down fish with the same amount of toxins.
In addition to this, fireworms are capable of rapid reproduction, unlike other bristle worms, and run the risk of taking over your tank.
Their food source is not limited to decaying matter left behind by other tank inhabitants,
What Does a Fire Worm Look Like?
It is easy to identify fireworms if you have seen a normal bristle worm. Fireworms normally have bristles with a red base. They also have red, brown, yellow, or green gill filaments, and white-tipped bristles. These can of course range a good bit, as there are several species of fireworms.
In addition, if you happen to see some type of bristle worm wandering around in the middle of the day, it’s likely a fireworm.
Other bristle worms are very timid and prefer to come out to scavenge at night, but fireworms will come out during normal feeding times to look for any possible food.
It is best to remove these as soon as you see them, as they are a threat to nearly everything in your tank and present very few, if any, benefits.
Bobbit Worms
Bobbit worms, while rare, are some of the worst possible things you can discover in your tank. They are highly aggressive and dangerous and do not act like any other bristle worms.
While most other bristle worms spend most of their time hiding in rocks, only coming out at night to scavenge, bobbit worms are aggressive ambush predators.
They bury themselves in the sand, with only their large jaws and mouths sticking out.
When a fish passes above them, they rapidly shoot out of the sand, grab the fish with their jaws, and pull them under the sand. They kill fish by striking fast and hard. One can move up to 20 feet per second, slicing fish in half.
If you see any sort of worm sticking out of your substrate in your reef aquarium you need to move it immediately.
However, you must be extremely careful. Remember that they can slice a fish in half, so they can also do some serious damage to your hand and arm.
This is the only bristle worm where the actual bristles are not the issue.
They are powerful and can grow well over ten feet in length in proper conditions. If they are left in the tank, your fish and invertebrates will be killed and consumed by this creature.
Bristle Worm Sting Treatment
When you get stung by a bristle worm or fireworm, you will not have trouble finding the affected area; it will hurt, and you will know exactly where you were stung.
The bristles will remain in the skin, so be careful not to touch other flesh or drive them in deeper.
Start by applying vinegar to the affected area.
Carefully remove the bristles with either tweezers or tape. There will be somewhere between several dozen to several hundred bristles, and you must remove them all to alleviate the pain.
After you have removed all the bristles, apply hydrocortisone ointment to the affected area, and take over-the-counter painkillers, especially if you were stung by a fireworm.
Do not be afraid to go to your doctor to treat this wound if need be.
The bristles of a bristle worm are hollow, which means there is a good possibility that bacteria from your tank are in them.
While most of these bacteria cannot survive out of the aquarium, it is possible that some of them will cause the wound to become infected.
If the redness or swelling persists for more than a few days, or pus appears, you should schedule a visit with your doctor immediately.
How to Remove Bristle Worms from Your Aquarium
There are several methods you can use to be rid of bristle worms or fireworms in your fish tank:
Manual Removal
The first, and one of the easiest methods, is to manually remove any of the worms you see. While you can do this with tweezers, it is often easier to target the rock that the worm is hiding in.
Once you know which rock the worm is in, lift the rock out of the water and dip it into a bucket of dechlorinated tap water.
This will shock the worms and cause them to fall to the bottom of the bucket, though a few tricky ones may still need to be removed with long tweezers.
However, only experienced aquarists should attempt a freshwater dip.
If you are not experienced in this area, there is a good chance that you will do much more harm than good. In this case, it is best to wait until you see the worm, then grab it with tweezers.
When it comes to disposing of the worm, surprisingly, a good number of people are actually in search of some bristle worms.
You may be able to ship it to someone searching for one, and even make a small profit off of the worm.
The same can be done for bobbit worms, as a few wish to have tanks dedicated to this predator, but almost no one is searching for fireworms.
Bristle Worm Traps
While there are many types of worm traps available on the market, even specialized ones for Bobbit worms, you can easily make your own worm trap.
You will need empty soda bottles and straws.
You can make a bristle worm trap with just a soda bottle, or one with a soda bottle and straws. Both seem to work, so there doesn’t seem to be much difference between them. You need to place some kind of food into the bottles to draw the worms in.
For the soda bottle one, simply cut off the top of the soda bottle and invert it, so that the part you drink from is now inside the bottle.
The worms will crawl up the sloped side and drop into the bottle, but because the entrance is above them, they will no longer be able to get out.
For the trap with straws, you can set the bottle either upright or lengthwise.
If you place it upright, drill holes along the bottom, but at least ¾-1” above the bottom of the bottle. If you place it lengthwise, make sure the holes are ¾-1” higher than the ground will be when you place it down.
Drill several holes, two or more, into the sides of the bottle. The straws will go into these holes, so it is best to make the holes slightly smaller than the straws to ensure a secure fit.
Similar to the previous trap, the worms will crawl into the straws and land in the bottle but will be unable to get out.
Predator Introduction
There are many types of fish that find bristle worms tasty. But there is a good chance that they will find other invertebrates in your tank tasty as well.
If you happen to have a tank without other invertebrates or small fish, you may be able to find a predator that will fit your tank.
Understand that once you add a new fish, you may have to tear down your tank in order to get it out. Be sure that the fish you pick can be a permanent member of your aquarium.
Some of the more popular choices of predators for these worms are an arrow crab, triggerfish, the sunset wrasse, and the dottyback. Of all these, the arrow crab most reliably eats bristle worms.
Just make sure that any predator you choose specializes in eating these worms so no other invertebrates will be harmed.
How to Prevent Bristle Worms from Getting in Your Aquarium
The easiest method of “removal” is to prevent them from ever getting into your aquarium in the first place. After all, bristle worms usually hitchhike in on live rock.
Live rock is essential for all saltwater aquariums. But not all of these rocks are entirely safe for your aquarium, purely due to the hitchhikers.
The best way to prevent anything unwanted from getting into your aquarium is to quarantine everything.
Most of the time, this refers to quarantining fish and invertebrates to prevent the possible spread of diseases. But for saltwater aquariums, this should also apply to the live rock. It is unlikely that you will ever need to quarantine live sand.
To quarantine the live rock, simply set up a separate, cycled, quarantine tank, and add normal fish food.
Check on the tank periodically; if you see that the food looks like something has been snacking on it, something probably has. It is then up to you to determine whether it is a fireworm, bristle worm, or something else entirely.
This will also help you keep better track of the smaller invertebrates in your tank. Hermit crabs and other such creatures also hitchhike in. Most are harmless but there are a few other invertebrates that can cause issues later on.
Quarantining everything that comes into the tank is the only way to ensure that bristle worms stay out.
In Conclusion
Seeing a bristle worm crawling around might make you worry if you aren’t used to these spiky bottom dwellers. Do bristle worms need to be gotten rid of? The answer is: usually not. Bristle worms are detritivores that are mostly harmless and rarely cause issues for a reef tank.
It is only when you have one of the predatory species or have so much detritus that they grow out of control that there is an issue. In that case, introducing a predator like arrow crabs can help keep their numbers down.
More Frequently Asked Questions about Bristle Worms
This guide to bristle worms covered a lot of ground, but maybe you aren’t satisfied. If so, let’s take a look at a few more frequently asked questions about bristle worms.
Are Bristle Worms Harmful?
Most bristle worm species are not harmful and even help your reef tank by eating dead organic matter and other waste. Bristle worms are valuable members of your saltwater clean-up crew. But a few bad bristle worms, such as fire worms, can cause problems for your other aquarium residents.
What Happens if You Touch a Bristle Worm?
If you touch a bristle worm it might try and bite you as many have large and powerful jaws. Not all bristle worms do, though. Bristle worms also have cactus-like tiny bristles that are loaded with venom. These can cause a painful sting and skin irritation that lasts as long as several days.
Should I Remove a Bristle Worm?
If you believe you have good bristle worms then it is best to leave them inside the tank. They can eat leftover food and help keep the water conditions good for the other animals. Only remove them if you suspect you have a predatory species. Or the bristle worm population has started to grow out of control. Even if you do have too many bristle worms it is better to cut down on their food source. This is a lot less work than tearing down the entire fish tank and even better for your water quality.
How Do Bristle Worms Get in Your Tank?
The common bristle worm is most often a hitchhiker that hides within pieces of live rock or among coral frags. Once they are introduced to a new aquarium they can be nearly impossible to get rid of. Fortunately, the majority are harmless and even helpful to your fish tank.
