bristle worm by prilfish

The bristle worm

If you have ever seen a bristle worm in your reef tank, you would probably agree they can be ugly little creatures. I’m pretty sure we’re genetically programmed to be creeped out by millipede-looking creatures. There is a lot of energy and discussion about this lowly polychaete in reef aquarium circles. The bristle worm is a live rock hitchhiker that reef tank owners love to hate. Is there merit to the debate, or is it a bit over-blown?

Let’s dig into the topic and see if we can separate the dirt from the detritus.

If you have a specific question and want to jump right to that section, you can do so, here:

polychaete worm in quarantine tank hungry and looking for a meal
This bristle worm is hungry, looking for a meal

What is a bristle worm?

A small, cryptic segmented worm that often lives in the live rock and sand of a reef aquarium. They eat detritus and decaying stuff in the tank. This is what they look like when they are small:

bristle worm in specimen container
Caught these two in my filter sponge

They belong to a family of segmented worms called polychaete, which means ‘many hairs’ in Latin. The family name would be olychaetep in Pig Latin, of course, but I don’t recommend using Pig Latin, if you want to impress–or even keep–your friends.

The hairs are what give this polychaete its common name. If you look closely at some of the larger pictures on this page, you can see that each segment on its body has bristles sticking out of both the left and right sides. Those hairs protect the worm from predators…and your fingers…if you stick your hand in the tank without thick gloves.

What do bristle worms eat?

Bristleworms are detritivores. That means they dig through the muck, gunk, and detritus in our tanks and eat the stuff that is rotting away and spoiling the water quality. Their preferred food is food waste, biological waste, even the rotting carcass of that missing damselfish you haven’t seen for a few days. Any of those items would proudly anchor the menu at Chez Bristleworm–but good luck getting a table on the weekend. Any of those items, if left to rot, would otherwise add ammonia, nitrites, and nitrates to your aquarium water–spoiling those pristine water parameters you work so hard to maintain.

A polychaete (bristle worm) - geograph.org.uk - 1087795
Whoah, that’s a big one

Most of us happily pay good money to add a cleanup crew to our reef tanks. So why is there so much fuss about this pesky polychaete?

Some of it, likely, is from a negative stigma associated with the fact that they dine on our biggest disasters–but let’s continue exploring the many aspects of this many-haired worm.

Where do they come from and how did they get in your tank?

The most likely scenario is that they found their way into your tank by hitchhiking their way in on a piece of live rock. As mentioned earlier, they are nocturnal creatures and spend their days hidden inside the many crevices within the live rock. In general, rocks don’t tend to move on their own–so it’s a pretty good strategy–unless, of course, that rock is headed to an aquarium store or someone else’s reef tank.

I’ve even seen them attached to live rock rubble or small coral frag plugs.

bristle worms all balled up
I tried to pick them up, for a photo (originally in water) but was surprised to see this hedgehog behavior when I tried to pick them up with this plastic tie

Can bristle worms kill fish?

Bristle worms are scavengers that eat left-over food and dead stuff in your aquarium. If you happen to see them in your tank eating a dead fish or dying coral, it is very likely the animal died first and the smell of decay drew the bristle worm to the feast.

What do bristle worms do?

These pointy polychaetes do the job that nobody else wants to do. They are the cleanup crew that removes the dead bodies and decaying biomass in the tank. No kidding. They do the work that a cleanup crew should do, with none of the fanfare and none of the fame.

bristle worm face
Can you spot the difference between a fire worm vs. bristle worm? A fire worm is shown above, they are much more photogenic

Are bristle worms parasites?

No, the common polychaetes you may find in a saltwater tank are not parasites. They are detritivores, meaning they sift through and eat things in the detritus, including dead and decaying matter. A parasite, by comparison, lives in or on a host and feeds off the host. There are other parasitic polychaetes in nature, but the common critters in your tank are not likely to be them.

Are bristle worms dangerous to humans?

The vast majority of bristleworms you are likely to encounter in a saltwater aquarium are regular old polychaetes that are NOT dangerous to humans. However, there is a small chance that a more dangerous species (the fire worm) made its way into your tank. It is a good idea to wear thick gloves when you have your hands in your tank anyway.

bristle worm striking a hedgehog pose
This ball of bristles was a little shy for the camera

Are there good and bad bristle worms?

Fireworm vs. bristleworm

While the nature of this article isn’t intended to delve into the ontological merit of these organisms or the philosophical value of a cleanup crew detritivore on the food chain (um, what?), but the simple answer is that, from a self-interested reef aquarists’ perspective, yes, there are good and bad bristle worms. Luckily for us, the most common variety is the good kind, and the bad kind is very rare.

Here is a video that shows the difference between a good and bad bristle worm, side by side:

And another video that shows the predatory behavior of one of the bad bristle worms:

Is having too many good bristle worms a problem?

Having too many bristle worms in a reef tank is a symptom of a bigger problem, not really a problem of its own right. You see, if you have a ton of bristle worms, it means you have a ton of dead animals or left-over food rotting in your tank. Because without all that food, you wouldn’t have all those bristle worms. So in one way of thinking about it, they are a very, very good thing—because, without them, you’d have very foul water. Instead of having polluted water, you have a zillion segmented polychaetes chowing down on the imperfections of your aquarium.

What happens if you touch a bristle worm?

Touching a bristle worm is a lot like touching a small cactus or getting a series of splinters. I can tell you, from first-hand experience. The tiny hair-like bristles will get stuck in your skin and have to be removed with tweezers. The affected area will be tender for a while. Here is an image of me pulling bristles out of my finger.

what happens when you touch a bristle worm? You get bristles stuck in your finger

Bristleworms may be in your sand or under your live rocks, usually in places you can’t see. That is one of the many reasons why it is important to always wear gloves when you have your hands inside the aquarium.

Yes. Those are my hands. You can read about that fun experience here.

How do I get rid of bristle worms from my tank?

There are three primary ways to get rid of bristle worms:

  1. Physically remove them when you see them (remember, don’t touch them)
  2. Trap them inside of a bristle worm trap (either store-purchased or DIY)
  3. Add a predator to eat them

Physically removing them

You need some sort of grabbing or scooping gadget to remove them safely without getting stuck. But the process is fairly straightforward. See ’em, get ’em. Sometimes you can suck them up with a gravel vacuum or a turkey baster.

4 bristle worms on styrofoam removed from tank with turkey baster
These worms were feasting on some detritus when I cleaned the tank and removed them. Nothing beats manual extraction

Building your own DIY bristle worm trap

If you are interested in trapping the little buggers while you sleep, here are two potential designs to make your own bristle worm trap using plastic stuff around your house. Please be careful when making them, they involved cutting thin plastic, which can actually be quite sharp, in my experience.

Design 1: DIY Soda bottle bristle worm trap

The soda bottle conversion into a bristle worm trap is a classic design you can use to trap lots of underwater critters, including polychaete worms. Since these creatures are pretty small, I recommend using the ~12-20 ounce size. Perhaps this is sharing a bit too much, but learning how to make a trap like this has actually been a fairly handy skill for me. I’ve used the exact same design to trap all sorts of things…from fruit flies to minnows and shrimp, and of course, bristle worms.

Step 1: Cut off the top of the bottle with scissors or a knife. You don’t have to worry about keeping the line super-straight, although the trap will look nicer and have fewer jagged edges if you manage to keep the cut fairly straight.

DIY bristle worm trap
Be careful the edges will be sharp

Be careful, the plastic edges will be sharp!

Step 2: Unscrew the cap, so that the top of the bottle is and remains open and invert the top part of the bottle so that the screw top end goes inside the body of the bottle

trap for bristle worms
Invert the top and put it inside

Step 3: Superglue or silicone the sides so that the bottle trap is completely sealed (remember, these are tiny creatures, you don’t want them to escape. When I’ve used this design to trap other things, not in my reef tank, I have used duct tape to seal the edges. It is way easier than glue. The problem is that I don’t know what’s in the glue there and don’t want to recommend that, at all, for your reef tank. I haven’t ever put duct tape in my water and don’t want you to. But just think of that as a concept–you want to seal the edges as best you can with your reef-safe materials.

It’s also possible to wedge the plastic in well enough that you can get a pretty decent seal without any glue or silicone. In my experience, one side or more will pucker, buckle or fold, creating a small gap, that a small invertebrate can clearly escape through, so you can focus your plug on that spot, or even try it out and see if you trap (and keep) any little guys without the extra adhesive step

bristle worm trap 1
This is the hardest part, because you don’t want to leave gaps that hey can escape through

Step 4: Place some rotting, stinking bait inside the trap. Congratulate yourself on making a bristle worm trap, lay the trap down sideways on the bottom of your tank, and go to sleep.

See what you can catch the next morning. You may need to weigh the trap down to get it to stay in one place. The basic premise of the soda bottle design is that the worms will climb in to get the free meal and some/most won’t climb back out…hopefully.

Don’t leave the rotting food in there too long (eg. not for multiple days at a time), you don’t want to ruin your water quality.

Design 2: DIY food storage container bristle worm trap

I also found a DIY bristle worm trap design on Reef Builders that involves taking a small, rectangular food storage container with a lid. For this design, you want to use the flattest container you can find. You cut an X pattern into the center lid with a box-cutter tool or a sharp knife. As a result of cuts, you will create 4 triangularly shaped flaps (that are sharp now, be careful). By bending those flaps inward, you create a similar funnel design where it is easier to get in than it is to get out.

Step 1: Cut an x through the top of the lid, creating 4 sharp, plastic triangles/pyramids that flex in the middle

bristle worm trap 2 step 1


Step 2: Push each of the parts down and bend them so that you create a semi-permanent gap large enough to catch some worms. Be careful when you do this, the sizes of the plastic will be thin and sharp–they can and will slice your finger.

bristle worm trap 2

Step 3: Bait your trap with something gross (like frozen food that was left out all day), set it on the bottom of your tank, and see what you get

Each of these DIY bristle worm trap designs works in a similar way–you put some bait in a container, you make it easy for small things to slip in, and make it difficult, but not impossible, for them to get back out. The invertebrate likely doesn’t realize it was trapped and likely didn’t pay much attention to how it got in, so it likely won’t find its way back out, besides some occasional random luck or persistent exploration.

What eats bristle worms?

The eat-or-be-eaten underwater world fascinates me. Here we have a small, segmented worm with tufts of stinging bristles sticking out of each segment for protection. Those bristles sting–I can attest to that. Yet, while these defenses certainly do discourage some reef inhabitants from nibbling on them, there are several predators who will happily dine on them.

Invertebrates eating invertebrates

If you are interested in creating some natural pressure on the polychaete population, there are two pretty good choices–arrow crabs and coral banded shrimp will both catch and eat bristle worms.

coral banded shrimp (top) and arrow crab (bottom) shown on same image
Coral banded shrimp (top) and arrow crabs (bottom) are effective bristle worm hunters that will keep the population down

Saltwater Fish that eat bristleworms

There are also a few good saltwater aquarium fish options that will seek out and eat bristleworms in your tank. Wrasses, like the 6-line and Melanurus, are effective predators. Hawkfishes love to eat small invertebrates and may help the cause, as well as Dottybacks (orchid, neon, etc.).

It is worth adding a disclaimer here that when you choose a natural biological control, like adding shrimp, crabs, or fish to take care of your bristle worm (or any other natural problem), you’re probably not going to get total, rapid elimination of the problem. Individual results certainly will vary. Sometimes your predator just won’t eat them. Other times they’ll just do an imperfect job.

4 fish that eat bristle worms

Should I get rid of them or not?

Just because you could remove the bristle worms doesn’t mean that you should. That is a decision that only you should make–but since you asked–I’ll give you my perspective on the matter. Bristle worms are ideally suited to aquarium life.

That’s why they grow and reproduce so well in our tanks. It would be rare and unusual for an aquarist to pay money, intentionally, for a bristle worm. We don’t invite them into our tanks, they just show up. But many of us (I’m raising one guilty hand right now) is a little bit sloppy with our husbandry; we feed a little too much and clean the tank a little less often than we should.

The bristle worm population in your tank helps create a little bit of a natural, biological buffer–a cleanup crew that you didn’t intend, but mother nature developed specifically for this purpose. In addition to that, think about all the biological diversity and invertebrate life going on in your tank. How many rotifers, copepods, stomatella snails, hermit crabs, starfish, and other snails live and die in your tank in a given week, month or year?

I suspect you’re pretty good about removing large, dead organisms, like an unfortunate fish–but what about all those other critters? Do you catch and remove them all? Or do you need (or want) a little help?

The bristle worm is nature’s cleanup crew, so my vote is that you leave them alone. Monitoring the population should give you some insights into how much waste is really in your tank (and free for the bristle worms to consume), but otherwise, these segmented polychaetes are a good thing, in most tanks. The only time I would recommend removing them is if you have the larger, problematic species–or if you absolutely just can’t stand the sight of them.

bristle worm by prilfish

Photo credit: Prilfish

Other similar articles

If you want to dive deeper into the world of live rock hitchhikers, check out the:

Live Rock Hitchhikers Guide (Don’t panic)

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