The bristle worm that many of us see in our reef tanks can get right up there with spiders in terms of their fear factor. Like most other scary creatures in nature, though, once you learn about their biology, their outward appearance usually makes more sense and become less scary.

To this day, I still crack up when I see people’s first encounter with a bristle worm on social media. It’s usually a half blurry picture taken when their tank lights are off of a worm crawling on the sand of their tank looking like a creature from a sci-fi movie. The caption usually reads, “What are these?”  or “Are these going to eat all my corals and fish?”  The good thing is that in most cases, the answer is no.

As we dig deeper though into the world of bristle worms, it is easy to see why they can get a suspicious look from almost any reef hobbyist. Let’s take a closer look at the biology of our common little tank mates.  We’ll learn why they are all over our reef tanks and what they do. You’ll also find out if they are harmful to corals and fish.  Let’s look at little deeper at the world of this scary looking annelid.

Bristle Worm Biology

Bristle worms are polychaetes, a form of annelid worm that is typically found in marine environments.  Part of the reason I think they get a bad rap is that there are actually over 10,000 species of polychaete bristle worms. The sheer number of bristle worm species should lead one to think that they can pretty much live anywhere in the ocean, and that turns out to be exactly the truth.  These worms have been found from the coldest abyssal plains in the arctic to the hydrothermal vent communities that pop up under extreme temperatures and pressures. The diverse nature of their distribution makes sense when you consider these worms reproduce as broadcast spawners that release their gametes into the ocean’s currents to be spread far and wide.

Bristle worms are segmented worms with a body length of about four inches on average but can range from one millimeter to almost 10 feet. Their coloration varies greatly, but many of the species we see in our reef tanks will have a pink, iridescent, or even luminescent hue to them. Often these colors are correlated to where the worms live, which can vary from sandy substrate to liverock. Their bodies have sharp little bristles that are called parapodiaon on the left and right. These parapodia serve both as a sensory and a defense mechanism. Anything that tries to touch or eat a bristle worm is going to get stuck with parapodia. Sometimes, this is not a big deal, but in other times, it can pack a punch. Every time I don’t wear gloves when moving around some liverock, the result is going to be red fingers that look like they have been stabbed with 1000 pieces of fiberglass insulation thanks to bristle worms. 

What Are They Doing in Our Reef Tanks?

To truly understand the function of a bristle worm within your tank, I think it is important to zoom out and take a look at the entire reef as an ecosystem. As much as we try to replicate a complete reef environment in our tanks, we are still looking at a cross-section of diversity. Parts of our tanks will mimic various reef niches, including the reef crest, reef flat, lagoon, and even sometimes mangrove and wetland ecosystems.  Knowing that all of these communities are necessary for a reef to thrive in the ocean is important when you are setting up your tank as well as when you are trying to diagnose a problem or predator in an established tank.  In my opinion, doing some of this ecosystem planning before you go to your local fish store to pick something out will give that creature a better chance at a long life in your tank when you bring it home.

Getting back to the bristle worms, we typically see a smaller subset of the 10,000 species. The most common of these are the fan worms. These are commonly found in your sump or as a spiral worm in a piece of porites coral. Fire worms, Amphinomids, are also common. Just don’t touch these guys or you will have shards of stinging fiberglass like spines making your fingers throb. Lastly, there is on occasion, the coral-eating fireworm, Hermodice Carunculata. This is a rare find in the reef hobby but can pack a heck of a punch if you were to touch it.

bristle wormBy in large, bristle worms are not put on this earth to eat our corals and fish.  Most are scavengers looking for loose scraps of food and detritus in our tanks. In my opinion, this makes them the best free member of our reef clean up crew you will ever find.  They live in our sandy substrates as well as in the caverns of our rock work.

Personally, my favorite contribution from these worms is their monthly reproduction that spews tons of eggs and sperm into the tank water just before the lights go out. Think about it, how much do you spend on coral food supplements you have to mix and dose in your tank?  Here is a monthly uber-dose of live gametes and subsequent larvae that you don’t have to pay a penny for.  Count me in!

You may witness this in your tank someday too. The worms climb to the highest point in your reef they can reach. Then, one at a time, they will start puffing their gametes into the current. This will build until 3-5 are doing this a second, and your tank starts to look like milk. I like to turn my skimmer off for a day and drop an extra air stone in the sump to make sure the oxygen stays high, yet I’m not siphoning off all the free coral food.  You will also notice your corals looking happy over the next few days as a result of the feast. We were lucky enough to catch this on video last year.

Should We Remove Them or Embrace Them?

I think by now, you are starting to get the picture that these little members of the reef community should, for the most part, be cherished rather than removed or hunted.  Yes, they do have a slight “I’m going to eat you”  kind of look to them, but in the big picture of reef ecosystems, bristle worms are a pillar of the community. If you do find yourself with one of the less than desired members of the Polychaete family, there are some measures you can take to try to reduce their population.  I say, reduce the population, as completely removing all the adult and larval forms of a particular species is almost impossible.

How to control bristle worm populations

If you find yourself trying to remove a certain species of worm or control its population naturally, there are a couple of steps you can take.  I prefer to add a natural predator as it is more of the “set it and forget it”  long-term mentality, but others prefer to manually trap and remove certain coral or fish-eating worms.

Building, borrowing, or renting a trap from your local fish store is probably the easiest means of manual removal out there. These traps are usually clear and have some fishing line attached to a door or a spring-loaded door that will shut behind what you are trying to capture. Since we are talking about worms here, I think the spring-loaded type of trap is a bit overkill, but it will work regardless. Night is the best time to catch worms as they are usually nocturnal in their foraging. Place a simple piece of food in the trap, which you can let sit for a while as the smell travels through the tank and signals an easy meal for the worms. Upon your return, you should be able to catch one or many worms with this method and then reload the trap with food if you so choose.

My favorite method of removal, predation, revolves around finding and adding the missing part of the food chain to your tank. Since almost every creature in the ocean has some sort of predator, something out there considers bristle worms to be a tasty, albeit spicy, meal. A word of caution here is to make sure that the predator doesn’t also like to eat your other tank mates, such as snails, shrimp, clams, or other fish. My two favorite bristle worm predators are the pencil wrasses and members of the hawkfish family.

The Final Word on the Bristle Worm

I firmly believe that if we look at our tanks through the lens of an ecosystem and not as a simple cause and effect experiment, we will gain both a better understanding of the total biology living there. This can help you as you decide what to do with the bristle worm population in your tank. You want to make sure that you use them to the greatest benefit while also ensuring they do not get too overpopulated and take over.