A Bristleworm Hunter to Thin Out the Populations

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One of the most abundant and viewed threads on aquarium forums would have to be the ID threads. For those that don’t know, ID threads are equally used by many experienced and newbies. You snap a good picture of a organism that you might not exactly know its identification, then post it in said ID thread, where others can chime in and help you figure out what this organism might be. In all of these forums, there is a specific ID thread we see used the most. That would be the hitchhiker ID thread. Everything from flatworms to corals hitchhikers are presented, and because they can come in all forms and sizes, most of the time hobbyists don’t know if they benefit their aquariums or cause a major downfall in those systems.

There is a specific hitchhiker that most of us have seen. Some of us don’t mind them and some of us can’t stand them. The hitchhiker in question tends to leave a trail of “bristles” in your finger, so when you get a grip of that live rock, it makes sure to prick you and later irritate your skin. Yes we are talking about bristleworms. Bristleworms can be as small as a couple centimeters and sometimes as long as a foot, or more. One of the biggest questions about bristleworms is if they are beneficial or a pest. Well from our experience in can go both ways. Bristleworms are great, if not one of the best detritivores we can introduce in our systems. They live amongst the sand and rocks, scavenging for uneaten food. There is a downside to this scavenging characteristic, as bristleworms will also quickly reproduce and can possibly invade a system because there is usually so much food for them to eat. In my experience, I had bristleworms coming out during feeding time and snatching food from corals. While I did not really mind at first, it quickly got out of control. Once the lights were off there were worms everywhere, so I knew that I had to take action. So I cut down on the number of feedings in hopes to slow down their reproduction, and started to manually remove as many as I could, with the help of long tweezers of course. At least once a week I was bristle worms hunting. Somedays I removed three or four and others I was removing dozens. After so many weeks of doing this, I really did not see a diminish in their numbers.

Another problem we saw with manual removal was many of the worms where quick to withdraw back into their worm “lair”. On top of that some of the worms had a good grip and ended up tearing in two. While we got one half out, the other half stayed in to just simply form into a new worm.

The only options we had where, one to break down the tank and restart with all new sand and dry rock to make sure we got them all, or go about it a more natural way. Honestly though, after you have had such great coral growth and your tank has been established for years, who wants to break down said tank? Unless you are upgrading to a bigger tank, you don’t want to go through the hassle of taking down an established system, so we went about it the natural way.

By natural way we mean introducing a natural predator of the bristle worms to help reduce their numbers. There are a couple of fish out there that aquarist swear eat bristleworms. The two that come to the top of our heads are the target mandarin and the six line wrasse. I voted against both of these fish simply because I attempted to eradicate red planaria with these them and had no success, so we figured we would try something different more on the invertebrate side.

This invertebrate looks as if it came out of a Syfy film. It sports a pointy and serrated lance at the top of its head, it also has very long legs that are almost spider like. It has a very unique mouth that opens like a small draw bridge and also secretes it’s waste in a similar fashion by opening a hatch that is located on its lower end. We are talking about the arrow crab, and it is known for being a great scavenger. It is also said that they might dig around coral with their claws, not necessarily to harm the coral but foraging for food. It is something to think about if you are weary of it irritating your corals. For us, it was something we where ok with because this crab does not only scavenge, but it is known to love the taste of bristleworms, which is exactly what we were looking for.

We partnered up with the arrow crab and we keep manually removing the worms, all while the crab chows down on bristleworm after bristleworm. It has been roughly a month since we introduced our friend, and while we still see one or two worms roam about during feeding time, we no longer notice the mass horde of worms we had before.

By no means are we saying bristleworms are complete pests. They are great janitors in our aquariums. The problem rather is when their numbers increase in a great amount that they start snatching food away from our corals. We must add that we have not noticed our arrow crab isn’t too prone to digging around our corals. It mainly stays on the sand bed and something exploring around the rock work. Occasionally it does accidentally step on some zoanthids, but nothing major.

While the debate goes on wether bristle worms are desirable in our aquariums or not, we found that many aquarist actually don’t mind them as long as their numbers don’t get out of hand. The arrow crab is an addition to our clean up crew that we don’t regret, and to best illustrate their effective feeding strategies, we leave you with a short clip of our friend snacking on a bristleworm (above).

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  • If you have a bristle worm problem then typically your aquarium is being overfed. This is our experience after working on thousands of aquariums and working with hundreds of aquarium owners who love to feed their animals every time they walk by their tank. If you remove the worms you would likely just end up seeing an explosion of mini sea stars or just more hair algae and so on. Using a crab to remove worms is worse then using chemicals to remove cyano when your RODI has no TDS meter…. (worse because the crab could rip open prized LPS corals to feed itself) Find the root cause of the issue before figuring out a solution to the symptoms of the issue. Just my two cents.

  • Daniel

    This is not always the case. I have had this crab for half a year and have kept them before and they have never touched any of my coral. I also do not over feed my tank I actually probably underfeed it. I feed when I remember and that is about once a week. I do weekly water changes and I don’t ever feed more than the fish can eat. So while over feeding sometimes is the cause of the problem it is not always the truth.

  • Daniel

    The article also states that feedings were cut down even further and worms where removed manually before deciding to add a natural predator.

  • Ralph S.

    A Bristleworm, like any creature, needs a food source to survive and multiply. In my experience, even if feeding is drastically reduced, it can take a while before the population declines…but it will.

    For those who are impatient or just want a cool invert, an alternative to the larger Arrow Crab is the smaller Galathea Crab.which actively consumed Bristleworms. The one I had did not bother any of my corals while the Bristleworms were available.

    Most crabs and shrimp are opportunistic and they will often go after food recently consumed by a coral if they are not well fed.

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