Re:Strange living? thing in my tank, help!!

Pete Giwojna

Dear Cherie:

Shorty is absolutely correct — your fuzzy little caterpillar-like critter is a marine polychaete better-known as a bristleworm. They are primarily nocturnal and tend to hold up during the day, and when you see one of them you can be certain there are several more hiding out as well. They are harmless as long as you don’t touch them — the bristles are very sharp, easily penetrate your hand or fingers, and are quite irritating — and are generally safe for seahorses as long as they don’t grow too large or become too numerous, as discussed below:

In general, bristleworms are benign, even beneficial inhabitants of a seahorse tank that perform a useful service as scavengers. But when their numbers get out of control, there comes a point when an overabundance of bristleworms becomes problematic as far as seahorses are concerned. That point is generally when the exploding population of bristleworms become too aggressive at feeding time, actively seeking out the frozen Mysis even during daylight hours, no longer content with cleaning up leftovers, and begin invading the feeding station. Too many bristleworms lingering too long at the feeding station brings them in direct contact with the hungry seahorses who come to the lunch counter for their favorite food as usual. The galloping gourmets may accidentally brush up against the encroaching bristleworms, or even attempt to perch on them, and they may get a snootful of bristles when snicking at the same mysid a bristleworm has taken an interest in. Even if the seahorses don’t inadvertently snick at them, the bristleworms may shed a few of their irritating spicules while they are at the feeding station, and the hungry seahorses can then accidentally ingest such loose spicules when slurping up frozen Mysis. Captive bred seahorses are aggressive eaters that are accustomed to slurping up food from the bottom, and it seems at times this may also lead them to strike at baby bristleworms. I’ve also heard a few reports of seahorses that snicked up a tiny bristleworm and got them lodged in their snout or throat. It’s unclear in these cases whether the bristleworm was accidentally sucked up while the seahorse was targeting a piece of nearby Mysis or whether the seahorse actually mistook the tiny worm for something edible and deliberately struck at it, but this is another potential danger the seahorse keeper should be aware of.

I have seen pictures of seahorses with bristleworm spicules embedded in their tails and snouts as a result of such close encounters. These injuries are usually minor, easily treated by removing the spicules and administering antibiotics orally via gut-loaded shrimp to prevent secondary infections, but the more bristleworms there are, the more likely such incidents and problems are to occur.

So when you start to see bristleworms swarming the food station, it is a good idea to start thinning them out. You might consider using a form of biological control to reduce the bristleworm population. For instance, Arrow crabs (Stenorhynchus seticornis) are predatory on bristleworms. Large arrow crabs can sometimes be problematic for seahorses, but in my experience they get along well together. You might want to try a medium-sized Arrow Crab, which will predate small bristleworms and keep their numbers in check.

And you can also control bristleworms by trapping them. A number of bristleworms traps are available at aquarium outlets and can serve as the seahorse keeper’s first line of defense against these prickly pests. Here are some tips from Dee that explain the best ways she’s found for trapping bristleworms:

deemarie1234 <[email protected]> wrote:

I actually caught a bristle worm last night that’s massive!!! Only 4"
long but nearly 1/2" wide! I used a trap for him. I wasn’t going for
him. I knew there was a big one in my reef tank and had seen one
about 7" long. He was the one I was after. My hubby woke me up this
morning to come see the one I did manage to catch.

Besides him though I’ve caught quite a few smaller ones as well. I
like to check my reef tank about 1 1/2 hrs after the lights have been
shut off. Then when I find them I watch them as they flee from the
flahslight and know where they are hiding. I set the traps right next
to their hidy holes and catch them. If they are really narrow the
traps aren’t that good. They can get back out very easily so you
can’t wait until morning to check the traps. You need to check them
periodically through the night, if a small worm is in the trap, empty
it then don’t wait till morning or he’ll be gone. Even the huge one I
just caught managed to get into a hole about 1/8" in diameter to the
food chamber.

I’ve had better success with nylons. I buy knee high nylons and at
night drop a rock or two in the toe along with some food. Then I set
the toe area right near the holes. When they try to get the food they
get stuck in the nylons. In the morning I just pull it out of the
tank and get rid of them.

I’ve also used a turkey baster a few times. If you can get into the
tank quiet enough they won’t really move, squeeze the ball and when
you’re really close to the worm let go. The suction can be pretty
good sometimes depending on where they are.


> Anyone out there with any great successes at catching and
> eradicating your tank of bristles without the use of drugs. This is
> important to me since I have an amazing Coco worm and a few other
> things planned for my reef that will be harmed by the so
> called "wormicides". Ive been using a trap with on and off success,
> actually caught about 1-3/night for the first few nights, and not
> having much success catching the more cautious ones. I think there
> might only be one or two left but not able to coax them out.

The best bait for trapping bristleworms depends on what I have handy at the time 🙂

I found the brine shrimp and sometimes krill (both frozen) work well.
I usually add some to either the trap or the nylons and then take a
small container of water from my reef tank to thaw it out before
putting either the traps or nylons in my tank.

I’ve got two more tricks for ya, First the turkey baster. It doesn’t
really have a lot of suction power. Or at least I’ve never found one
that the ball was on tightly. So I bought a new one and took the ball
off. I wrapped the end with thick clear tape, smae diameter as duct
tape just clear tape. Then when I put the ball back on it was much
tighter giving me much better suction.

Also there is a different type of bulb that is very useful especially
for small worms. They make them for babies noses and you can also get
them anywhere they sell camera equipment. They are used to blow dust
off camera lenses. They are a good size to fit in your hand snugly
and have good suction. Again to make it better there is a small hole
in the bottom on the bulb. I used a piece of a silicone ear plug and
covered the hole. Now I’ve got twice the suction and it works great!!
Eeven though the spout end looks small it really can suck up some
good size worms.

Personally I’m fascinated with the night time world of my tanks. I
completely dewormed my rocks in my sea horse tank. But as for my reef
tank, I kinda like having some brstile worms in there. They really do
clean up a lot of stuff and in tiny little spaces where noone would
ever think would need cleaning. But when I start finding portions of
dead hermit crabs I get a bit cranky. I still can’t kill them though.
I’ve still got the huge one alive and well and am trying to figure
somewhere where I can put him so he can live. I’d love to watch him
closer. I also get a bit cranky when I reach into a tank at night and
don’t watch where I’m reaching. Oops! Got hit pretty hard by a
fireworm one night from a brand new rock. Ended up pulling out three
bristles and got a good sting that lasted 3 days.

Good luck catching your worms, once you start you’ll get hooked on
catching the little monsters 😉


Just don’t freak out about bristleworms and overreact at the sight of a few of the pests, Cherie. Bristleworms are not public enemy number one — in most instances, they are harmless or even helpful residents of your seahorse tank. But if you want to play it safe, no one here will fault you for trapping the little buggers or enlisting a medium-sized arrow crab to thin them out.

If you decide to try the latter, however, Cherie, and introduce a smallish arrow crab as a means of biological control for the bristleworms, be sure to observe the following precautions:

I would characterize arrow crabs (Stenorhynchus seticornis) as opportunistic omnivores. I have kept them in a number of my aquaria over the years, including a few seahorse tanks, without any problems. They never bothered my Hippocampus erectus at all, but they can be hard on sessile invertebrates in general and I certainly wouldn’t trust them with dwarf seahorses. Nor would I trust them with a small goby.

I kept a couple of large arrow crabs in my Monster Bin with a 14-inch African lionfish (Pterois volitans) and a couple of overgrown ribbon eels, and the arrow crabs proved to be fairly proficient at capturing the live minnows I fed to the lionfish, particularly after the minnows had been weakened by the saltwater. If the opportunity presents itself, they are quite capable of capturing small bottom-dwelling fishes like gobies.

Arrow crabs will happily devour any bristleworms they can catch but they won’t eradicate them from your aquarium. Too many of the bristleworms always remain inaccessible to them within the rockwork and sand for that, but a small to medium-sized arrow crab or two can help control the bristleworm population. A fairly effective way to reduce their numbers is to regularly trap large bristleworms after lights out along with keeping a young arrow crab to thin out smaller worms (providing there are no sessile invertebrates in the tank the crabs could harm).

In my experience, small to medium-sized arrow crabs are safe with large seahorses and can be used to help limit the number of bristleworms in your tank. Just be sure that you don’t pick out the biggest, baddest, bruiser of an arrow crab to do the job! Go with a smaller specimen, keep a close eye on it, and be prepared to replace it with a smaller individual after it molts once or twice. They grow fast and can nearly double in size after each molt.

Remember there are always exceptions to every rule, and large crustaceans are never completely trustworthy. Even the most harmless and seemingly inoffensive crabs can cause trouble under certain circumstances. For example, not long ago I heard from a hobbyist that had been keeping a decorator crab in his seahorse tank. All went well at first and there were no problems of any kind for months until, for no apparent reason, the crab suddenly began to quite deliberately amputate portions of the seahorses’ tails. It was not attacking the seahorses as prey or attempting to eat its mutilated victims, it was merely methodically harvesting portions of their anatomy with which to adorn itself! It was simply doing what all decorator crabs do — snipping off and gathering bits and pieces of its immediate environment to attach to itself as a form of natural camouflage. It just goes to show, with crabs you can never be sure how things are going to work out…

The following URL will direct you to an informative Fact Sheet about bristleworms you should find interesting, Cherie:

Click here: Bristleworm Factsheet (fwd)

Best of luck with your new seahorse setup, Cherie! As long as all you have observed is one small bristleworm, the little buggers have not yet reached the point where they pose any problem for your seahorses.

Pete Giwojna

America's Only Seahorse Aqua-Farm and One of Hawaii's Most Popular Attractions

Ocean Rider seahorse farm is a consistent Trip Advisor Certificate of Excellence Award Winner and "Top 10 Things To Do" Kona, Hawaii attraction. Our "Magical Seahorse Tours" are educational and fun for the whole family.

Tour tickets are available for Purchase On-Line. Space is limited and subject to availability.

small seahorse Ocean Rider, Inc. is an Organic Hawaiian-Based Seahorse Aqua-Farm & Aquarium that Follows Strict Good Farming Practices in Raising Seahorses and Other Aquatic Life.

Seahorse Hawaii Foundation

Inspiring ocean awareness by saving the endangered seahorse and sea dragons around the world from extinction through conservation, research, propagation, and education.

Help us save the seahorse and the coral reefs they live in with a tax deductible contribution to the Seahorse Hawaii Foundation. You will be helping to protect and propagate over 25 species of endangered seahorses, sea dragons and friends.

Make A Tax-Deductible Donation Today!

A Different Kind of Farm (Video) »

Ocean Rider Kona Hawaii

Ocean Rider Kona Hawaii
Seahorse Aqua-Farm & Tours

73-4388 Ilikai Place

Kailua Kona, Hawaii 96740

Map & Directions


Contact Ocean Rider

Copyright ©1999-2023
All Rights Reserved | Ocean Rider Inc.

My Online Order Details

Purchase Policy

Site Terms and Conditions