Tube worms are just that - a variety of marine worms that live in tubes. Most of them belong to one of two large groups, which are called sabellids and serpulids, and some specimens from each of these groups can be found regularly in aquarium shops. However, as neat looking and interesting as they are, they are definitely not for everyone.
The sabellid worms form a tube made of a tough but flexible material, which is sometimes coated with a layer of detritus, mud, sand, and/or small shell fragments, while the serpulids build rock-hard tubes made of calcium carbonate. Many of the "hard-tube" serpulids also produce a small plug or door-like structure, called an operculum, that they can use to seal themselves inside their tube, while the "soft-tube" sabellids do not. There are a few exceptions, but these are the general characteristics.
All of these worms also have specialized structures called crowns, which are extended from the tubes to collect food particles and for gas exchange. They act like a sieve and a gill at the same time. The typically circular-shaped crowns are made up of many branches called radioles, and the radioles themselves are covered by tiny hair-like branches. Thus, many look like some sort of small fan or feather duster.
As I said, the crowns capture food, and how they do it is quite interesting. The lower surface of each radiole is covered by tiny hair-like cilia that rhythmically move in such a way as to create water currents which move from the underside of the crown, upwards through the radioles, and away from the worm. Conversely, the upper side of each radiole has a series of grooves running down the middle from the tips to the center of the crown. So, the neat part is that when the bottom to top current is produced, small particles in the water are caught in eddies created on the upper side of the radioles and are caught and moved to the worm's mouth by more cilia the grooves. The cilia/grooves also sort the particles before sending them to the mouth, and any particles that are made of the wrong stuff, are too big, or are too small are rejected.
In addition, as they grow they tend to prefer different particle sizes. Very young, small worms have very small crowns/radioles/grooves and therefore prefer to capture and eat very small particles, like bacteria and single-celled phytoplankton. Conversely, big adult worms have bigger crowns/radioles/grooves and may feed exclusively on much, much larger multi-cellular zooplankton (still pretty small stuff, really). And, as you can guess, the preferred food size can depend on what maximum size an adult achieves. Tube worms can be very picky eaters!
Some Common Types: So, let's take a look at some of the common types of tube worms. There are lots and lots of little oddballs that show up in stores here and there, and many, many others that hitchhike on corals and live rock, but these are the four "regulars" you'll most often see for sale.
There are several relatively large sabellid tube worms called feather dusters, but Sabellastarte magnifica and spectabilis are the most common. They look so much alike that you can't ID them without removing one from its tube, but if you really want to know what you have the easy way to find out is to ask where it came from. S. magnifica is collected in the Caribbean, while S. spectabilis comes from the Indo-Pacific.
These two species' tubes may be 8 inches long and the worms have impressive crowns that are often 3 or 4 inches in diameter and nicely colored. Also note that the tubes are coated with mud/detritus particles, but not sand or other larger particles. These are indeed the most commonly offered tube worms by far and you will see them regularly at stores. This does not mean they are inherently hardy though, as they still require very fine foods despite their size.
Cluster dusters, primarily the Caribbean Bispira brunnea, are much smaller sabellids. They have soft tubes that are typically only a couple of inches long at most, and their crowns may be less than a half-inch in diameter when full-grown. But, they are very attractive nonetheless, and typically come in tightly spaced clumps made up of numerous individuals.
Oddly enough, these worms can reproduce asexually (as can many others), and if conditions are optimal they may increase their numbers rapidly. But, in most cases they are rather difficult to keep alive long-term at all, so hoping for them to spread may be overly optimistic for most hobbyists.
Christmas trees worms:
The serpulid Christmas tree worms, Spirobranchus spp., are quite unusual in that you're likely to only see them living in colonies of stony corals. And when I say "in" I mean they settle on corals when they are larvae and then their hard tubes are actually covered over by coral skeleton as the coral grows. Thus, the worm's tube becomes integrated with the coral. They really do live in the coral colony, which is most often Porites.
Their tubes, which typically coil in spirals, can be 3 inches or so in total length, and their unique crowns can be close to an inch at full size. I say these are unique because they spiral upwards from a wide base to a small tip in a corkscrew fashion, and are found in distinct pairs extended from the tube. Unlike the typical feather duster look, these crowns really look like two small trees, and come in all sorts of bright colors and neat patterns.
These used to be very common back in the "old days" when non-aquacultured Florida live rock was available, as they were often present on pieces of "wild" rock. But, now you can't get that type of rock and pretty much have to buy a coral to get the worms (which are coming from the Indo-Pacific). Thus, you'll have to make sure you can care for the worms properly - and the living coral, too. These are therefore suitable for reef-aquariums only!
The Indo-Pacific serpulid cocoa worm, Protula bispiralis, which is also often called the "hard-tube worm", is another big and beautiful tube worm. These may have a tube that is over a foot long and an inch in diameter, and they also have two typically brightly-colored crowns. The crowns are also tiered, and look more like those of the Christmas tree worms than the sabellid feather and cluster dusters.
They are seen with some regularity in stores, but unfortunately they are probably the least likely to survive for long. Again, despite their larger size, they'll require fine foods that are typically in short supply in most aquariums. Even in well-maintained reef aquariums these worms infrequently live more than a few months.
Some Common Issues: Let's also go over a couple of things you'll need to take care of and problems that tube worms sometimes have.
Not staying put:
Tube worms will not care for being rolled around on the bottom of an aquarium by currents, or just lying flat on the sand as it can prevent them from feeding. Instead, they should be placed in the rockwork where they will stay put, should be glued to a substrate, or should have part of their tube buried in sediments to hold them in place.
If placing a large tube worm in a crack or crevice in rockwork, be sure that you do not crimp, pinch, or bend the tube to make it stay put. This will surely make the tube's inhabitant unhappy. Depending on it's size, you'll also need to keep at least an inch or two of the tube sticking out to make sure the worm can fully extend its crown without having it in direct contact with the rock. Remember they move water from the bottom up and probably won't like having the bottom-side of their crown cramped in between some rocks.
Clusters of worms can also be glued to rock or shells by using some gel-type superglue. Clusters can be turned over, dabbed with glue, and then stuck wherever you like. The glue will set/stick, even underwater. Just be sure that you do not accidentally glue the worms shut in their tubes, or glue them to your fingers!
Lastly, if you have a deep enough sand/gravel bed you can try burying the tube, while leaving some of it sticking out. Again, depending on the worm's size, you'll need to leave plenty of tube out so that the worm can fully extend its crown.
It isn't common, but every once in a while some tube worms may decide to crawl out of their tube and do a little adventure traveling. However, you should not try to shove a worm back into its tube if this happens, as you will be putting it back where it obviously didn't want to be, and likely kill it in the process. Keep in mind that a worm is basically a soft, unprotected meal at such times, so if one decides to leave its home there must be a reason. Possibilities range from unfavorable currents that prevent it from capturing food to being nibbled at by some small predator. The good news is that if nothing eats them, they can sometimes find a location where they are happier and can build a new tube. So, as bad as it may look, the best thing to do is leave them alone.
Loosing the crown:
On occasion a tube worm will decide to dump its crown. Yes, the whole thing. The entire crown just comes loose and you'll find it sitting on the bottom somewhere nearby. This would seem to be a sure sign of death, but amazingly enough, they can grow them back. So don't pull out the tube! It may take a couple of weeks, but typically the crown will grow back. The new crown may be significantly smaller than before, though.
I don't know of any "proven" reason they do this, although it has been suggested that it is a sign of starvation and/or water quality problems. Those two things get the blame for most anything anyway. It is likely a bad omen if a worm does this repeatedly, and of course, there are times when the worm doesn't extend its crown because it is actually dead, too. But, you'll likely see nothing of the worm at all instead of finding the separated crown in such cases.
Yes, tube worms are part of the menu for some other animals. So, if you think you want the worms, better make sure that you take care of any incompatibility issues. Many sorts of butterfly fishes will eat them, as will a number of wrasses, and the flame hawkfish (Neocirrhites armatus) likes 'em, too. Then there are a number of crabs that will tear them up, and even some serpent stars will make a meal of them, as well. Fortunately, almost all butterflies, many wrasses, and the vast majority of crabs are not considered to be welcome in reef-type aquariums anyway, but you do need to be particularly wary of the hawks and serpents, as they are commonly keep in such tanks.
Starving to death:
Well, this should be an obvious one by this point. These worms eat relatively tiny foods and most aquariums simply cannot support them. If you have a reef aquarium with a deep sand/mud bed or some sort of refugium set-up you may be fine, but for most aquarists there is no sure-fire way to keep these worms alive. Even using high-quality "plankton in a bottle" may not work at all.
As mentioned, these different types of worms can be exceptionally picky about what they eat, therefore one type may absolutely love one food product while the same stuff may do absolutely nothing for another type of worm. Some (especially the smaller ones that eat the tiniest foods) may reproduce prolifically in reef aquariums when fed nothing at all, while others will slowly starve even if given a bath in food. Thus, you may try several different types of worms and have great success with some and no success with others. Unfortunately, in most any case it will be trial and error, unless you are able to provide an adequate supply of a wide range of food sizes - and that isn't easy. So, if you aren't ready to do this one way or another, I strongly suggest you leave all of these worms alone.
As a last note about these worms and food, do remember that they create currents that carry food from the underside of the crown/radioles to the upper side. So, if you choose to target feed a worm using a syringe full of plankton, don't squirt it at the worm from the top. It won't work! Instead, you can gently squirt such foods towards a worm from the side in such a way that the worm can direct particles upwards through the radioles in the proper manner.
Some shopping tips: If you do have a system that employs a well-stocked deep substrate bed and/or you use specialty foods for filter-feeders and want to try a tube worm, or a few, you still need to be careful when shopping. Choosing the best specimens is always a good way to improve your chances for success.
When you go shopping, the most obvious thing to look for is a worm(s) with a fully-extended crown. If it is only partially extended for the tube, or not "fanned out", the worm may be in poor health. If it is well-extended you need to give it a good looking over for any missing radioles or any other apparent damage that looks like it might lead to troubles.
You should also try to test an apparently healthy worm, just to make sure. These worms have eyes of various complexity here and there, and they are also very sensitive to touch. Thus, they should respond to a shadow passing overhead and/or a light touch by quickly retreating to safety in their tube. So, if they are out and you wave your hand (or something else) just over them and block the light, or give them a light touch on the crown, they should jerk into their tube. If they don't, there may be trouble.
Speaking of the tube, take a good look at it, too. Many worms with soft tubes may have some damage which can be repaired without too much trouble, but if their tube is torn up badly the worm inside may have unseen injuries from whatever event tore up the tube. The same goes for worms with hard tubes, and they can take much, much longer to repair a damaged tube, as well. So, it is really better to just choose a different specimen if you see such damage.
Lastly, as is the case with most things marine, try to keep a chosen specimen submerged from tank to tank. They have void space in their tube and when they are lifted out of water the tube fills with air. Then, when re-submerged some air bubbles may be trapped in the tube, which can create problems.
That's about it. Again, I must stress that if you are not prepared to make sure they are getting enough of the appropriate food, leave them alone. However, if you will do what it takes - enjoy.
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Fox, R. 2001. Invertebrate Zoology Online: http://www.lander.edu/rsfox/310labindex.html
Rupert, E. and R. Barnes. 1994. Invertebrate Zoology, 6th ed. Saunders College Publishing, Fort Worth, TX. 1140 pp.
Sprung, J. 2002. Featherdusters in the Aquarium. Advanced Aquarist's Online Magazine: http://www.advancedaquarist.com - August issue.