I've had a 12" long Medusa Worm in my tank for a couple months now, and have had a hard time finding info on it for health and longevity. It would stay out in the open when I first had it, but now seems content hiding and burrowing under the reef. Do you have any supplemental feeding suggestions other than to let it scavenge? I've also heard a rumor that it can excrete a toxin similar to a Sea Apple. Is this true? I had a Sea Apple poison a reef tank a few years ago, and I don't want to repeat that experience!
Medusa worms are actually legless (Apodid) sea cucumbers. These animals are members of the Family Synaptidae in the Class Holothuroidea and the Phylum Echinodermata. Unlike the other echinoderms in which the mouth is on the "bottom side" of the animal, the sea cucumbers lie on their side, and either stick their tentacles into the water column to suspension feed, or apply their tentacles directly to the substrate to deposit feed. The synaptid cucumbers fall into this latter group, and are the most active deposit feeders of the Holothuroids. They lack the tube feet and respiratory tree characteristic of other sea cucumbers, but are popular for their attractive pinnate feeding tentacles and high level of activity. Because they lack a respiratory tree and Cuvierian tubules (defensive structures with which the most potent toxins of sea cucumbers are associated), they are relatively nontoxic in comparison to some of their more potent relatives such as the sea apples (Pseudocochirus spp.). However, like all sea cucumbers, they do have a variety of nasty chemicals associated with the skin to deter predators from feeding on them. The topic of sea cucumber toxins and their effects on aquaria will have to wait for another time, however.
These sea cucumbers get their common name because they resemble a giant worm from which a "mop" of feeding tentacles are constantly being slapped across the substrate before being drawn into the mouth. These animals are impossible to positively identify by anyone other than an expert, and even then it usually involves knowing exactly where the animal was collected, and likely killing the animal to be certain of the species identification. Therefore, chances are that you'll never know which species are imported when you see one offered for sale in the local petshop. The most common Indopacific genera to be imported for the aquarium trade are likely to be Euapta, Synapta or Synaptula, while the most common Caribbean species are likely to be either Euapta lappa or Synaptula hydriformis. All of these animals look fairly similar, being rather soft and flaccid, with large rounded knobs across the body surface. These are shallow water animals (most species are rarely seen below 50ft) that are common on virtually all coral reefs, and many species can get quite large (some species can exceed 6 feet in length). Although these animals are common and can get quite large, they are rarely observed because they are almost entirely nocturnal (only active at night), although in the aquarium, they often lose this strict nocturnal schedule.
These animals have a wide range of feeding habits. Some species, such as the Carribean Euapta lappa, are generalist detritivores, which cruise the sea floor at night collecting tiny organic detritus from the reef rubble and base rock structure. Other species, such as the Indo-Pacific Synaptula lamperti, are highly specialized feeders which are only associated with living sponges, where it removes tiny organic particles from the surface of the sponge and appears to require the sponge metabolites to survive. Obviously, it will be much easier to accommodate the first animal in an aquarium than the second. Other species are found only grazing detritus off the blades of sea grasses, or only on fine sandy bottoms. The primary problem with getting one of these animals, however, is that you will have no idea of which species you are buying, so you simply have to trust your supplier about it's care and requirements. That is easier for some people than others. If you simply order an animal, you have a pretty fair chance that you will get a species that is a highly specific feeder, and for which you may have no way to provide food. For this reason I generally recommend that people avoid buying one of these animals unless they can first determine what they feed upon. Furthermore, the few studies that have been done on the feeding habits of these animals indicate that they are very active feeders, and being among the most active of echinoderms, they require a lot of food energy. Research indicates that they feed only at night, and that food processing is very rapid -- particles ingested by the cucumber are voided from the gut within about 1 hour! Even when animals are seen apparently feeding during the day, when collected it was found that they had empty guts. Even if you are fortunate enough to get a generalist feeder, unless you have a well established tank with plenty of fine organic detritus upon which the animal can feed, it is likely to starve to death. Like most marine invertebrates, these animals are capable of going long periods without food, and they slowly shrink while digesting their internal organs if denied food; the process of starving takes many months, however, and depending on the species and its initial condition, it could take more than a year before the animal may succumb to starvation.
Another consideration for getting one of these cucumbers is that these animals have a highly reduced skeletal system. Like all echinoderms, the skeletal system is composed of a series of tiny calcareous plates (ossicles) embedded in the skin of the animal. In synaptid cukes, these ossicles are reduced to simple hooks called "anchor ossicles" which project through the skin and give the animal adhesion to the substrate. Anyone who has handled one of these cucumbers can vouch for how "sticky" they are, and if you place the animals into a small container (like an aquarium shipping bag) they often even get snagged on themselves! Because they lack the tube feet that other echinoderms use to crawl about, synaptids must crawl about somewhat like a bristleworm (polychaete), and the anchor ossicles function in much the same way as the bristles of a worm they provide traction against which the animal can push to propel itself. The flabby soft body allows them to crawl about and extend or retract their body with great flexibility. Their "half-filled baggie" look is deceptive, however, because the animals are the fastest and most active of the sea cucumbers, and are capable of rapidly crawling around the aquarium, or quickly withdrawing into a crevice when disturbed. The standard feeding behavior of reef-dwelling synaptids is to anchor about 1/3 of the body into some secure hole, and then extend the anterior 2/3 of the body to feed. If something disturbs the animal, it can rapidly retract itself into that hidey-hole and avoid being eaten. The problem, however, is that those anchor ossicles may be laying across some of your other invertebrates, and when the cucumber retracts, it simply tugs those ossicles free of whatever it was previously anchored upon -- this can dislodge and/or cause damage to the soft tissue of whatever the cuke was lying on at the time...
After all of that, however, I have to admit that I have one of the generalist Caribbean species, Euapta lappa, in my reef tank, and it is doing just fine. If you have a well established deep sandbed and are feeding plankton on a regular basis, there ought to be plenty of organic detritus for the animal to find. My cuke spends most of it's time cleaning the underside of the rocks, and only comes out onto the surface of the sand and the rocks at night. You can supplement it's feeding by adding a couple of sinking shrimp pellets to the area it tends to hang out just before the lights go out. That will give the pellets time to soften and fall apart before the cuke starts to feed, and mine seems to really like the pellet mush (but so do the hermits, brittle stars and worms, and they all learn what the pellets are pretty quickly as well ). If well fed, these animals may even be able to reproduce in the aquarium. Studies on Synaptula hydriformis found that these animals are simultaneous hermaphrodites (contain both fully functional male and female reproductive tracts), which are likely capable of self-fertilization. The fertilized eggs are retained in the body cavity of the adult and develop internally until young are released as fully-functional juveniles at about 8mm long.
All-in-all, however, despite how fascinating these animals are, unless you are confident of the identification of the animal, can provide suitable conditions for the animal to feed, andare willing to live with the potential drawbacks of keeping one of these animals in an aquarium, they are not really recommended for keeping in a reef tank...