Sea cucumbers are all members of the Class Holothuroidea, but members of this Class can be surprisingly different from one another.  Within the Class Holothuroidea, there are three sub-classes: the Dendrochirotacea (largely suspension-feeding sea cucumbers such as the popular sea apple, Pseudocolochirus spp.), the Aspidochirotacea (largely deposit-feeding sea cucumbers such as the popular tiger-tail cucumber, Holthuria spp.), and the Apodacea (the unusual group of legless cucumbers to which the medusa worms belong).  Unlike the other Classes of echinoderms mentioned above, in which the mouth is on the "bottom side" of the animal, the sea cucumbers all lie on their side, and the mouth is located at the “font end” of the animal.  Although both ends generally look pretty similar to the casual observer, the sure way to identify the head is to see the oral tentacles with which the animal feeds.  Depending on the species of sea cucumber and what they eat, these oral tentacles can be either stuck out into the water column to suspension feed, or applied directly to the substrate to deposit feed.  The legless synaptid cucumbers fall into this latter group, and are the most active deposit feeders of the Holothuroids.  That tends to make the head end of these animals quite obvious, because they almost always have their feeding tentacles extended as they move about in search of particulate detritus to eat.  Now although I mentioned above that tube feet are one of the unifying characteristics of the echinoderms, these Apodid sea cucumbers are called “legless” because they lack obvious tube feet.  In this case, it is because the ancestor of these cucumbers actually had tube feet, but they have been lost through time in this group.  That is not to say that they are always absent: some Apodid cukes really do lack any detectable vestiges of tube feet, but in many species the tube feet are simply reduced to the point that they are not really visible.

It is also important to note that aside from their lack of tube feet, most apodid cucumbers lack the respiratory tree and most of the associated structures characteristic of the other sub-classes of sea cucumbers.  Instead, these animals tend to gain oxygen and expel carbon dioxide primarily across the body surfaces.  Because these animals lack a respiratory tree and the associated tubules of Cuvier (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 (Pseudocolochirus spp.) and spotted sea cucumbers (in particular, Bohadaschia argus and Actinopyga agassizii).  The Cuvierian tubules commonly found in species of Holothuria, Bohadaschia, Stichopus and Actinopyga liberate a highly toxic saponin compound known as holothurin.  This compound acts to quickly stun and even kill potential predators, which are effectively asphyxiated (suffocated) by the toxin.  The toxins of some of these species are so effective that South Pacific island cultures use the macerated bodies of these sea cucumbers to stun and capture fishes, crabs and lobsters as a method of traditional fishing (Frey 1951; Ruppert and Barnes 1994)!  Despite the fact that the medusa worms lack these structures and are considered ‘relatively non-toxic’ by comparison to many of these species, this does not mean that they are by any means non-toxic.  For example, the beautiful sea apples (e.g., Pseudocolochirus violaceus and P. tricolor) also lack these Cuverian tubules, but along with Bohadachia argus and Actinopyga agassizii, are considered among the most toxic sea cucumbers in the world.  In fact, according to Wilkens (1998), it takes only about 1g of tissue from any of these particularly toxic species to poison the fish in a 25g tank.  These toxins also affect humans, and many people suffer moderate to severe skin and eye irritation if they come into contact with these toxins (Cunningham and Goetz 1996).  Some cases in which toxins came into direct contact with the eyes have resulted in blindness, and deaths have even been reported in cases where people have eaten these animals without the proper preparation.

Despite the similarity in appearance, these animals have a wide range of feeding habits.  For example, the Caribbean Synaptula hydriformis appears to be primarily a generalist herbivore, feeding primarily on diatoms, with the occasional piece of red or green algal detritus ingested when they can find it (Martínez 1989), and these animals are common in a variety of habitats including coral reefs, sea grass beds, mangroves, and even inland salt-water lakes (Hendler et al. 1995; Pawson 1986)!  Other species, such as Synaptula lamperti, are highly specialized feeders which are only associated with living individuals of the sponge Ianthella basta (which I have never seen offered for sale in any pet shop).  These cucumbers live by ingesting tiny organic particles and exudates from the surface of the sponge, and appear to require these specific sponge metabolites to defend and nourish it (Hammond and Wilkinson 1989).  These two species of Synaptula appear fairly similar, but obviously it will be much easier to accommodate the first animal in an aquarium than the second!  Other species, such as the Caribbean Euapta lappa, are generalist detritivores that cruise the sea floor at night collecting any tiny particles of organic detritus within a given size range from the reef rubble and base rock structure.  Some species appear to graze from a wide range of sediment types and particle sizes, while others are highly specific (Hammond 1982a).  For example, Leptosynapta multigranula, graze organic detritus from only carbonate sediments of varying sizes, while other species, such as Leptosynapta tenuis or L.crassipatina appear to feed effectively only while burrowing through very fine sands and muddy bottoms (Hendler et al. 1995).  In some areas of the South Eastern US, Leptosynapta tenuis are the major consumers of detritus, and in suitable habitats, Ruppert and Fox (1988) estimated that these cucumbers can process as much as 25 metric tons of sediment per hectare per year!  In contrast, Chiridota rotifera is found actively grazing detritus from surfaces of tide pools, sea grass beds, calcareous algae, coral rubble fields, coral heads, and sandy mudflats – not exactly what anyone would consider a habitat specialist.

The primary problem with getting any of these animals, however, is that you cannot identify them by yourself, and will therefore have no idea of which species you are buying.  That leaves you with no option but to trust your supplier about its care and requirements.  That is easier for some people than others, and depending on how reliable and knowledgeable your supplier is you may be able to obtain a generalist detritivore that will thrive in a well-established reef aquarium.  If, however, you simply purchase an animal at random, there are more specific feeders than generalists in this group.  Therefore, you have a fair to excellent 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 regardless of whether or not they are active, these animals appear to feed only at night (Hammond 1982b), and food processing is very rapid – particles ingested by the cucumber are voided from the gut within only about 1 hour!  Even when animals are seen apparently feeding during the day, when collected the researchers found that these animals inevitably had digestive systems that were completely empty (Hammond 1982b).  This is where I come back to the earlier statement that although these animals are normally nocturnal, they often lose their avoidance of the light and are sometimes seen cruising around the tank in full daylight.  Although similar studies have not been done on any animals kept in a reef aquarium for any length of time, studies on natural reefs indicate that regardless of what looks like normal feeding behavior during the day, they were not actually ingesting any particles, and were therefore not feeding,.  We do not know why these animals lose their strict nocturnal schedule in captivity, but there are a couple of likely possibilities.  First, the animals may be removed from the predation pressure that they face on the coral reef, and begin to become bolder over time in captivity. This is possible, because I have also observed that the same tends to occur in shallow lagoons and other protected habitats where, in the absence of predators, these cucumbers are found in high density during daylight hours.  The other possibility is that the animals are starving to the point that they abandon any semblance of their natural behavior and start to feed during the daylight.  If we assume that it is the former explanation, then (like in natural populations) the animals may appear to be active during daylight hours, but are probably not actually feeding.  If we assume that it is the latter explanation, then the animals are doomed to slowly waste away in the absence of sufficient or appropriate food in the aquarium.