I have a reef tank with a 2" sand bed and was thinking about getting a cucumber to add to my tank. I was thinking of getting the kind that stays on the bottom and borrows through the substrate. I know that brightly colored cucumbers like sea apples are poisonous, but I have seen some that come in a variety of dull colors (greyish-black, beige & black and grey) at the local pet shop. I was wondering if these dull-colored cucumbers were poisonous and if they would do the job of stirring my sand for me?
The color of the cucumber doesn’t really tell you much about the animal at all. The real distinction you want to draw is between the suspension-feeding and deposit-feeding sea cucumbers. Suspension feeders are those cukes that stick their feeding tentacles into the water and collect particles out of the current as water flows past them. Deposit feeding cukes are those that slap their tentacles around on the substrate and ingest what sticks to them. For a sand bed, you’ll want one of the deposit feeders, but these can come in a wide variety of colors and some of the better “sand eating” cukes are actually quite colorful (e.g., Holothuria edulis).
As for being poisonous, I would have to say that there is really no such thing as a non-toxic cuke (they all possess some chemical defenses against predators), but the real question is how much of a risk is that animal to the rest of the critters in your tank. The defensive chemistry of sea cucumbers is pretty well known. There are a variety of saponins (soap-like chemicals) in the skin, and some more specific and toxic chemicals that are often specifically associated with the Cuvierian tubules (I’ll come back to explain what these are below). It is the release of these chemicals that is likely to have the disastrous effects that many warn of whenever sea cucumbers are mentioned.
Let's back up a step. When a cucumber is stressed, it can react in a variety of ways. First, like other echinoderms, they have a compound in their skin called catch collagen - this tissue is under neurological control and is capable of changing from a 'liquid' to a 'solid' form very quickly. This is one of the coolest things about echinoderms in general, and is how cukes manage to get themselves into tiny holes in rocks - they “goopify” their bodies (for lack of a better description), literally pour themselves into the hole they have chosen, and then solidify their skin to prevent anything from being able to remove them. The same is true of how urchin move and “lock” their spines, sea stars can exert continuous pressure on a clam to slowly pry it open, and so on... Sorry, I guess I’m getting off track... As I said, the cuke can change its consistency, and many react to stress by either becoming flaccid and goopy, or by ejecting all the water from its system and becoming a small, hard turd-like lump. Either of these responses is typical of animals that have been harmlessly disturbed (e.g., poked with a finger or such) or moved from one tank to another.
A second, and more drastic response is evisceration. In this case, the cuke simply expels a portion of its digestive system (guts) onto the substrate. Evisceration can be induced in a variety of ways (e.g., chemical stress, physical manipulation, crowding, etc.), and this “puking” response usually includes some or all of the digestive system (and in some cases other organs such as the respiratory tree and gonads), but is not necessarily accompanied by chemical discharge, thus even stressed cukes that eject their intestines may not have much of an impact on your aquarium, depending on the situation. Despite the fact that this stress response may not wipe out your tank, it is not trivial to the cucumber however – the cuke loses it's digestive capacity in the process and although it can regenerate the gut, it needs time, and excellent water conditions to do so. I suspect that many cucumbers that slowly shrink and eventually die in the aquarium despite the best efforts of the aquarist have likely been mishandled at some point and have eviscerated; the repeated insults of being collected, shipped and eventually placed into an aquarium probably does not fall under the category of “ideal conditions” for regeneration....
The most likely problem for your tank as a whole, however, is when a stressed cuke expels its Cuvierian tubules. These tubules are a series of long, spaghetti-like tubes which are associated with the hind gut of sea cucumbers, and are thought to be primarily defensive in function. They are located near the anus, and branch off from the base of the respiratory tree (the branched “gills” of a sea cucumber). Now, if you’re reading this carefully, you should wonder why the “gills” of a sea cucumber are near its butt. Well, the answer is simple – sea cucumbers actually breath through their anus! That’s right – many people make the mistake of watching a sea cucumber breathe (it is quite obvious as the anus opens to allow water to flow in and then pinches down as the animal “exhales” the water it just “inhaled”) and thinking that the opening that they are watching is the mouth. In cucumbers however, respiration is done through the anus, and the respiratory tree is associated with the other end of the digestive system. Why does any of this matter? Because when a cucumber is really threatened (it thinks it is about to be eaten), it can respond by inhaling a bunch of water and physically rupturing (literally exploding) the hind gut to expel these tubules and a soup of defensive chemicals that are intended to prevent the predator from ever wanting to mess with a sea cucumber again. These chemicals usually work well to discourage predators, and unfortunately are also likely to seriously impact, and potentially wipe out everything nearby in an enclosed tank (the so-called “cuke-nuke”).
Of course the amount of this chemical soup and the exact identity and toxicitiy of the chemicals a cucumber has vary from species to species, which makes it hard to make reliable generalizations about which cucumbers are safe for a reef tank and which are a potential nightmare. In general, it is probably reasonable to say that "sand-feeding turd cukes are less toxic than the colorful filter feeders,”or more technically, the concentrations and variety of toxic chemicals found in the Aspidochirotiacea – the group that contains the sand-feeding cucumbers – is generally lower than those of the Dendrochirotacea – the group that contains most of the colorful suspension feeding cucumbers. However, there are plenty of exceptions to that generalization (as I mentioned above), and the fact that there is a strong behavioral component makes it impossible to predict the effect of any given cuke in any given tank when something particular happens.
The best generalization I can make is that for most drab deposit-feeding sand cukes the risk of a tank wipe-out is quite low (although certainly not all -- there are some such cucumbers that possess particularly nasty toxins). In fact, even with many of the filter-feeding cukes, the risk is low as long as you take certain precautions to prevent the animal meeting an unpleasant end (as explained in the section above). Protecting powerhead intakes before adding a crawling animal like a cuke seems like a pretty reasonable request for any animal being kept in our tanks, and should not be viewed as a downside to keeping them. The same can be said of anemones, snails and other animals that frequently blunder into the intake of unprotected powerheads – if you plan to keep motile animals in your tank, you should take precautions that they do not blunder into what amounts to a blender for them. The fact that most sea cucumbers possess toxins which could impact an entire tank makes this concern that much more important with these animals, however. A good strainer will prevent most animals from being sucked into a powerhead - the problem is that many strainers are not well designed (they are too small), and others break or fall off too easily. Ideally a strainer should be large relative to the animal you're trying to keep out of it (about equal size or ideally larger), and should be securely attached, so that it cannot be knocked off accidentally by any of the animals in your aquarium (personally, I hot glue my strainers on so that they never come off by accident). The problems generally occur when the baskets are small enough that the animal can cover most of it or flimsy enough that it can be knocked off by a roaming animal. Then a powerhead can do some real damage, and your tank may be at risk from that...
OK, having established how cucumbers can react to stress, and why some reactions can be more serious than others, lets move on to some specifics about the deposit-feeding cucumbers you would like to add. These animals require a lot of organically enriched fine sediments, and without that, they slowly digest themselves as they starve to death in tank. Like most marine invertebrates, these animals are capable of going very long periods without feeding, and it can take several months for the animal to show any signs of starvation. If your gravel is too large (if it can be called gravel rather than sand), or your tank is too clean (new, frequently vacuumed or bare bottom), a cuke is likely to be doomed in your aquarium. I generally suggest not adding one unless you have a well established tank -- lots of detritus -- with sugar-sized or finer sand. I suspect that because cukes don’t move around very much, and are not considered interesting or attractive to most people, they are probably not watched quite as closely as would a fish or a coral in the tank. Because of that, many people may not know the difference when the animal dies – any large animal dying in the aquarium and left to decay will obviously have a dramatic impact on water quality, and I would bet that at least some reports of "cuke nukes" could be simply explained by the animal dying in a tank that did not provide the proper conditions for it, and people not knowing the difference between a live cuke and a dead one until it begins to rot...
Although cukes get a bad reputation because of the reports of "tank nukes," in reality such events are actually pretty rare. They are reported a couple of times a year on various newsgroups, but generally happen when people are away, or they didn't notice the cuke disappearing, or do not really know how a cuke is supposed to look, etc. Most sand-dwelling cukes won't cause a tank wipe out even if they hit a powerhead (which is a pretty easy thing to prevent and really shouldn't be a concern in a well maintained tank), and even in worst-case scenarios with sea apples (among the most toxic of the cukes we see in aquaria), the problems with toxin release are effectively avoided by water changes, heavy skimming and carbon addition if caught immediately. In fact, many soft corals, sponges and tunicates kept in reef aquaria are much more toxic than are these cucumbers, but because these other species are not crawling around and potentially hitting unguarded powerheads, they pose less of a threat than do sea cucumbers.
Personally, I do not think that there is anything wrong with keeping sea cucumbers in a reef tank as long as you take the necessary precautions to protect them from an untimely death -- I keep cukes in all my tanks. Ultimately, though, you are the one who has to decide if you are willing to keep the animals in your tank, whether your tank conditions are right for one, and whether the benefits of keeping one are worth the potential risks...