Aquarium Invertebrates: Sea Cucumbers - Part II
I decided to continue on with the subject of sea cucumbers for this article, because there are many subjects that I mentioned but never really discussed at any length in the last article about medusa worms (Toonen 2002). In fact, I am going to do one more article on sea cucumber after this one, but I’ll focus specifically on the colorful and popular sea apples ( Pseudocholchirus spp.) in that next article.
So, let’s start off by backing up a step from my introduction last time and actually explaining what a sea cucumber is. Well, sea cucumbers are members of one of the most familiar groups of marine invertebrates, the Phylum Echinodermata. I say that echinoderms are one of the most familiar groups of marine invertebrates because some members, in particular the sea stars (Class Asteroidea), have become common symbols of life in the sea throughout popular culture. In contrast to the sea stars, their less attractive and well-known cousins, the sea cucumbers (Class Holothuroidea) are largely unknown in popular culture. There are roughly 6000 known species of Echinoderms alive today, and approximately 900 of those described species are sea cucumbers (Ruppert and Barnes 1994). The Class Holothuroidea is further divided into three main groups, although only the first two are commonly sold in the pet trade: the Dendrochirotacea (or filter-feeding sea cucumbers), the Aspidochirotiacea (or deposit feeding sea cucumbers), and the apodacea (or medusa worms). The medusa worms were the subject of my last column (linked above), and will therefore not be covered again here. Instead I will discuss the other two groups of sea cucumbers that are more common in the trade, but focus primarily on the deposit feeding Aspidochirotiacea in this article.
Sea cucumbers can vary dramatically in size, with some of the smallest animals reaching a maximum size of less than a 2 cm long, and the largest can reach more than a meter (3 ft) in length and be nearly 1ft in diameter (24cm)! A meter long cucumber is a big animal, and obviously anything that reaches more than 3 feet in length is not very practical for a miniature reef aquarium at home. Regardless of whether the animals look it or not, there is a ‘belly’ (ventral side) and a ‘back’ (dorsal side) to a sea cucumber, and the animals will always attach with the ‘belly’ side toward the substrate: if you flip a sea cucumber onto the side opposite that with which it is attached, it will right itself before reattaching. This is easier to see in some species than others, but for the most part sea cucumbers are motile enough that even if they are placed into an aquarium upside down, they will quickly right themselves and find a comfortable location to feed.
A comfortable position in which to feed will vary depending on the species of sea cucumber you are planning to add to your tank. Although there is an enormous amount of variation within the broad categories of deposit-feeding and suspension-feeding sea cucumbers, most of the animals that are commonly imported for sale will generally perform a similar function in the aquarium. So, with the full acknowledgment that there are many exceptions to the rule that are not commonly imported at this time, I will treat each of these groups on the whole in this article and discuss some of the more common species in detail later on. I will discuss some of the specifics of suspension-feeding cucumber husbandry in the next article in which I go into detail about the captive care of sea apples. In this article, I will instead concentrate on some generalities about deposit-feeding sea cucumbers and end with a discussion of what risk a sea cucumber poses to a reef aquarium. One of the most common deposit-feeding sea cucumbers found in the pet shops these days are the ‘tiger-tail cucumbers.’ This is one of the many cases in the pet trade where a well-known common name is applied to a wide variety of species, many of which don’t act or even look similar. In this case, however, most of the cucumbers that I have seen offered for sale under the name ‘tiger-tail’ fall into the same general life history mode and will actually perform a similar function in the aquarium: they are generalist detritivores that mop the sediment surface for fine organic detritus (Erhardt and Moosleitner 1998). The majority of the species of deposit-feeding sea cucumbers that are offered for sale in the petshops are usually members of the genus Holothuria. Perhaps the most commonly offered for sale under the name tiger-tail are H. hilla from the Red Sea and Indo-Pacific, and H. impatiens or H. thomasi from the Caribbean. Other commonly sold species include H. floridana (a mottled and highly variable-colored Caribbean cucumber that is common along the Gulf shores southward along the coast of Central America as far as Colombia), H. atra (a black cucumber from the Read Sea and Indo-Pacific), and H. edulis (the edible sea cucumber which is black along the upper surface, and bright pink along the belly, also from the Red Sea and Indo- Pacific). Each of these species performs a similar function in the miniature reef aquarium: they consume organic detritus and ingest fine-grained sands to digest off the bacteria, microalgae and diatoms that cover the surface of each sand particle. In this way, they help to maintain a cleaner and more attractive aquarium, which makes them a useful addition to your aquarium. Because these animals feed on the same food, however, it is important that they not be overstocked in an aquarium, or they will rapidly consume all the available food and then starve. Despite the fact that they don’t look like much, sea cucumbers are relatively large animals, and they generally require a lot of organically enriched fine detritus to survive. In the absence of sufficient food, they will of course starve to death in an aquarium.
Feeding And Starvation In Sea Cucumbers
Like most marine invertebrates, sea cucumbers are capable of going for long periods of time without feeding before they show any outward signs of a problem. Even when they do show obvious signs of starvation, most people do not know what to look for in these unusual animals, and will likely miss the warning signs. Because sea cucumbers are able to slowly digest themselves to cope with periods of starvation, they are often able to last for many months without any outward signs of starvation. The first time that people notice something is probably wrong with their cucumber is usually when they realize that the cucumber is much smaller than it was when it was first purchased. Slowly shrinking is a sure sign that the animal is not getting enough food, and is bound to waste away in your tank if they do not get more food. Deposit-feeding sea cucumbers can be either selective or not. The non- selective species that make the best detritivores for the aquarium feed primarily by eating sand or mud and digesting away the organic component before pooping out perfectly clean sand castings (often looking almost like a spaghetti noodle) behind them. If you do not see these sort of castings from your sea cucumbers on a regular basis, chances are that they are not getting enough food.
There are three common reasons why a sea cucumber may not get sufficient food in captivity. The first is quite simply overstocking. As I mentioned above, these animals need a lot of organic detritus, and an average reef tank can only support one or two sea cucumbers. Sprung (2001) recommends a rule-of- thumb of three inches of sea cucumber per 20 gallons of reef aquarium to prevent overstocking, which seems like a reasonable level for a well- established and stocked aquarium, but is probably still a bit heavy for a sparse or newly setup reef tank. A second problem comes from placing sea cucumbers into tanks with inappropriately-sized sediments. The deposit- feeding sea cucumbers seen in the hobby typically come from areas of fine sands (such as mangrove swamps or seagrass beds) rather than areas in which gravel predominates (Erhardt and Moosleitner 1998; Hendler et al. 1995). Although the Caribbean tiger-tail cucumber ( H. thomasi ) is capable of ingesting particles as large as 2mm in diameter when fully-grown, they are restricted to much smaller particles when they are small (Erhardt and Moosleitner 1998), and few people have anything close to a fully-grown cucumber in their tanks. Because these animals often feed by simply ingesting everything of the appropriate size and digesting what they can from those particles, placing a small sea cucumber into a tank with coarse gravel (the size of crushed coral rather than sand) often prevents them from feeding appropriately. Thus, 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 sea cucumber is almost certain to be doomed in your aquarium. I generally suggest that people not add a sea cucumber unless you have a well established tank – or at least lots of organic detritus -- with sugar-sized or finer sand. The final reason is one which you as an aquarist have little control of - the animal may not have a gut, and regardless of how well-suited your tank is, a cucumber without a gut will not be able to feed.
Some of you may be asking yourself ‘how can the animal not have a gut!?’ Well, violently expelling part or all of the gut (evisceration) is one of the defensive mechanisms of sea cucumbers (I’ll come back to this in more detail in the section below), and if they are not handled with great care during the collection and shipping process, there is a good chance that the cucumber may have ejected its gut in response to that stress. Evisceration is a common response among the sea cucumbers to severe stress (Ruppert and Barnes 1994), and I suspect that most tropical sea cucumbers are likely to experience some stress in the course of being collected and shipped around the world. For example, during the time that I did research on sea cucumbers ( Parastichopus ) in the Pacific Northwest, I often found that after collecting many individuals on a dive, the entire group would eviscerate when I brought them to shore to be weighed and measured. This was a result of simply picking the animals up, sticking them into a bag underwater, and then bringing them to shore. If the tropical species that are imported for our reef tanks behave anything like these animals, then it would not be at all surprising if many of the sea cucumbers that slowly shrink and eventually die in the aquarium despite the best efforts of the aquarist have been roughly handled at some point and have eviscerated as a result. Given the repeated insults of being collected, shipped and placed into aquarium after aquarium during that time before finally being added to a reef tank thousands of miles from where the animal was collected, it would be surprising to me if the animals did not experience extreme stress during that time...
However, even if the animals have an intact gut, are housed in a tank that is not overstocked, and has great water conditions and sufficiently fine sands that the animal can feed, some of them will still starve. This may be because some species are actually specific feeders, and although they may ingest foods, they may not be able to digest them. For example, the cold-water sea cucumber that I studied, Parastichopus parvimensis, ingests a wide range of organic detritus but it turns out that the plant components of that detritus remain undigested even after passing through the animal’s gut (Ruppert and Barnes 1994). Obviously, if you were to add a species such as this to your aquarium, and the primary source of organic detritus in your tank was from various micro- and macroalgae, then this cucumber would not fare particularly well. Fortunately for us as aquarists, the majority of species that are commonly sold in the trade are relatively non-selective, and prefer to feed on any nutrient-rich particles (Erhardt and Moosleitner 1998; Hendler et al. 1995).
Thus, for the most common species of deposit-feeding cucumbers in the trade, the primary concerns are: 1) avoid overstocking, 2) provide properly-sized sediments, and 3) try to obtain a healthy animal when you purchase one...
Sea Cucumber Defense Responses
I would not be doing anyone a favor if I did not point out that most popular aquarium texts have dire warnings about the dangers of including one of these animals in your reef tank (e.g., Delbeek and Sprung 1994a; Delbeek and Sprung 1994b; Fenner 1998; Paletta 1999; Tullock 1997). So why is it that these animals are so feared by many aquarists? Well, the simple answer is that when they are injured in the aquarium, some of these species are capable of releasing a toxic chemical soup that can poison an aquarium (reviewed by Paul 1992; Pawlik 1993). The possibility of having one of these animals poison an aquarium (commonly called a ‘cuke nuke’ on the internet) is enough for many people to shy away from ever adding one of these animals to their aquarium. However, I think that this fear is somewhat misplaced. I’d like to take a bit of time to explain the chemical defenses of sea cucumbers in some detail and hopefully dispel some of the myths surrounding these animals.
Let’s start at the beginning: when a sea 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 (Brusca and Brusca 1990; Motokawa 1984a; Motokawa 1984b; Ruppert and Barnes 1994). This is one of the coolest things about echinoderms in general, and is one of the reasons that this group has been so successful. The ability of the catch collagen to change from liquid to solid form at will is how sea cucumbers manage to get themselves into such tiny holes in the live rock structure - they are able to ‘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 (Motokawa 1984a; Motokawa 1984b). The same is true of how urchins move and ‘lock’ their spines, or how sea stars can exert continuous pressure on a clam to slowly pry it open without getting tired, and so on. Sorry, I guess I’m getting off track... OK, so as I just said, the cucumber 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 (at least that seems to be the description I hear most often from the spouses of aquarists who own a sea cucumber). Either of these responses is typical of animals that have been moved between tanks or harmlessly disturbed (e.g., poked with a finger, nipped by a fish or such).
A second and more drastic response is evisceration - the violent expulsion of the gut I mentioned above. In this case, the cuke basically expels a portion of its digestive system (guts) onto the substrate. It may be the fore or hind gut depending on the species in question, and may or may not be a serious trauma to the cucumber (Ruppert and Barnes 1994). Evisceration can be induced in a variety of ways (including such factors as chemical stress, physical manipulation, crowding, etc.), and in some species, every individual in the population appears to be at the same stage of regeneration, suggesting that evisceration may be a normal seasonal phenomenon in some species (Ruppert and Barnes 1994). It is unclear why individuals go through the evisceration, but possible explanations include periods of inactivity when food is naturally rare, or the elimination of toxic wastes that have accumulated in the internal tissues (Ruppert and Barnes 1994). Whatever the cause, this ‘puking your guts out’ 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 (Brusca and Brusca 1990; Ruppert and Barnes 1994). Because there is not necessarily any chemical discharge that accompanies evisceration, even a highly stressed sea cucumber that ejects its 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 certainly not trivial to the sea cucumber – the cuke loses its digestive capacity in the process and although it can regenerate the gut, it needs time, rest, and excellent water conditions to do so. If the cucumber was stressed enough to eviscerate in your aquarium in the first place, chances are slim that conditions are ideal for them to regenerate their gut, either.
The final and most drastic response possible 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 certain sea cucumbers, and are thought to be primarily defensive in function. Not all species possess these defensive structures, and even those that do, generally do not eject them without dire provocation. The Cuvierian tubules 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 be wondering why the ‘gills’ of a sea cucumber are so near to its butt. Well, the answer is simple, although unusual – sea cucumbers actually breathe 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. The amount of defensive chemistry and the specific combination of chemicals that are produced varies by species from just under two to nearly three and one-half percent of the dry weight of the cucumber (Bryan et al. 1997). In general, the chemicals which appear to protect the sea cucumbers from being eaten are saponins (soap-like compounds) which are derived from triterpenoids (Ponomarenko et al. 2001; Stonik and Elyakov 1988). These chemicals usually work well to discourage many generalist predators, and unfortunately are also likely to seriously impact, and potentially even wipe out all their tankmates in a reef aquarium (the so-called ‘cuke-nuke’).
Of course the amount of this chemical soup and the exact identity and toxicity 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. As for being poisonous, I would have to say that there is really no such thing as a completely non-toxic sea cucumber, because nearly every species that has been tested to date appears to possess chemical defenses against at least some species of predators (e.g., Bryan et al. 1997, reviewed by Paul 1992; Pawlik 1993). However, having said that, the defensive chemicals not only differ among species of sea cucumbers, but the same set of defensive chemicals also tends to have very different effects on different species. For example, Bryan et al. (1997) found that incorporating the defensive chemicals from the body wall of three different sea cucumbers ( Holothuria lentigenosa, H. thomasi & Isostichopus badionotous ) into food pellets led to all of them being spit out by both pinfish ( Lagodon rhomboides ) and killifish ( Cyprinodon variegatus ). However, the exact same suite of chemicals incorporated into food pellets offered to the arrow crab ( Stenorhyncus seticornis ) had no detectable effect on the crabs appetite. Thus, there is variability both among species of sea cucumbers in the exact chemicals that they use to defend themselves, and the effect of each of those chemicals on potential tankmates (e.g., Paul 1992; Pawlik 1993).
So Which Cucumbers Are Safe For A Reef Tank?
Well, to recap what we’ve learned so far, different species tend to have different specific sets of chemical defenses, and those chemicals tend to have different effects on different potential predators. So, obviously it is virtually impossible to make any real generalizations about the specific toxicity of a given species of sea cucumber or how their chemical defense is likely to impact your reef tank. In general, it is probably reasonable to say that sand-feeding turd cukes are less toxic than the colorful filter feeders, or more specifically, 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 (Paul 1992; Pawlik 1993). However, there are plenty of exceptions to that generalization, and the fact that there is a strong behavioral component makes it impossible to predict the effect of any given cucumber in any given tank when something particular happens. Ultimately, the real question that most likely matters to you is not which sea cucumbers are more or less toxic, but rather how much of a risk is some particular sea cucumber to the rest of the critters in your tank.
The Dreaded ‘Cuke Nuke’
Well, as I’ve said now several times, there is simply too much variability for me to make any realistic generalizations about the risk of any specific sea cucumber. The best generalization that I can make is that for most drab deposit-feeding sand cucumbers the risk of a tank wipe-out is quite low (although there are certainly some drab cucumbers, such as H. atra, that possess particularly nasty toxins). In fact, even with the most toxic species of sea cucumbers, the risk of having a serious episode in your reef aquarium is relatively low as long as you take certain precautions to prevent the animal meeting an unpleasant end. Why is that? Well, as I explained above, the sea cucumbers do not release their defensive chemicals on a whim - they must be pretty threatened or stressed to react in that manner. If you take responsibility for the safety of your animals, and properly cucumber-proof your tank, then there is no reason that any cucumber should ever be in a sufficiently stressful situation that it would release those defensive chemicals. Furthermore, cucumber-proofing your tank is relatively simply, and we have no excuse for not doing so. For example, properly protecting pump intakes and overflows before adding any crawling animal (such as a sea cucumber) seems like a pretty reasonable request for our tanks, and in my opinion this should never be viewed as a downside to keeping such animals. I would argue that it is simply our responsibility to provide an adequate home for any animal that we purchase. The same precautions ought to be taken prior to adding an anemone, snails or any other animals that crawl actively around the aquarium and could blunder into the intake of an unprotected pump. If you plan to keep motile animals in your tank, it is your responsibility to take adequate precautions that they cannot wander into what amounts to a blender for them.
While I stick by the assertion that equivalent precautions ought to be taken with any motile animal, the fact that most sea cucumbers possess toxins which could impact an entire tank makes this concern that much more important with these animals. A good strainer will prevent most animals from being sucked into the intake of a pump, powerhead or overflow. What many aquarists find is that the problem with many strainers is that they are either poorly designed (i.e., they are too small, or they break), or they are difficult to keep where we want them (i.e., they fall off or are dislodged by the activity of the animals themselves). Ideally a strainer should be large enough to diffuse the flow sufficiently that you cannot feel any suction at all, and it should also have holes small enough to protect the animal that you're trying to keep out of it, but at the same time, the holes must be large enough that they do not become plugged. The strainer must also be able to be securely attached, so that it cannot be knocked off accidentally by you or by any of the animals in your aquarium. Personally, I hot glue my strainers on so that they never come off by accident. Problems with strainers generally occur when either the baskets are small enough that your animal can cover most of it or flimsy enough that it can be knocked off by a roaming animal. The intake of a pump, a powerhead or an overflow can do some real damage if they are unprotected and the animal gets into it, and that is what leads to an animal being stressed enough that they will release their defensive chemicals.
Although sea cucumbers get a bad reputation because of the reports of ‘cuke nukes’ that are common on the boards, in reality such events are actually pretty rare. They are reported a couple of times a year on various newsgroups, but generally seem to happen when people are away, or they didn't notice if or when the cucumber disappeared, or the cucumber looked fine before the problem but died along with the rest of the tank, and so on... The fact of the matter is that many times it is impossible to determine the exact cause of the problems in the tank, but because so many people have heard the warnings about sea cucumbers, they are often an easy target for unexplained problems. In fact, I suspect that because sea cucumber don’t generally move around very much, and are not considered the most interesting or attractive pets by most people, they are probably not watched quite as closely as they should be. The simple fact is that people probably don’t check on their sea cucumber quite as often as they would a fish or a coral in the tank. Because of that, many people may not necessarily know the difference whether or not their sea cucumber is actually alive... Obviously, any large animal dying in the aquarium and left to decay will have a dramatic impact on water quality, and I would be willing to bet that at least some reports of ‘cuke nukes’ could be simply explained by the animal dying in a tank. Perhaps the tank did not provide the proper conditions for it, or perhaps the animal was not healthy to start with, but whatever the reason, because many people may not know what to look for, they may not know the difference between a live sea cucumber and a dead one until it begins to rot...
The bottom line is that most sea cucumbers won't cause a tank wipe out even if they hit a powerhead (which as I said, is a pretty easy thing to prevent and really shouldn't be a concern in a well maintained tank). The usual exception to the bad experience with a sea cucumber being the obvious cause of a tank wipe out is when something happens to a sea apples ( Pseudocholchirus spp.), because they are among the most potentially hazardous of the sea cucumbers that we see in the aquarium hobby. Even if a cucumber ends up being damaged badly enough to release their toxins in the aquarium, there is not necessarily any serious repercussions if the aquarist is attentive. The primary danger from a cucumber releasing its toxins in the tank comes from the fact that the defensive chemicals continue to circulate, and remain undiluted in the aquarium. The die-offs that result from this situation can be effectively avoided by water changes, heavy skimming and carbon addition if the problem is caught immediately. In fact, many soft corals, sponges and tunicates kept in reef aquaria are actually much more toxic than are these sea 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. That is why I usually tell people that the biggest danger of keeping sea cucumbers in our tanks comes from our own negligence rather than from the sea cucumber itself...
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 at least one sea cucumber in every one of my own reef 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...
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