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By Bob Goemans
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Q&A - Sea Apples

Authored by: Rob Toonen

Rob,

I recently purchased a sea apple, and wanted to find some more information about how best to care for it. A friend of mine has had his sea apple for about 4 months, and it seems to be doing well. He target feeds it by using a turkey baster to squirt enriched baby brine shrimp (soaked in Selcon for an hour or so) a couple of times a week, and aside from that he says it catches some of the bits of adult brine that he feeds to his fish and larger corals. I know that there is a risk of them poisoning the tank if they die, but I really like the thing and really want to have one in my tank. Is this crazy, or is it really as easy to keep as my friend says? Thanks,

Michelle

Michelle,

Well, I hate to say it, but I think your friend is fooling themselves into thinking their cucumber is doing well, because your description of what they do is wrong on a variety of levels. It’s going to take me two parts to complete my answer to this question, because it touches on several topics and common myths that I would like to comment on. First, let me back up a step and explain what a sea apple is for anyone who doesn’t really know. Sea cucumbers are members of the Class Holothuria, and along with the asteroids (sea stars), ophiuroids (brittle stars), echinoids (sea urchins and sand dollars), and crinoids (feather stars and sea lilies) make up the Phylum Echinodermata. There are about 900 described species of sea cucumbers (Holothuroidea) which into three main groups, although only the first two are commonly sold in the pet trade: the Dendrochirotacea (or filter-feeding cucumbers), the Aspidochirotiacea (or deposit feeding cucumbers), and the apodacea (or medusa worms). Sea cucumbers can vary dramatically in size, with 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 about 1ft in diameter (24cm)! Regardless of whether the animals look it or not, there is a “belly” (ventral side) and a “back” (dorsal side) to the animals, and the animals will always attach with the “belly” side toward the substrate: if you flip a sea apple onto the side opposite that with which it is attached, it will right itself before reattaching.

Sea Apples are commonly imported suspension feeding sea cucumbers belonging to the genus Pseudocholchirus. The most common species found in pet shops is the Indonesian Sea Apple, P. axiologus although the more colorful Australian Sea Apple (P. violaceus) has become a much more common import recently as well. Both species are obligate filter-feeders and have obvious rows of tube feet which it uses to attach itself to some substrate in an area of high flow in order to expose it’s feeding tentacles to the greatest amount of passing water. These animals are capable f greatly inflating depending on the species and their mood, and a full grown animal would probably average about 10cm (4") in length. When disturbed or unhappy, however, they can inflate themselves to more than double their normal size (some can reach the size of a volleyball) and either ride the current, or “gallop” around the aquarium (at least insofar as a sea cucumber can gallop).

Ok, having said that, you are correct in that sea cucumbers have a potent chemical defense that can be quite toxic in an enclosed aquarium. However, as I explained in a previous article, sea cucumbers have a variety of defenses that are generally tried before they resort to using their toxins. When a sea cucumber is stressed, it can react in a variety of ways. The cuke can react to stress by 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. Another common response of an unhappy sea apple is for the animal to take on great deal of water and swell to an enormous size. This swelling allows them to ride currents and move into a different area (which will hopefully suit them better) much more quickly than if they were to crawl there themselves. The most drastic response a cucumber can make is to expels its Cuvierian tubules (a series of long, spaghetti-like tubes leading off the hindgut -- or colon equivalent -- at the base of the respiratory tree -- or the equivalent of gills) by rupturing the hindgut, because this is almost always accompanied with the release of a soup of defensive chemicals that are likely to seriously impact, and potentially wipe out an enclosed tank. I have discussed cucumber responses to stress and the likely causes and effects of toxin release in the past, so I won’t go into it in detail again here. However, in my experience, reports of “cuke nukes” often occur when the tank is small, runs a small (or no) skimmer, does not use or regularly replace carbon, or when the tank is not checked on a daily basis (such as when someone goes away for vacation and has a friend look after their tank). I have had some sea cucumbers (including sea apples) meet an unpleasant end in a powerhead in my aquarium over the years, and have never suffered a wipe out from these unfortunate events. I attribute this to catching it early, doing extensive water changes and increasing skimming and carbon use to remove the toxins as quickly as possible.

Now, if you were reading carefully there, you should have caught the fact that I mentioned the cuvierian tubules are "leading off the hindgut at the base of the respiratory tree" -- but that must mean that the breathing structures (respiratory tree) is attached to the hindgut (basically their butts), and how can that be? Well, one of the most interesting features of sea cucumbers for most people is that fact that they do, indeed, breathe through their butts! Often when people first see a sea apple, the animal is closed up and simply hanging out waiting to detect phytoplankton so it can begin feeding (I’ll discuss this in detail next time). If you watch the animal closely for a while, you’ll see a small hole opening and closing as the animal first “inhales” water into the respiratory tree and then “exhales” it again. This behavior (breathing, or more correctly, respiration) is very familiar to us, and people typically assume that the end which is doing that breathing is the head, but in this case, the respiratory tree is attached to the cloaca (a common opening for waste from both the urinary tract and the gut found in most animals other than mammals, which have a separate opening for each excretory function) and it is the anus which is opening and closing as the animal inhales and exhales. The rate of water exchange is dependent on temperature, but is generally quite slow in these animals. On average it takes 5-10 “breaths” (a cycle of dilation and contraction of the cloaca) to completely fill the respiratory tree, and depending on the temperature and the size of the animal in question, each “breath” could take as much as one minute. Once the tree is completely filled, the entire volume of water is typically expelled in a single large contraction. It is during such contractions that wastes (including both urine and feces) are generally expelled in a jet away from the animal. Observing an animal defecate is a very good sign, because if you never see your sea apple poop, there is a good chance that it is not getting enough food.

While on the subject of anal respiration, there are a variety of fishes that are specialized to take advantage of this constant flow of water and protection by living in the respiratory tree of sea cucumbers, and I have occasionally responded to questions from reef keepers who suddenly discover a thin silvery fish, closely resembling a small freshwater knife-fish (e.g., Notopterus or Xenomystus spp.) that only appears at night in the aquarium. This is most likely a slender pearlfish (e.g., Carapus bermudensis) which make their daytime home in the trunk of one of the branches of respiratory trees of many sea cucumbers. At night the fish emerges from the anus of the sea cucumber and searches for food (such as mysid shrimp and amphipods); after feeding, the fish return to the protective shelter of the sea cucumber respiratory tree for the daylight hours.

The last thing I’ll discuss in this portion of the reply is that your friend has kept their cucumber for four months, and you consider that a success. I must say that I do not. Sea cucumbers, like most marine invertebrates, require relatively little energy to sustain themselves, and are quite tolerant of starvation by our standards; a healthy, well-fed sea cucumber imported into an aquarium will likely take something on the order of 6-18 months to starve to death (depending on its size and reserves, and what, if anything it can pick up in the aquarium). There is a good possibility from what you describe that your friends cucumber is starving but just hasn’t shown any obvious effects of starvation yet. This could be because the cuke was well provisioned when your friend bought it, or because the rate of starvation is likely slowed by the cuke ingesting some of the Selcon in which the brine shrimp are being soaked prior to feeding (because your friend is not enriching their brine shrimp properly, either – I’ll come back to this in the next column). Many marine invertebrates show very few obvious signs of starvation, and many deal with periods of low or no food by digesting internal organs while waiting for food to appear again. You would not be able to tell that the animal is starving until you notice that it seems to have shrunken a little since you first bought it. It is only once the animal shrinks significantly that most people decide that something is wrong with the way the animal is being kept, and more often than not, it is really too late to save the animal by that time. As I mentioned above, a good rule of thumb is that if you don’t see an animal poop, it’s most likely not eating enough!

Next time, I’ll continue on with the remainder of the issues you raise in this question and try to give you an actual answer to what conditions you require to have a reasonable chance of keeping a sea cucumber healthy in your aquarium....

Rob

Michelle,

I’m sorry I couldn’t complete the answer to your question in a single column, but there were simply to many issues I wanted to deal with in your question. Last month, I explained a little about sea cucumber defensive chemistry and under what conditions the cucumber toxins were likely to pose a threat to your tank. I also explained a little about starvation in marine invertebrates and tried to convince you that four months was still not long enough to declare success with keeping an animal like this in an aquarium, because even if it had never eaten since your friend brought it home, it should easily survive four months in the aquarium. Lastly I mentioned that your friend was not enriching their brine shrimp properly, and promised to come back to that subject this month.

Well, I’ll start with enrichment of brine shrimp before moving on to actually answer your question about the conditions necessary to keep a sea apple healthy in an aquarium. When Artemia nauplii first hatch (Instar I), they do not have a complete gut, and it is not until they develop to Instar II that the baby brine start to feed. The time of development to Instar II depends on a variety of factors (most importantly temperature, but also salinity), but occurs within 6 to 30 hours after hatching. At 28ΕC (roughly 82ΕF) it takes about 8 hours for the newly hatched nauplii to begin feeding, and as the temperature drops, that time becomes much longer. As soon as the nauplii hatch, they begin to digest their yolk and become less nutritious with time after hatching, however, once they reach the second instar, they can begin to feed, and you can enhance their nutritional value by feeding them a highly unsaturated fatty acid (HUFA) supplement that provides high concentrations of essential fatty acids for the health of most marine animals. Your friend has the right idea, but you’re not trying to soak the nauplii in the stuff, you’re trying to get them to eat it, so that they are basically swimming bags of nutrients when you feed them to your reef tank. The problem is that baby brine shrimp are very inefficient feeders, and it takes a long time for them to ingest enough of the HUFA supplement that they become “enriched.” For newly hatched brine at the second instar, it takes a minimum of 12 hours to get the best results – in fact, ideally it’s better to enrich Artemia nauplii for two 12 hour periods with a cleaning in between to limit the growth of bacteria. To clean the nauplii, simply pour the culture through a “brine shrimp net” (one of the fine white mesh nets available at any pet shop) and rinse them a couple of times before setting them up in clean water and adding new HUFA. After the two feedings of the HUFA supplement, the nutritional value of the nauplii is much greater than when they are newly hatched. The same is true of adult brine, and enrichment with Selcon greatly enhances the nutritional value of the live adults as well. Larger Artemia can be enriched in shorter times due to the increased efficiency of filter-feeding as they grow. In fact, adult Artemia can be significantly enriched in as little as about one or two hours.

Ok, now that we’ve covered those issues, let me move on to care requirements for your sea apple, should you decide to get one. Despite the fact that they are quite hardy animals, and until the recent popularity of Tiger Tail cucumbers (Holothuria hilla) were almost certainly the most common sea cucumbers sold in the hobby, their survival record is quite poor, and a recent poll of aquarists on the internet suggested that very few people had kept one of these animals alive for 3 years or more. In large part, I suspect this is due to the passage of misinformation such as that your friend has provided you – sea apples are suspension feeding cucumbers that specialize on phytoplankton (microscopic algae better known as “greenwater”), and even foods that appears small to us, such as newly hatched brine shrimp, are simply too large for the animals to capture and ingest. Without accurate information about the needs of the animals in your care, it is not surprising that most people ultimately fail with these animals. I’m sure that your friend told you about the care of sea apples with the best of intentions, but they have gotten poor information from someone else, and passed along that misinformation along to you. Sea apples have a remarkably poor survival record in aquaria, and together with the potential toxicity of the animal if it meets an unpleasant end or your fish eat the eggs of a spawning animal (as I discussed last time), makes sea apples a relatively poor choice for inclusion in most aquaria.

Despite that, I must admit that I have a couple in my reef tanks that I really like, and more often than not, it is one of the first things people notice when looking into the aquarium. They are magnificent animals and are a beautiful addition to an aquarium if they can be properly cared for and maintained. Most often when I see an animal in the local pet shop, they are inflated and move frequently around the aquarium, rarely if ever stopping to extend their tentacles to feed. This activity is taken as a good sign of health by most shoppers, but in this case it means that the animal is unhappy and searching for a better location. It is perfectly normal for the cuke to climb around to a position of reasonably high flow, and then sit and wait for some food to be blown by before opening to start feeding. In experiments with suspension feeding cucumbers kept in large flow-through aquaria fed by unfiltered natural seawater, the animals were observed to feed on a regular daily schedule that coincided with the maximum amount of phytoplankton in the water. When researchers began to filter the seawater, the feeding pattern broke down. The researchers subsequently found that animals in flowing seawater could be induced to feed at any time by simply adding cultured algae to the tank water. In my tank, the sea apples will close for days if no algae is added to the tank, but will start to feed actively within literally seconds of adding algae cultures to aquarium. I have actually done the same thing with a sea apple from one of the local petshops as a demonstration, and despite the insult of being pulled from the tank and stuck into a bag, by simply adding a squirt of phytoplankton to the bag, I got the animal to open up and immediately start feeding.

Until recently the ability to regularly feed phytoplankton to a reef tank was quite a feat, and involved complicated and time-consuming culture techniques to raise greenwater at home. Recently several suppliers have started to market phytoplankton for home aquaria and you can now buy live, frozen or spray-dried phytoplankton cultures to feed your reef in much the same way as you add other prepared foods. Although I have written several articles in various places on how to culture phytoplankton at home, it’s time consuming and such a hassle that with the introduction of these products, I have stopped culturing algae myself and now buy phytoplankton to feed to my reef tanks rather than growing my own. If you’re a real do-it-yourselfer, then it is certainly possible and cheap to culture algae at home, but if you’re like me, and would like to spend your time enjoying your tank rather than maintaining green-water cultures to feed it, I’d suggest purchasing your phytoplankton and let someone else do the dirty work of culturing the stuff for you.

In any case, you should also tell your friend that unless they begin to add phytoplankton to their tank on a regular basis, there is a good chance that their sea apple will slowly starve to death over the period of a year or so (as I explained last time). As for the amount and frequency of feeding, it is harder to say. The specifics depend on your aquarium, the number and type of suspension feeders and such, but following the instructions on the phytoplankton products ought to suffice. A good rule of thumb is that if you cannot see the animal poop, it is not getting enough food. The obvious gauge is that if the animal grows it is being fed enough -- if it shrinks, it is starving, and you need to increase the feeding frequency and/or amount.

One more detail that I think is worth explaining is that like most suspension feeders, these animals use mucus to trap particulates from the water column. However, like other sea cucumbers they lack mucus secreting cells on the feeding tentacles themselves, and instead have a small sac-like structure inside the mouth where the tentacles are covered in mucus before being stuck forth into the water flow to trap more particles. Once a tentacle becomes covered with phytoplankton, that tentacle is popped into the mouth and sucked clean (just like us using our fingers to eat from a honey jar) before being covered in mucus within that sac and deployed once again. This is what leads to the stereotypic feeding behavior of all holothurians in which a single tentacle is withdrawn and sucked clean while the rest are left extended.

Rob

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