Ok, coming back to the issue of feeding, after water quality, one of the most important determinants of how well an animal is likely to do in a given aquarium is its nutritional condition.  Not surprisingly, if an animal is not getting enough food, chances are slim that it will survive for long regardless of how good the water quality is kept in the tank.  Whether or not a sea apple is getting enough food in the aquarium is typically a function of three primary factors: 1) is the correct food available and in sufficient quantity for them; 2) flow rates – like most suspension feeding sea cucumbers, these animals are able to feed most efficiently under a relatively narrow range of specific water flow speeds; and 3) the stocking density of the animals in the aquarium – is it sufficiently low that all the animals are able to get sufficient food?  I will discuss each of these in further detail below, but first I want to emphasize that 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, energy reserves, and whether or not it can occasionally pick up a little food in the aquarium).  I often see people on various bulletin boards exclaiming something to the effect of: “sea apples are easy to keep, I’ve had mine for 6 months and it’s doing great.”  Unfortunately, many marine invertebrates show few or no obvious signs of starvation, and they deal with periods of low food by digesting their internal organs while waiting for food to become abundant again.  The first sign of difficulty for many sea apples is that people notice that it seems to be a little smaller than when they first bought it, but some don’t even notice this.  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 pooping, it’s most likely not eating enough!  Although I am focussing primarily on sea apples in this column, the recommendations regarding their care and feeding in the aquarium could be equally well applied to most of the suspension feeding sea cucumbers in the hobby (such as Colochirus or Pentacta species).          

So, what exactly is the correct food for these animals?  Well, if you search around the internet you’ll find a variety of recommendations for what to feed a sea apple.  Some of the most common are commercially prepared invertebrate foods and newly hatched baby brine shrimp. Sadly this information is utterly incorrect, and I suspect that the failure of most hobbyists to successfully maintain these animals in captivity is due in large part to the continual offering of this sort of misinformation.  The simple fact is that sea apples are suspension feeding cucumbers that specialize on phytoplankton, and even foods that appears small to us (like the baby brine shrimp that are so commonly recommended), are much (I emphasize MUCH) too large for these animals to capture and ingest.  Studies in which researchers sifted the gut contents of a variety of suspension-feeding sea cucumbers found that all but one species ate only particles of less than 53 μm in diameter (for comparison, newly hatched brine shrimp are approximately 400 µm, and my survey of commercially prepared invertebrate foods found such products contained a mean particle size of 365 ± 696 µm, see Toonen et al. 2002).  Researchers have found that the majority of the diet of suspension feeding sea cucumbers such as sea apples consist of phytoplankton cells (primarily larger species of phytoplankton such as Coscinodiscus, Chaetoceros, Skeletonema, and Thalassiosira), with occasional ingestion of tiny invertebrate eggs and larvae (Hamel & Mercier 1998).  In fact, even full-grown rotifers (which average between 75 and 300 μm, depending on the strain) are too large for most suspension feeding cucumbers to eat, and regardless of the advice offered on the web and by many petshops about what to feed your sea apple, if you are not supplementing your tank with phytoplankton on a daily basis, your sea apple is almost sure to slowly starve to death.  In fact, it would be better if you were feeding your tank with phytoplankton at least a couple of times a day to keep these animals healthy.  Researchers found that sea cucumbers fill and empty their guts 2-3 times per day in the wild (e.g., Hammond 1982, Sewell & Bergquist 1990), and in order to meet their metabolic demands, these animals need to be fed daily at the very least in the aquarium.  Because suspension feeders are generally adapted to continuous feeding, most will generally be healthier if fed small amounts more often than high doses of planktonic food less often.  This is a very good reason to dose your tank with half the recommended amount of phytoplankton twice per day rather than adding a heavy dose every few days as many aquarists tend to do…

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.  There are a number of good articles available online for how to culture phytoplankton at home (e.g., Marini 2002, Toonen 1998a), but it is now so simple to buy phytoplankton to feed to a reef tank rather than growing your own that unless you’ve got a very large tank and are using a lot of phytoplankton, most people opt to simply buy it.  If you’re a real do-it-yourselfer, then it is certainly possible and cheap to culture algae at home, but you have to do it correctly or the phytoplankton can not only be non-nutritious, under some improper culture conditions, it can be downright toxic (see, Toonen 1998a).  If you want to culture phytoplankton, then take the time to read some of the articles above and do it properly.  Otherwise, I’d suggest purchasing your phytoplankton and let someone else do the work of culturing the stuff for you. Sea apples, like most suspension feeding sea cucumbers tend not to waste energy trying to feed if there is nothing around for them to eat.  The animals are very sensitive to both flow conditions and the presence of phytoplankton in the water.  In my tank, the sea apple will close for days if no phytoplankton is added to the tank, but will start to feed actively within literally seconds of adding phytoplankton to the aquarium.  I have actually done this demonstration with a sea apple from one of the local petshops which had not opened since it arrived, and despite the stress 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 in the bag.  In nature, suspension feeding sea cucumbers such as sea apples tend to feed on a regular daily basis.  In experiments using large flow-through aquaria fed by unfiltered natural seawater, the regular daily feeding schedule was shown to coincide with the maximum amount of phytoplankton coming through the system each day (Rabindra et al. 1998).  However, when researchers began to filter the seawater, the regular timing of feeding in these cucumbers quickly broke down.  The researchers subsequently found that hungry animals in flowing seawater could be induced to feed at any time by simply adding cultured phytoplankton to the tank water (Rabindra et al. 1998).

This sea apple (Pseudocholochirus) has extended its tree-like feeding tentacles into the water column to capture tiny planktonic food.  Although these animals have a reputation for being extremely dangerous to a coral reef aquarium, they only cause problems in self-defense.  A sea apple can make an attractive and interesting addition to well-designed aquarium in which the animal is protected from pump intakes and receives    appropriate and sufficient food. Photo by Julian Sprung

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.  If the tentacles are not being withdrawn to the mouth on a regular basis, the cucumber is not actually feeding, and the frequency of tentacle withdrawal can be used as a reasonable gauge for how well an animal is feeding.  On average, these animals should be sucking a tentacle clean and re-deploying it about once per minute, and it will be faster if the animal is provided with high quality food, such as live phytoplankton, and slower if offered a low quality food such as a commercial invertebrate food based on pea flour and yeast.  If the tentacle withdrawal rate is significantly slower than one per minute, then the animal is not getting enough or the proper quality food to feed effectively, and you can be confident that it will eventually starve in your aquarium.  If your sea apple is not extending its feeding tentacles, there is a serious problem with either the flow regime or the food that it is being offered.