Most marine aquarists are only too happy to have something different in their aquariums, as long as that something fits in with the current population.
The apple being talked of here is the Sea Apple which is definitely different! As can be seen it’s colourful too. An aquarist who has one will usually be fascinated by ‘the thing in the corner’. The Sea Apple has been available to aquarists for many years. I feel that the beginner aquarist should let experience build before keeping one, as a pure guess say about a year, or at least until the common general husbandry mistakes are no longer made and seawater quality is consistently high. This is important as will be seen.
‘Sea Apple’ is the common name; the proper one is Pseudocolochirus axiologus. They need the usual correct introduction to the aquarium, that is, acclimatization, and should not be exposed to air. They need to be handled gently but this of course is the same for all livestock.
The Sea Apple does not require reef lighting as it is not a coral but a species of sea cucumber. This can of course be confusing, the common name talks of ‘apple’ yet the species is ‘cucumber’. (This seems to happen from time to time, such as the Foxface fish (Lo vulpinus) that belongs to the rabbitfish family.) The Sea Apple is usually able to grow to about 4″ (circa 10cm) though there are larger species that grow to about 6″ (circa 15.25cm) or more though these are not so readily available and are more expensive.
The Sea Apple is able to move slowly around the aquarium and will do so until it is happy with its position, and this depends a lot on seawater flow. The flow doesn’t need to be very strong but moderate to brisk. This is because from the top centre a crown of feathery tentacles will appear and too strong a flow could cause problems. As said, the Sea Apple will make its choice and the final place could be a viewing glass, a corner, a rock or even the tube from a tubeworm. The aquarist has his or her preference for position, so keep those fingers crossed. The final position is likely to be between half way and the top of the aquarium. Once the Sea Apple has come to rest it should be left in peace unless the position is causing trouble to other livestock or presents a danger to the Sea Apple itself. It should not be irritated by the aquarist; this could be a real problem as will be seen.
The tentacles are used for feeding and are extended into the seawater column for this purpose. Fine particulate matter is captured and, one by one, the tentacles are passed into the mouth for removal of any food and then withdrawn, it’s a bit like a human sucking food off the fingers. This is a fairly slow process and is fascinating to observe.
The Sea Apple is quite hardy in a high quality environment and can live a long time. Many do fail though and this is to do with feeding, or rather the lack of it. In most aquariums there is insufficient suitable fine particulate matter in the seawater and therefore the Sea Apple doesn’t get enough to eat. This causes it to slowly shrink until it dies. Target feeding is needed, the frequency of which is best twice a day or more if the need is observed. There are plenty of commercial fine foods available that are suitable. All that is needed is for this food to be squirted into the seawater with a suitable instrument a little upstream from the Sea Apple when it has the tentacles extended, the food will then drift over them. If the seawater current is too high causing the food to pass too quickly, turning off the pumps for a short period will not do any harm. Any of the fine food that is not taken will drift off and could well be captured by corals.
Overall I would suggest that a reef system is the best aquarium for the Sea Apple to be introduced into. The reason is that there is a greater likelihood that there will not be any fish present that could nip at the tentacles, though this is not always the case. If the Sea Apple is excessively harassed then serious trouble could occur.
The problem that a Sea Apple could cause is very serious. If it is excessively harassed and stressed a toxin could be released that could, and likely will, kill the other livestock. Not a pleasant thought. That is why the Sea Apple should not be harassed excessively by the aquarist and its tank mates should be suitable. In addition it would be a good cautionary measure to protect the intakes of circulation pumps etc so that the Sea Apple cannot be accidentally damaged, this should be done before introduction.
Having said that, the Sea Apple should be a good introduction to a suitable high quality environment provided the aquarist has reasonable experience and feeding is given suitable attention.