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I wonder if anyone has had any success in feeding the Sea Apple. I was recently advised that they are easy to keep by a LFS, as long as light was OK, but I got home and read they are hard to feed, and sure enough mine has after four days decided to close up, I think from lack of food. Has anyone any experience with these also is there any way to get it to come out to feed now its closed?

I think the LFS you went to did not give you credible advice.  Sea apples, a species of sea cucumber, are suspension filter feeders.  Unless there are good plankton or particulate levels in the tank, they would require additional feedings.  However, even this can be difficult, as there isn't much information available on suitable foods for them and other suspension filter feeders.

Commercial invertebrate target foods would probably not work, you might stand a better chance with finely blended seafood, whatever available plankton you can find, and maybe yeast cells.  As far as I know of, sea apples are not Zooxanthellae, therefore lighting has nothing much to do with their nutrition.  These animals rank up there with flame scallops, feather stars and carnation corals in the "really hard to feed and keep" category.

Also, I guess that the LFS did not tell you about the lethal effects if the sea apple releases it's toxins, either when distressed or in decomposition.  A complete tank wipe-out would be a very likely outcome if this happens.  At the moment, the best thing you can do is to remove this "time bomb" from your system, especially since it is showing signs of decline.  The best place to drop it off would be the LFS you bought it from, and you can gauge from their reaction on this return if this deal was an honest one in the beginning.

I've had one for more than a year now, and it's still doing well. They like a reasonably high water flow, but they will move around by themselves until they find the most suitable spot. My sea apple is also very sensitive to any change in light intensity, and/or to external movement that could throw a shadow or change intensity from outside the tank. He will then immediately pull in his tentacles, and stay "hidden" for anything from a few minutes to more than an hour.

I used to feed him some liquid "invert food", but have stopped that quite some time ago (maybe 6 months ago, not too sure). Since then, I have not fed him anything directly. I have lots of small copepods, etc. in my tank, and it is quite likely that he is eating some of their newly hatched offspring.

They do posses some potent "chemical weapons". Mine once touched my large carpet anemone, without any harm. I then tried to move it away, and this caused the anemone to grab it, and to close up in its normal "feeding mode". After about 2 seconds it opened up, seemed to spit the apple out, and spent the next few hours in an obviously agitated state, seemingly with a very bad taste in it's mouth. Neither the apple nor the anemone showed any lasting effects, though.

They move about either by crawling around on their numerous "feet", or by inflating  their bodies, and just floating around in the current. They also have a tendency to settle on one's best coral, and make themselves comfortable there, to the detriment of the coral.

An Update on my sea apple

I moved it as it was obviously unhappy and two days there after he popped out his feeder tentacles, I syringed some food onto him and he really perked up. I did so again about an hour later, he then fully opened up and when I woke up this morning somehow he has moved himself up the rockwork onto a ledge near a power head so he now seems OK. If anyone else gets one, they seem to need a lot of attention when it comes to feeding, although they are a lovely colour.

I'm sorry to report that my Sea Apple died after about 18 months in my tank. In the end it DID start to "shrink", and must have died from hunger even though I tried to give it enough food. So, based on my experience, I must conclude that they are very difficult to keep alive if one does not have the correct size and type of food it needs.