Missing Nudibranch (12/23/2003) I recently added a Nudibranch, don't know the name - dark blue with yellow strips, which disappeared over night.  The next week I added two lettuce Nudibranchs, one had bites out of him the next day and was almost completely gone the next day, the other disappeared the following day. <Nudibranchs should definitely be put in a quarantine tank for a month before going in the display tank.> At the same time I added a yellow cucumber.  It has been over two weeks without a sighting of it.  I am fighting an algae problem right now so it is very easy to see if the substrate is disturbed by anything, I have see no sign of the cucumber since the second day.  From reading anything I can find about these types of animals I would expect to find a dead fish if they were eaten by a fish, I have not. <Lettuce Nudis are not toxic. In general, the brighter the colors, the more toxic the animal. The cucumber (Colochirus robustus) may just have found a good spot where you can't see it. Mine has not moved for more than a month. This particular species is not likely to poison your tank if it dies, but it pays to be careful with a system that includes any sea cucumber.>  I have a 75 gallon with 125 lbs live rock, just removed the bioballs from my wet/dry and replaced it with more live rock.  I have 1 coral beauty, 1 purple pseudo, 2 false percula clowns, 2 yellow tail blue damsels, 4 blue/green Chromis, 2 yellow damsels, 2 brittle stars, 2 Condy anemones, 1 coral banded shrimp, 2 emerald crabs, 1 sally lightfoot and 1 tiger cowry.  I also have bubble corals, pulsing xenia and a hammer, assorted snails and hermits.  What could eat these Nudibranchs and live? <I'd suspect the Coral Beauty.>  I find them very interesting and would like to keep them successfully. <First off Alan, I strongly recommend against buying any Nudibranch other than the lettuce Nudibranch or Berghia. 99% of all others die because they are virtually impossible to feed in an aquarium. Not only that, many are toxic and can kill your fish when they die. Berghias are often purchased to eat Aiptasia and then starve once they've eaten it all. Lettuce Nudibranchs eat macro algae and will often survive. I do suspect that the coral beauty is a problem here--likely to eventually eat the lettuce Nudibranch. I can tell you like inverts (as do I), so I recommend that you buy and read "Reef Invertebrates" by Bob & Anthony. There is so much useful information in there about all kinds of interesting critters. Hope this helps, Steve Allen.> Thanks,  Alan