I've also owned a horshoe crab which came with a used set-up I purchased. They don't seem to be the most intelligent creatures. This one would occassionally surface out of the sand and swim/drift inverted all over the tank. A number of times it ended up in my bubble tip anemone and froze, probably from the stings. After a while I got tired of intervening and rescuing the foolish thing and noted that the anemone would eventually release the ugly critter. It would always recover fairly quickly and start it's inverted swimming routine. They have two sets of eyes but I swear the thing was blind, that's how clumsily the thing moved around in the tank. A few months ago I found it expired in the corner of the tank. It was a good sand sifter while it lived but I don't think I'll be getting another one.