Another thing that we have never added to the exhibit is crabs, but there are crabs aplenty in there. My supervisor and I were looking at the various inverts when she spotted something odd and I identified it as a hermit crab without a shell. It was pretty large, which is probably why it didn't have a shell--the only snails in the exhibit that get bigger than a dime are limpets, which aren't much use for crab homes. It was looking understandably agitated. She managed to round up some shells that had just been removed from another exhibit, one of which turned out to be still occupied by another hermit--more on it in a moment. She put the other four near the original crab which grabbed the closest. It seemed to be having trouble getting in, so she nudged the others closer. Normally, having a Big Thing poking around in the water near it would be cause for a crab to leave, but this one had a single agenda. It settled into one shell and proceeded to examine the other, now that its soft abdomen was protected.
She placed the other hermit crab a bit of a way away so there wouldn't be any conflict over ownership. However, a smaller hermit crab that was using a bit of broken tubeworm casing as a shell immediately dashed out from under a rock pile and was ready to fight for the new ones' shell. My supervisor moved the smallest of the new empty shells over to the scene and the pugnacious little thing promptly relocated into it.
No-one had been aware that there was such an issue and we'll be seeding the exhibit with a variety of shell sizes from here on. Less drama but more hermit crabs is fine.