Welcome to the land of the believers- I refer to all crabs as terrorists. This includes hermit crabs. I can't figure out why they seem to be a “necessity” when there are so few of them on a reef.
From
http://web.archive.org/web/200104100...=&RecordNo=166
"The downside of this is that the food they are eating is often small animals, such as larvae, small polyps, small snails and smaller crustaceans. Additionally, they often attack recently fed corals, sea anemones and other animals removing their food and consuming it. In the process, they undoubtedly damage these animals.
Hermit crabs are relatively rare in natural reef situations; they simply are never found in dense numbers on coral reef rocks. The corals and anemones in these areas are adapted neither to the crab’s competition for food nor to the physical damage occurring when the crabs move over their surfaces. The pitter-patter of little pointy feet can be very destructive to delicate coral tissues."