A week ago I added a horseshoe crab to my 70 gallon reef tank. Last night after lights out, I was startled when I heard my pistol shrimp popping his claw furiously. I found that the new horseshoe crab was attacking the pistol shrimp using his tail spike like a sword. (I have since named him Zorro!) All this, right at the opening to the shrimps den. I was only able to detur this behavior by relocating a couple more rocks around the dens opending. I always thought that these crabs were peacefull, sand shifting & only ate deturis & other organic matter in the sand bed. :evil: