Where I catch them there isn't a crushed coral substrate. It's all sand. These are south pacific varieties so don't take my advice if your shrimp is from the caribean, or somewhere other than the south pacific. As for diet I see scattered shells of blue-eyed crabs, they are very abundant, hermit crabs and I have personally seen the one in my tank go after a small spiny urchin. The shrimp was very persistent in bothering the urchin until it ultimately moved to another rock. Until that day I didn't even know that urchins moved around after they had attached into the crevice of a rock. I removed the first mantis from my tank after getting mad at all the dead crabs I was finding, about one every other day. Then I realized it was no different that keeping any other predator and when I saw a second about 2 weeks ago I have let him be. After all, all the crabs I want are about a 5 mintue drive away.