You want to keep a lookout for Gorilla Crabs (xanthid species) and Stone Crabs. These pesky buggers are predators that can cause damage to your reef, eating anemones, clams, and anything else they can get their claws around. They might look cute when they're small, but it's just not worth putting them into the tank. If you really want to keep them, you could put them into your sump/fuge (if you have one).
The sure-fire way to spot one of these is to look for the black tips on their claws. As Gorilla crabs get larger, they become quite "hairy" all over. You can smash these guys pretty easily using a chopstick. Take a look at the catalog of organisms for pictures of other "good crabs" that may hitchhike on your rock. (From Tampa Bay Saltwater - sells rock) http://tbsaltwater.com/thepackage/im...orillacrab.jpg
He is BIG! He's probably about 3.5 inches wide. He's been skulking around my 300g for quite a while. I saw him one day about 3 weeks ago.. and then couldn't find him. I thought maybe I had imagined seeing a hairy black claw...until today. I nabbed him with my steel tweezers during feeding time.
I've lost so many various corals, but with all the travel, it was hard to tell if it was tank params or something munching on the corals. I'm working on setting up my 75g display right now, and working on the 300g params with water changes, general cleaning and maintenance. The 300g is going to be unloaded and all rock dipped, and anything that doesn't resemble a fish, a snail or hermit crab is going to find a new home...or grave.
If you are interested in learning about reefkeeping or have a problem with your reef, our reefkeeping community is here to help. Feel free to ask a question or search our site. We have lots of experienced reefkeepers that are willing to provide free reefkeeping advice!
Besides being a great resource for all levels of reef aquarium hobbyists, CaptiveReefs is a social experience that will enhance your enjoyment of reefkeeping. CaptiveReefs is committed to connecting reefkeepers with the support and information they need to grow beautiful coral reef aquariums.