The Pistol Shrimp can form a symbiotic bond with several different types of Goby including the Watchman and the Yasha. The shrimp is a great digger, and uses his large claw much like a bulldozer... but he is nearly blind.
Poor eye sight can be a problem in the wild, to dig holes he must venture out into the open to "dump" each load of sand and gravel, making him vulnerable to large fish and other predators. This is where the Goby fish comes in! The goby will sit at the entrance of the cave keeping an eye out for predators, while the shrimp keeps in physical contact using one of his tentacles. When he feels the fish rush back into the burrow, he knows it isn't safe outside..
This pair makes an excellent and interesting reef tank spectacle, just be sure your rockwork is built well as all that digging in search of food can move a LOT of sand around!!!
To see this symbiotic relationship in your own aquarium, you need a 30 gallon tank or larger, with at live rock and at least 2 inches of fine live sand, though 3-4 inches would be more fitting.