An alpheid shrimp digs and maintains a deep burrow. While underground, the shrimp is safe. Above ground, it is vulnerable to predators. A goby fish lives in the burrow with the shrimp. The goby fish sits at the entrance, keeping watch for predators, and signals the shrimp with a flick of its tail when it is safe to come out. Or, if a predator swims by, the goby darts into the burrow and the shrimp retreats further inside. These two animals are completely dependent on each other—the goby benefits by getting a burrow to live in and the shrimp knows when predators are near. M for mutualism P for parasitism C for commensalism