An alpheid shrimp digs and maintains a deep burrow keeping it safe. Above ground, it is vulnerable to predators. A goby fish lives in the burrow with the
shrimp and sits at the entrance, keeping watch for predators. It signals the
shrimp with a flick of its tail when it is safe to come out; if a predator swims by,
the goby darts into the burrow and the shrimp retreats further inside. The goby
benefits by getting a burrow to live in and the shrimp knows when predators are
near. What type of relationship is this?