The shrimp lives among the tentacles of a sea anemone, which supplies protection and marks a fixed location on the reef. Fish learn these locations and return to them. When a potential client approaches, the shrimp performs a stereotyped display of antennal whipping and body rocking, which experiments have shown increases the rate at which fish adopt a posing posture and accept cleaning.
The transaction is real. Cleaner shrimp remove ectoparasites, including gnathiid isopods, along with damaged tissue, and fish will hold open their mouths and gill chambers to allow access to places they cannot reach. Clients that have been cleaned show measurable reductions in parasite load, and cleaning also appears to lower stress responses in some client species.
One caution belongs with the picture. Because a cleaner shrimp is conspicuous, white-antennaed and stationary, it is often assumed that any small striped shrimp in an aquarium will clean. Cleaning behaviour is specific to particular species and situations, and shrimp deprived of anemone hosts often stop performing the display altogether.