Cleaner shrimp are true cleaners of injured fish

Reef fishes sustain injuries from various behavioural and environmental interactions. Injured fishes have been observed frequenting cleaning stations to be attended by different cleaner fishes. This symbiotic relationship between injured fishes and cleaner fishes has only been observed in the wild and has never been demonstrated empirically for cleaner shrimp. We present the first investigation of cleaning of injured fish by cleaner shrimp, and the first controlled laboratory trial investigating the functional relationship between injured client fish and cleaners. We tested whether the cleaner shrimp (Lysmata amboinensis) cleaned injured sea goldies (Pseudanthias squamipinnis) following a standardised, unilateral superficial skin lesion. We recorded the cleaning behaviour between shrimp and fish and determined that the fish regulated the cleaning and reduced the amount of cleaning time by the shrimp of the injured side immediately post-injury, corresponding with previous literary evidence of the rapid onset of re-epithelialisation of the injury to seal it in the first 24 h in injured fishes. Thereafter, injured fish showed no cleaning preference between injured and uninjured sides. Image analyses determined that the cleaner shrimp reduced the redness of the injury, representing rubor, associated with the inflammatory response in fishes. Injuries in fishes are susceptible to invasion by secondary pathogens, and the reduction of injury rubor by shrimp may suggest that cleaning by these shrimp could reduce the success of opportunistic infection. Cleaner shrimp neither aggravated existing injury, nor created additional injury, measured quantitatively. The cleaning of injured fish by cleaner shrimp thus likely involves true cleaning behaviour.