| Cleaning behavior is considered to be a classical example of mutualism. However, no studies, to our knowledge, have measured the bene?ts to clients in terms of growth. In the longest experimental study of its kind, over an 8 year period, cleaner ?sh Labroides dimidiatus were consistently removed from seven patch reefs (61–285m2) and left undisturbed on nine control reefs, and the growth and parasite load of the damsel?sh Pomacentrus moluccensis determined. After 8 years, growth was reduced and parasitic copepod abundance was higher on ?sh from removal reefs compared with controls, but only in larger individuals. Behavioural observations revealed that P. moluccensis cleaned by L. dimidiatus were 27 per cent larger than nearby conspeci?cs. The selective cleaning by L. dimidiatus probably explains why only larger P. moluccensis individuals bene?ted from cleaning. This is the ?rst demonstration, to our knowledge, that cleaners affect the growth rate of client individuals; a greater size for a given age should result in increased fecundity at a given time. The effect of the removal of so few , small ?sh on the size of another ?sh species is unprecedentedoncoralreefs. |