Long-term cleaner fish presence affects growth of a coral reef fish

Abstract

Cleaning behaviour is considered to be a classical example of mutualism. However, no studies, to our knowledge, have measured the benefits to clients in terms of growth. In the longest experimental study of its kind, over an 8 year period, cleaner fish Labroides dimidiatus were consistently removed from seven patch reefs (61-285 m(2)) and left undisturbed on nine control reefs, and the growth and parasite load of the damselfish Pomacentrus moluccensis determined. After 8 years, growth was reduced and parasitic copepod abundance was higher on fish 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 conspecifics. The selective cleaning by L. dimidiatus probably explains why only larger P. moluccensis individuals benefited from cleaning. This is the first 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 fish on the size of another fish species is unprecedented on coral reefs.

Figure 1.

Fish growth of P. moluccensis, derived from the regression of log fish standard length (SL) and log age, from reefs with (black circles, solid line) and without (white circles, dashed line) cleaner fish L. dimidiatus. Annual ages were jittered to reveal underlying data. Arrows indicate outliers omitted. Sample sizes in brackets.

Figure 2.

Parasitic copepod juveniles on P. moluccensis, summed across 10 fish sampled per reef, compared with mean fish SL per reef on reefs with (black circles) and without (white circles) cleaner fish L. dimidiatus.