Most decapod crustaceans are nocturnal. However, because cleaner shrimp at cleaning stations act in concert with their hosts, they clean debris and parasites off the body surfaces of diurnal hosts during daytime. It is not known how cleaner shrimp physiologically accommodate diurnal environments and perform cleaning behaviors. We examined the effect of light cues on the daily rhythmicity of the banded coral shrimp Stenopus hispidus (Decapoda, Stenopodidae), a cleaner shrimp in coral reefs. Shrimp were individually kept in aquaria under conditions of 12-hr light and 12-hr darkness (LD) and constant darkness (DD). Double-plotted actogram analyses revealed that shrimp under LD were active during scotophase and inactive during photophase. Locomotor rhythms were observed in most shrimp under DD. Periodogram analyses showed a weak circadian rhythm in shrimp under DD. Our results show that this species is nocturnal and that its locomotor activity is controlled primarily by the LD cycle. Day-night differences in locomotor activity were reduced in shrimp under LD with weak irradiance, and they remained active during photophase. Shrimp under LD using blue or green light-emitting diode light, but not red light-emitting diode light, showed a day-inactive and night-active rhythmicity. These results indicate that this species can be active under environments with low green and blue spectra-for example, during twilight hours, at depth, or on cloudy days-even during daytime, and this weakness of the circadian clock may be advantageous in their role as cleaner shrimp. It is concluded that in addition to the presence of visiting hosts, light conditions at cleaning stations are likely to influence cleaning activity.
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