Spectral irradiance and foraging efficiency in the guppy, Poecilia reticulata

The spectral composition of the light environment can have important implications for visually mediated behaviours. We examined how spectral irradiance influences the behaviour of guppies foraging for live zooplankton prey. Daphnia are semitransparent, transmitting human-visible wavelengths but absorbing strongly in the ultraviolet (UV). We first tested the hypothesis that UV wavelengths contribute to foraging efficiency. We then used lighting conditions that selectively blocked regions of the spectrum (UV, short-wavelength, medium-wavelength and long-wavelength light) to determine the effect of removal of these particular spectral regions on guppy foraging rate. Guppies foraged equally effectively under UV-present and UV-absent conditions, suggesting that UV wavelengths are not particularly important for detecting and locating prey under our experimental conditions. Similarly, foraging effectiveness was not significantly affected by the removal of short-wavelength information. In the absence of long wavelengths, however, foraging rate was significantly reduced. This suggests that long-wavelength information is particularly important in this foraging task. We used image analysis to measure the contrast of Daphnia against the background. Contrast was reduced in the absence of UV or long wavelengths but increased when short wavelengths were removed. Variation in contrast cannot, however, fully account for the results observed. It is also possible that the removal of long wavelengths hindered other factors such as motion perception.