Shoal choice and ultraviolet reflections in stickleback populations from different photic habitats

Abstract

Grouping is a widespread phenomenon in the animal kingdom and the decision to join a group is a function of individual and environmental conditions, meaning that any advantages and disadvantages have to be pondered constantly. Shoaling decisions in fishes are communicated via a variety of factors, such as colour signals, amongst other ultraviolet (UV) signals. The sensitivity for ultraviolet signals is assumed to be costly and a function of the predominant ecological conditions. The island of North Uist, Scotland, comprises bodies of water that possess great variation in their spectral distribution, especially in the UV spectral range. We examined different populations of three-spined stickleback (Gasterosteus aculeatus L.), which is known to use UV for visual tasks, consisting of three populations from tea-stained lakes and four from clear-water lakes, concerning their preferences to join a shoal viewed under UV-present and UV-absent conditions. Nonreproductively active sticklebacks from tea-stained lakes significantly preferred the shoal under UV-absent conditions, whereas sticklebacks from clear-water lakes did not show a significant preference. Reflection measurements showed that the UV chroma (intensity) of sticklebacks from tea-stained lakes was higher than that of sticklebacks from clear-water, most likely contrasting maximally against the UV-poor background or compensating for a stronger attenuation of the signal.