Ultraviolet light influences habitat preferences in a fish under predation risk

Environmental light conditions are of general importance in predator-prey interactions. In aquatic systems, prey individuals experience different levels of predation risk depending on the properties of the visual environment, such as structural complexity or water transparency. To reduce the threat of predation, prey should move to habitats providing better protection against visual predators. We studied the role of UV wavelengths in habitat choice behaviour under predation risk in a fish, the three-spined stickleback (Gasterosteus aculeatus) that uses UV signals in different contexts of intraspecific communication. In a laboratory experiment sticklebacks were exposed to a predatory threat and given the choice between two escape habitats, one providing full-spectrum conditions including UV light (UV+) and one without UV wavelengths (UV-). Fish from two rearing treatments were tested, one group had been raised under natural lighting conditions (UV+), the other group under UV-deficient lighting conditions (UV-). Sticklebacks from the UV+ group preferred the UV- habitat as a refuge which suggests that predator avoidance behaviour is UV-related in this species with UV- conditions presumably being advantageous for prey fish. However, individuals from the UV- treatment group were equally attracted to both presented light habitats. It is possible that these fish could not discriminate between the two light habitats due to physiological limitations caused by their rearing conditions. Further control trials with neutral-density filters revealed that the UV- habitat preference of UV+ fish in the main experiment was rather not influenced by a difference in achromatic brightness between the UV+ and UV- habitat.