ABSTRACT. The significance of ultraviolet light as a cue in diel vertical migration of free-swimming copepodids of the ectoparasite salmon louse Lepeophtheirus salmonis (Krnyer) and adults of the holopelagic Calanus finrnarchicus (Gunnerus) were examined in the laboratory. Copepodids of salmon lice were photopositive in 1 m deep water columns when illuminated with visible light (VIS) only, but when VIS was combined with ultraviolet light, with a spectral irradiance maximum at 313 nm, the copepodids gathered significantly deeper in the water column. In similar expenments C. finrnarchicus did not show upward migration as a response to VIS, and a combination with ultraviolet light did not alter their normal behaviour. In 250 ml beakers, however, C. finmarch~cus showed avoidance behaviour upon being exposed to UV-light; the copepods swam vigorously with prosome pointing downwards as if they were trying to escape from the irradiation. We conclude that these copepods must have sensory structures not yet descnbed which can detect UV-radiation. Possible implications of these findings are discussed in relation to vertical rnigration and to host-finding in the salmon louse.

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