Susceptibility to ultraviolet radiation in Calanus finmarchicus and Lepeophtheirus salmonis and the adaptive value of external filtering (Crustacea: Copepoda)

Abstract

Copepods were exposed to UVB with a spectral irradiance maximum at 313 nm and additional visible light. In the holopelagic Calanus finmarchicus, with a carotenoid concentration of 0.85 μg mg-1, the LD50 dose was 29 kJ m-2 whereas in the parasitic copepod Lepeophtheirus salmonis the LD50 was 84 kJ m-2. The ability to photorepair was demonstrated for both species. The survival data were Weibull distributed, and this result is discussed relative to a mechanistic 'Weibull model'. Attenuation of UVB (306 nm) was measured in the inner Oslo fjord (Kd = 3.9 m-1). The adaptive significance of behavioural avoidance is assessed in model simulations; the results of the survival experiments are used in connection with the Morowitz formula. Two cases were compared; when Calanus was assumed incapable of active external filtering the specimens would be safe if the depth of the mixed layer was deeper than 7 m in the inner Oslo fjord, however, with the ability of behavioural avoidance, Calanus would be protected if they avoided the upper 1 m. The adaptive value associated with behavioural avoidance of toxic radiation, and the implications relative to vertical migration are discussed.