BROWMAN, H.I.*; SKIFTESVIK, A.B.; NOVALES FLAMARIQUE, I.; BOXASPEN, K.; BELANGER, M.: Ontogenetic changes in visual responses of the parasitic salmon Louse, Lepeophtheirus salmonis, to ultraviolet and polarized light

The salmon louse is an ectoparasitic copepod of salmonid fishes whose life cycle involves two broadly-defined free-living larval stages, the nauplius and the copepodid. After settling on a host, the copepodid goes through various transformations to become a mobile adult. We recorded the 3-D swim path responses of free-swimming salmon lice at the naupliar, copepodid and adult stages to the onset (ON) and offset (OFF) of lights of varying spectral irradiance and polarization. Nauplii showed a prominent swim-up OFF response across the spectrum 352-652 nm, but no ON response. Copepodids exhibited a swim-up ON response and a passive (sinking) OFF response across the same spectral range. Adults showed active swim-up responses to both ON and OFF stimuli, although the OFF response was proportionately stronger. The spectral range of the adult ON and OFF responses was the same as that for nauplii and copepodids. Absolute sensitivity of the OFF response under white light was 2.36 x 10-17 for nauplii, 1.24 x 10-13 for copepodids, and 1.85 x 10-14 photons-1 m2 sec for both the ON and OFF response of adult female lice. This suggests that the naupliar visual system is best suited for host detection (i.e., response to a shadow) under a bright light field (daylight hours), while copepodids and adults may be more specialized for host detection at crepuscular periods and during the night, when light levels are low. None of the developmental stages exhibited directed swimming behaviour in response to a linearly polarized light field.