Ontogenetic changes in visual sensitivity of the parasitic salmon louse Lepeophtheirus salmonis

The salmon louse, Lepeophtheirus salmonis, is an ectoparasitic copepod of s almonid fishes whose life cycle involves two broadly defined, free-living l arval stages, the nauplius and the copepodid, After settling on a host, the copepodid goes through various transformations to become a mobile adult. W e recorded swimming 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 s wim-up OFF response across the spectrum 352-652nm, but no ON response. Cope podids 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 stro nger. The spectral range of the adult ON and OFF responses was the same as that of the copepodids and slightly greater than that of the nauplii, which did not exhibit responses at 652nm, The absolute sensitivity of copepodids under white light (approx, 10(-13) photons m(2) s(1)) was higher than that of nauplii (approx, 10(-17)photons(-1)m(2)s, OFF response) and that of adu lt female lice (approx, 10(-14)photons(-1)m(2)s). This suggests that the na upliar visual system is best suited for detection of shadows (e.g, the host ) under a bright light field (daylight hours), while copepodids and adults may be more specialized for host detection at crepuscular periods and durin g the night, when light levels are low. None of the developmental stages re sponded to the rotation of the plane of polarized light or exhibited any di fference in directed response when polarized light was used in place of dif fuse light.