| Summary: | Visual function was investigated in the antarctic fish Pagothenia borchgrevinki (Boulenger), Trematomus bernacchii Boulenger, T. centronotus Regan and T. hansoni Boulenger. All Trematomus species have large anterior aphakic spaces (indicating a forward feeding vector), whereas Pagothenia does not. Pagothenia and T. hansoni , which both feed in the water column, lack the corneal iridescence displayed by T. bernacchii and T. centronotus . This is thought to relate to the importance of downwelling light for image formation in Pagothenia and T. hansoni . Absolute sensitivity thresholds to white and monochromatic light were measured using electroretinogram (ERG) responses in light- and dark-adapted Pagothenia and dark-adapted Trematomus species. Dark-adapted fish (retinal screening pigment withdrawn from rod outer segments) had thresholds of l.3x10-3µEm-2s-1 to a 200ms pulse of white light, whereas that of lightadapted Pagothenia (rod outer segments covered by the retinal screening pigment) was l.9x10-2µEm-2s-1. We suggest that the thresholds approximate threshold stimuli for rods and cones, respectively. Measurement of thresholds of light-adapted Pagothenia made using a behavioural measure (feeding responses) gave a threshold of 5x10-3µEm-2s-1. Limits for photopic and scotopic vision are predicted to be reached at depths of 20-40 and 30-60 m, respectively, under snow and ice conditions typically encountered at this time of year. ERG-determined spectral sensitivity curves peaked around 500 nm in all four species and matched the spectral irradiance under the ice. Shifts to longer wavelengths in spectral irradiance caused by sub-ice phytoplankton growth may degrade visual ability. Flicker fusion frequencies (FFFs) reached a maximum value of 15 Hz in Pagothenia at the maximum stimulus intensity used, but at environmentally realistic light intensities they were less than 8 Hz. FFFs were lower in all Trematomus species than in Pagothenia .
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