J Comp Physiol A (2009) 195:643–650 DOI 10.1007/s00359-009-0439-2 123 ORIGINAL PAPER Aerial visual acuity in harbor seals (Phoca vitulina) as a function of luminance Frederike Diana Hanke · Guido Dehnhardt Received: 5 February 2009 / Revised: 19 March 2009 / Accepted: 22 March 2009 / Published online: 10 April 2009  Springer-Verlag 2009 Abstract In this study, we measured aerial visual acuity in harbor seals. As a Wrst approach to the hypothesis that harbor seals can obtain acute aerial visual acuity mediated by the interaction of the vertical slit-shaped pupil and the corneal Xattening although refractive measurements had revealed aerial myopia, visual acuity was tested as a func- tion of luminance and pupil dilation. We analyzed aerial visual acuity (minimal resolvable stripe width) in three har- bor seals in a two-alternative-forced-choice discrimination experiment. Our results further support the hypothesis that harbor seals possess an aerial visual acuity comparable to the acuity in clear waters if the vertical slit pupil does not exceed the zone of corneal Xattening in bright light. When the pupil dilates with decreasing luminance, visual acuity decreases which might be due to deXected light from the stronger curved peripheral cornea. Keywords Harbor seal · Phoca vitulina · Visual acuity · Luminance · Corneal Xattening Abbreviations IR Infrared SD Standard deviation Introduction The amphibious lifestyle of harbor seals (Phoca vitulina) is a challenge for all sensory systems. The most serious prob- lem of amphibious vision might be the gain in refractive power when the eyes are lifted above the water surface. This is due to the cornea being optically nearly ineVective under water but contributing large parts to the overall refractive power of the eye in air. Harbor seals seem to be primarily adapted for underwater vision, where essential activities, such as hunting, are performed. Therefore, it was speculated that the eye is emmetropic when immersed in water mediated by a spherical lens (Jamieson and Fisher 1972; Hanke et al. 2008) and myopic in air. Refractive measurements in harbor seals supported this hypothesis and, additionally, revealed a high degree of astigmatism against the rule, meaning that the horizontal meridian was refracting the light stronger than the vertical meridian (Johnson 1893, 1901; Jamieson 1970; Hanke FD et al. 2006). Interestingly, in our study on harbor seal refraction (Hanke FD et al. 2006), the degree of aerial ametropia depended on pupil size. The high degree of aerial myopia was measured in widely dilated pupils, however, analyzing intermediate stages of pupil constriction, vertical slits, myopia tended to decrease with decreasing vertical pupil diameter rendering the eyes near-emmetropic (Hanke FD et al. 2006). It was hypothesized that this relationship can in large be understood when looking at corneal topography (Hanke FD et al. 2006). The cornea displays a central Xat stripe with radii of curvature up to 60–80 mm in the vertical meridian but a less pronounced Xattening in the horizontal meridian (central radii of curvature 25–30 mm) rendering the eye highly astigmatic. Thus, when restricting the optical zone to the corneal Xattening by the vertical slit pupil and F. D. Hanke (&) Department of General Zoology and Neurobiology, University of Bochum, ND 6/33, 44780 Bochum, Germany e-mail: frhanke@yahoo.de G. Dehnhardt Institute for Biosciences, Sensory and Cognitive Ecology, University of Rostock, Albert-Einstein-Strasse 3, 18059 Rostock, Germany