Underwater visual single target acuity in harbor seals (Phoca vitulina)

Single target acuity (STA) is a key measure for understanding visual object perception in harbor seals. While STA was already determined with stationary and moving compact single targets of different contrasts in air, we herein assessed underwater STA with the same stimuli as used for aerial testing for comparison. A two-alternative-forced-choice task was conducted with two trained harbor seals under controlled light and turbidity conditions. Underwater STA was determined as 0.30 deg for stationary and 0.27 deg for moving single targets, and thus matched the previously documented aerial STA for one harbor seal. This result confirms that our test individual can resolve compact objects equally well in both media under our experimental conditions. Underwater STA decreased when contrast was reduced, reaching 0.63 deg at the lowest contrast condition tested. Performance for low-contrast stimuli was likely impacted by caustic effects, resulting from the movements of the harbor seal's head in and out of the testing basin. Additionally, as it was already found for aerial STA, no difference between underwater STA for stationary and moving single targets was documented. Our findings contribute to the understanding of amphibious vision in harbor seals and are informative regarding the perception of objects, such as prey fish.