International Journal of Comparative Psychology

In Experiment 1 underwater visual acuity was assessed in two Florida manatees, Trichechus manatus Latirostris , using grating stimuli in three conditions: Vertical gratings presented in freshwater, vertical gratings in saltwater, and horizontal gratings in saltwater. All stimuli were tested in a free-swimming format from a minimum distance of one meter. Substantial differences were found between the two subjects. One subject’s minimum angles of resolution (MAR) were 56 min for vertical stimuli in freshwater, 38 min for vertical stimuli in saltwater, and 24 min for horizontal grating stimuli in saltwater. When only trials under brightest light conditions were analyzed, MARs improved for vertical stimuli to 24 min in freshwater and 21 min in saltwater. No improvement was found for horizontal stimuli with brighter light conditions. The MARs of the second subject measured over a degree for all test conditions. In Experiment 2 only the first subject was tested from a closer viewing distance. He showed no improvement when allowed to approach targets to within 30 cm. The limited resolution of both subjects and absence of increased acuity at closer distances in one suggests that manatees use vision for intermediate or longer distance inspection of large objects. The disparity in visual resolution between subjects has possible implications for variability in acuity within the species.