Comparison of visual acuity and visual axis of three flatfish species with different ecotypes

The visual acuity, visual axis and visual accommodation of pointhead flounder, slime flounder, and red halibut were determined to obtain basic knowledge for developing appropriate fishing gear and fishing methods for sustainable fisheries. Each of these species has a different ecotype in terms of habitat, depth and prey species. Thus, it was hypothesized that they may differ in terms of visual acuity, visual axis and visual accommodation. Few studies have compared these characters in flatfishes from different ecotypes. We used histological methods to determine visual acuity (i.e. cone cell density) and visual axis (i.e. cone cell distribution) in each of these species. The maximum visual acuity was 0.127 in pointhead flounder (total length, TL 344 mm), 0.092 in slime flounder (TL 372 mm) and 0.109 in red halibut (TL 336 mm). Based on the cone cell distribution in the retina, the visual axis was upward and forward in pointhead flounder, forward and downward in slime flounder, and downward in red halibut. Finally, the mean angle of lens movement was -2° in pointhead flounder, -13° in slime flounder and -32° in red halibut. This measurement of lens movement indicated that the average near-point distance was 0.87 x TL in pointhead flounder, 0.65 x TL in slime flounder and 1.02 x TL in red halibut. At similar TL (336-355 mm), the visual acuity of these species differs depending on the direction in which they are looking.