Fish of the genus Anableps (Anablepidae, Cyprinodontiformes)
have eyes that are adapted for simultaneous aerial and aquatic vision. In
this study we investigate some of the corresponding retinal
specializations of the adult Anableps anableps eye using retinal
transverse sections and wholemounts. The linear dimensions of the retina
were found to be asymmetric with a greater representation of the dorsal
compared to the ventral visual field. The total number of neurons in the
ganglion cell layer of the ventral hemiretina was on average 3.6 times
greater than the values obtained in the dorsal hemiretina. Isodensity
contour maps revealed a prominent horizontal visual streak in the ventral
hemiretina with an average peak cell density of 18,286
cells/mm2. A second less-well-developed horizontal visual
streak was also observed in the dorsal hemiretina. A sub-population of
large cells with soma areas between 74 and 188 μm2 was
identified and found to be distributed evenly across both hemiretinas.
Together, these results show that the sampling gain of the ventral retina
is significantly greater than the dorsal segment, that retinal
specializations important for mediating acute vision are present in the
parts of the visual field immediately above and below the surface of the
water, and that visual functions related with the large ganglion cells
require more even sampling across the visual field. The relevance of these
retinal specializations to the feeding and other behavioral strategies
adopted by Anableps is discussed.