The Multiple Bank Rod Fovea of Bajacalifornia drakei, an Alepocephalid Deep-Sea Teleost

The bathypelagic alepocephalid teleost Bajacalifornia drakei has large eyes with sighting grooves and rostral aphakic gaps. An object in front of the fish will be imaged in both eyes through the full aperture of the lens in the temporal periphery of the retina, where a convexiclivate fovea with a prominent fovea externa is located. The fovea externa is composed of many banks of rods, up to 28 in a 39 mm specimen, compared with two banks in the peripheral retina. Estimates of foveal rod outer segments, nuclei and synaptic spherules show that there are almost twice as many outer segments as nuclei, and that the number of spherules is much fewer than either. It is likely that new banks of rods are added to the retina, disproportionately many in the fovea, as the fish grows. The paucity of synapses suggests that many rods lose contact with bipolar cells, which may make new contacts with successive banks of rods. The possible mechanism of formation of the new banks, consequences of the multiple bank arrangement, and optical function of the fovea are discussed.