Biological Oceanography

(^) Eyes (Fig. 12.10) of the bigeye smooth-head (Bajacalifornia megalops, lengths to
28 cm) are optimized for spotting and localizing point light sources almost directly
ahead (Locket 1985). Large specimens of this fish are bathypelagic, collected below
1200 m and as deep as 3000 m, far below levels with any sunlit scene. Juveniles can
be caught at depths as shallow as 250 m. Each eye views the water ahead and slightly
to the opposite side along a deep swale in the snout. Pupils are round at the back but
extend anteriorly to allow light from those angles into the lenses. Light focuses onto a
fovea (a retinal patch with refined image resolution) at the posterior extremity of the
retina. The foveae are pits with curving side walls lined with extreme densities of rod
cells with very long outer segments, the part of the cell with photosensitive pigment.
Locket hypothesized that the relative shapes of a flash image across the two foveae
(Fig. 12.10) could provide information about the location of the source ahead. As in
many deep-sea fish, neural impulses from many rod cells are accumulated by
ganglion cells in the retina before transmission to the brain, gaining sensitivity at the
cost of resolution. Locket observed in sectioned retinas that B. megalops has up to 28
layers of rod cells, but that not all of them remain attached to ganglia. He proposed
that the layers with lost connections are remnants of a developmental progression
during an ontogenetic migration to deeper and deeper levels. However, several layers
do remain active, those with the longest pigmented segments and, therefore, the
greatest light-absorbing capacity.
Fig. 12.10 Dorsal–anterior aspect of a bigeye smooth-head (Rouleina attrita),
showing ranges of light rays focused on the posterior foveae. The circles show
hypothesized focal patterns of a point source of bioluminescence on each fovea.
Patterns could provide information about source location. Beyond A, only one fovea
would receive a focused (and more skewed) image.
(After Warrant & Locket 2004.)