spherical aberration to provide sharp images at the retina. Sensitivity increases with
pupil size (more area, more gathered photons). In some mid-water fish, the pupils
extend to the full diameter of the lenses, and those protrude through the pupil to admit
light from ∼180°. Maximum contrast (and thus discernibility) of objects in the visual
field is against the brightest light, which is above, and it falls off dramatically to the
sides and below. A frequent adaptation to that geometry is tubular eyes looking up,
with the retina restricted to the bottom of the tube. Their effective angle is only about
50°, but that provides the sharpest possible images at the nearly flat retina. Focal
length sets the overall eye size for a given lens size. Thus, large pupils and lenses
require large and distant retinas, dictating that vision in many mid-water fish and
squid dominates both space in the head and capacity in the brain. The eyes of the
giant squid (Architeuthis) are as large as 37 cm in diameter, with a salad-bowl-sized
retina. There are many variant eye plans among fish (Collin 1997), but we can only
present two examples.
(^) Adults of the circumglobal tropical–subtropical pearl eye, Scopelarchus analis are
as large as 12.6 cm. They are captured below 500 m in daytime and ascend to as
shallow as 275 m at night. Like many mesopelagic fish, their tubular eyes are aligned
dorso-ventrally in the head, looking up. The protruding lens (Fig. 12.8) focuses the
scene above on a nearly flat retina at the bottom of the tubular vitreous space. In
addition, light entering the lens from the side, some of it passing to the lens through a
clear patch of body wall (a “lens pad”), is focused on an auxiliary retina in the medial
wall of the tube, providing information about contrasts, or more likely flashes, located
laterally. Similarly upward looking, tubular eyes occur in many families, and the
addition of a retinal patch for lateral viewing has also evolved repeatedly. In
Bathylychnops exilis the lateral patch has become a pouch-like subsidiary eye with its
own lens (a thickened corneal protrusion) in the body wall looking ventrally (Pearcy
et al. 1965).
Fig. 12.8 Orientation and anatomy of the eye of Scopelarchus analis. (a) Photograph
by Shaun P. Collin. (b) Vertical section: AR, accessory retina; L, lens; LP, lens pad;
MR, main retina; ON, optic nerve; as drawn by Partridge et al. (1992).
(^) (After Pointer et al. 2007.)