Mirrors – Silvered Sides
(^) Silvered sides occur in many fish of near-surface waters: oarfish, sardines, tuna, and
others. These mirrors are panels of guanine crystals arranged so that light reflected to
the eye of a predator approaching at any angle tends to come from the same direction
as if the prey were not there (Fig. 12.2). The silhouette of the opaque fish nearly
disappears against the diffuse light of the background (Franz 1907). To make this
work, the guanine panels parallel the vertical (dorsal to ventral) axis of the body,
regardless of the curvature of the sides (Fig. 12.2). In mid-water fish, such mirrors are
only useful on the side planes that are almost exactly vertical, so mirrored scales are
reduced to a single row. Deeper yet, they are useless, and fish do not have them. They
are not a feature of mesopelagic invertebrates.
Fig. 12.2 Transverse section of a fish body showing the orientation of reflective
guanine plates and the resulting camouflaging effect. The predator eyes at the right
and lower left receive reflected light to replace light that it would have seen if the fish
wasn’t there. Effects of countershading are implied by eyes above and below.
(^) (After Denton 1970.)