Partridge et al. (1992) determined absorption spectra in retinal cells of S. analis.
They found two pigments, λmax = 405 nm (violet) and 507 nm (green) in the main
retina, the latter only in distal portions of some rods. Both of those were found (Fig.
12.9) in the auxiliary retina, along with a third pigment with λmax = 479 (blue) found
alone just in some cells. Indeed, Pointer et al. (2007) have applied molecular genetic
techniques to S. analis, showing that there are genes for three visual pigments
(opsins): two rhodopsins and a third most closely similar to cone pigments of shallow-
living fish. The measured absorption maxima for S. analis are certainly approximate,
but the fact that there are three functionally distinct pigments, proteins coded by
substantially different genes, suggests that this fish may discriminate among shades of
blue, possibly between sky light and bioluminescence. Whether the neural circuitry
for this color distinction is also present remains to be shown.
Fig. 12.9 Absorption spectra (from microspectrophotometry) of visual pigments in the
accessory retina of Scopelarchus analis. A pigment with λmax at 443 nm is found in
outer segments (o.s.) of both shorter and longer rods, shown diagrammatically. Part of
the o.s. of relatively long outer rods has a pigment with λmax at 503 nm. Those two are
also found in rods of the main retina. Compare the lens transmission spectrum (Fig.
12.7). A third pigment, λmax = 479 is only found in outer segments of short rods of the
accessory retina.
(After Partridge et al. 1992.)