Biological Oceanography

(^) The evolutionary back and forth between prey defensive strategies and predator
counter-strategies has passed through many cycles in mid-water, as in most habitats.
Ventral countershading provides an extended example. While Argyropelecus has
evolved a very precise spectral match, not all mid-water animals have achieved it;
they produce countershading with a substantial spectral “shoulder” above 490 nm. A
few predatory fish (Chlorophthalmus, Argyropelecus, Scopelarchus) and squid
(Histioteuthis) have developed yellow eye lenses which probably enable them to take
advantage of this slight mismatch. Yellow pigment in the eye absorbs blue light right
at the water-transmission maximum, acting as a sharp cut-off filter that removes the
dominant wavelengths. However, it passes light greater than 490 nm (Muntz 1976;
Fig. 12.7), which must cause the ventrally luminescent animals above to appear as
pale shapes.
Fig. 12.7 Comparison of the ventral bioluminescence spectrum of a typical myctophid
fish, Myctophum punctatum (dashed line), which is not filter-corrected as in
Argyropelecus (Fig. 12.4), to the downwelling spectrum at 500 m (bell-shaped curve)
and to the light-transmission characteristics (L) of the yellow eye lens of the pearl eye,
Scopelarchus analis. By eliminating the dominant spectral component (blue), the
predator can see the excess of the prey bioluminescence at longer wavelengths (blue-
green). This matches the λmax = 503 nm of one rod pigment of S. analis (Fig. 12.9).
(^) (After Muntz 1976.)