8. VISION AND BIOLUMINESCENCE
257
length maxima a t 494-502 mp). I n other words, the population living
in shallower water is living in an ambient light of greater intensity and
of a greener colour. An examination of the spectral sensitivities of the
eyes of individuals in the two populations (Fig. 94) shows that the eyes
of the shallow water individuals are relatively more sensitive to green
light (515 mp) than to blue (460mp), a shift in relative sensitivity
which adapts the eye to the spectral distribution of the ambient light
in which it lives. Boden and Kampa suggest that this adaptation may
be effected by storing more astaxanthin in the eyes and that here may
be a mechanism of adaptation to the photic environment through
differential deposition of pigment, possibly a screening pigment. This is
the only instance of this adaptation so far described. If, however, it is a
general feature of euphausiids then it means that an individual species
is not restricted to a specific isolume but, if other characteristics of the
environment are limiting-in the case of Saanich inlet, a layer of oxygen
deficient water below lOOm-then the animals may adapt to higher
light intensities or, presumably conversely, to lower light intensities.