Countershading
(^) Being dark dorsally and light ventrally is important at all lighted levels of oceans and
lakes. You have noticed that fish tend to be dark dorsally, silvery or white ventrally.
This “countershading” appears in a range of near-surface organisms: fish, surface-
floating snails (Janthina), the chambered Nautilus. It cannot work perfectly, because
there is always less upwelling light than downwelling light. Thus, reflected upwelling
light cannot exactly match downwelling light. In mid-water, another possibility arises,
and many organisms have evolved this adaptation. At levels where the downwelling
light is reduced to an intensity possible to match with bioluminescence, arrays of
photophores on the ventral sides of fish, squid, shrimp, and euphausiids match the
downwelling light, thus filling in their silhouettes. This reason for ventral photophores
was suggested independently several times, starting with Dahlgren (1915–1917).
(^) Young and Roper (1977) and Warner et al. (1979) provided experimental
demonstrations that this must in fact be the function of ventral photophores (which
could have other functions, like attracting appropriate mates). They showed that
squid, fish, and shrimp closely balance their ventral bioluminescence with the
downwelling light; there is an exact match. Young and Roper placed squids or fish in
a tank above a mirror at a 45° angle, so that an observer off to the side could watch
both the overhead illumination and the ventral side of the animals. A photomultiplier
tube (PMT) in line with the observer’s eye permitted measures of light levels.
Animals were conditioned at a variety of light intensities for 5–10 minutes. Then the
observer, who could not see the animal, quickly lowered the intensity until he could.
Next, he raised it again until the animal disappeared, indicating a match. The intensity
of the match, that is, of the animal’s light output, was read from a meter. This required
that the matching response be fairly slow, multiple seconds rather than, say,
milliseconds. Long response times must be short enough in the slow-moving, mid-
water world. The results were a sequence of comparisons of the conditioning
intensities and the matches called by the observer (Table 12.1).
Table 12.1 Observed ventral light output for four squid held at different training
intensities of downwelling irradiance (“relative light value”).
(From Young & Roper 1977.)