To achieve a countershading match to the downwelling intensity, the animal must (i)
measure the downwelling light (eyes); (ii) produce variable levels of light ventrally
(photophores); and (iii) be able to determine the ventral output intensity to establish
the match. For requirement (ii), the bioluminescence must not only match the
intensity of downwelling light, it must match its spectrum. However, the output of
luminescing molecules, or luciferins, “discovered” by an animal’s biochemical
evolution may not provide an exact match. For example, the spectrum (Fig. 12.4) of
light produced by the ventral photocytes of the hatchetfish, Argyropelecus aculeatus,
is broader than the downwelling spectrum, particularly toward longer wavelengths. To
perfect the match, the light passes through filters arrayed below the tube of photocytes
before it is emitted through a bank of dispersing reflectors. The final match is
excellent.
Fig. 12.4 (a) Arrangement of light-producing organs in the hatchet fish,
Argyropelecus. Light is generated in photocytes in a tubular organ along the ventral
side. It is emitted ventrally after passing through a conical filter and is spread below
the body by an array of angled mirrors. (b) Above, comparison of the light spectrum
generated in the photocytes (gλ) to the spectrum of downwelling light (solid line);
below, spectrum of emitted light (after passing the filter) in four specimens.
(^) (After Denton 1991.)