visible colors, fades away first. Blue
light, which has the shortest wave-
length in the visible light spectrum,
travels farthest through the water.
So, a red flish appears black in
pitch-black water because red light
does not reach these depths, and
the fish absorbs the other colors of
the visible spectrum.
At 20 meters (65 feet), red light
does not penetrate and, at this
depth, a red fish absorbs all wave-
lengths of color and is very difficult,
if not impossible, to see (Fig. 1).
All objects that are not transpar-
ent or translucent either absorb or
reflect nearly all of the light that
strikes them. When struck by white
light (containing all colors), a red
fish reflects red light and absorbs all
other colors. White objects appear
white because they reflect all colors
of light in the visible spectrum.
Black objects appear black because
they absorb all colors of light.
Black fish also do not reflect any
light, because they absorb all colors
of the visible light spectrum. So,
in the twilight zone, many fish are
either black or red. At these depths,
these animals are not visible, so they
predominate, compared to fish of
Lighting up under
Other sea creatures use their light to escape
predators. Instead of making themselves
invisible, they flash their lights at would-be
predators. How does it work? Siphonophores,
Some animals use their light to hunt. The
female anglerfish, a fish with a fierce-look-
ing face, dangles a light organ on top of its
head that acts as a searchlight. It uses it
to signal to a mate and to attract prey. The
light organ contains millions of bacteria
that emit blue-green light.
All of these marine animals emit light
by a process called bioluminescence,
in which light is produced by a chemical
reaction between two substances. The
light-producing substance is called a
luciferin. The other is an enzyme called a luciferase. An enzyme is a molecule produced
by a living organism that acts as a catalyst
to increase the rate of a chemical reaction
in that organism.
Bioluminescence occurs when luciferin and
oxygen react together in the presence of lucif-
erase, leading to an oxidized luciferin, along
with carbon dioxide and light (Fig. 2). This
remarkable reaction produces light:
luciferin + O2
➔ oxidized luciferin + CO2 + light
The light comes from an excited inter-
mediate state resulting from the reaction of
luciferin with oxygen. This intermediate state
contains excited electrons, which go back to
their original energy levels when the interme-
diate state “de-excites” by releasing energy in
the form of light. It ultimately leads to oxidized
luciferin and carbon dioxide.
Figure 2. In the presence of the enzyme
luciferase, which catalyzes the reaction, a
luciferin such as coelenterazine, reacts with
oxygen (O2) to form an intermediate compound
that decomposes to form an excited state of the
oxidized luciferin, in this case coelenteramide,
which emits a photon of blue light.
shark (below) uses
its sharp teeth
(left) to remove a
chunk of flesh
from its prey.
Figure 1. In the ocean, the light quickly fades as it goes
deeper under water, but light of certain wavelengths fades
away before others. Infrared and ultraviolet light (left and
right, respectively) fade away first, and among the light
colors in the visible spectrum, red penetrates the least and
blue penetrates the most.
Photos on left page: Top center: A female
anglerfish. Bottom right: A yellow and blue
jellyfish. Bottom left; A siphonophore, a
floating colony of polyps (saclike animals)
that are transparent or lightly colored.
(No light reflected) Blue
of the sea
a luciferin found in crystal jellyfish
coelenteramide, an oxyluciferin