PHYSICS

Glaucoma arises from an abnormal increase in fluid pressure
inside the eyeball. The pressure increase can lead to a swelling and
distortion of the lens causing pronounced myopia.
22.3. BIOLUMINESCENCE
The ability to emit light is found in a diverse cross-section of
organisms - bacteria, peridineae, medusae, crustaceans, molluscs,
tunicate, fish, and insects. Bioluminescence is an enzymatically
catalyzed chemiluminescence - luminescence caused by
chemical reactions. There is considerable biochemical diversity in
the systems responsible for bioluminescence. Single cells (photocytes)
to complex glands (photophores) may be used by animals.
Fig. 22.13. Firefly (Pyrophorus),
The most common biochemical process involves the luciferin-luci­from the American tropics, with two
[erase system (as in bioluminescent bacteria). The reaction involves
light organs on its thorax behind
the oxidative breakdown of a high-energy, complex organic molecule
(luciferin) by an enzyme (luciferase) which releases photons
the eyes (Hader, 1987)
of light.
The best-known role of bioluminescence is in communication
among sexual partners (e.g., fireflies). Each species emits a
specific light signal pattern that consists of single flashes or a modulated
emission from 5 to over 40 flashes per second (fig.
bacterium phosphoerum, Pyrocystis,
22.12).
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tJ
~
lO:
=
e
.... =
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~ 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Time, s
Fig. 22.12. Light signals of a male Photinus evanescens from Jamaica:
11 pulses per second with a frequency of 20 Hz
Another important behavioral factor is the timing of signaling
which differs among species. For example, each species of firefly
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has its own time niche. Precise timing increases the chances of
sexual partners finding each other, while a short time period of
activity reduces the risk of being found by predators. Bioluminescence
is an adaptive mechanism utilized by a number of species
which inhabitant dark environments - such as many deep-sea fish
species. At least 700 genera are known to use bioluminescence. Various
explanations of the adaptive advantage conferred by bioluminescence
are: attraction of other animals, which are captured
and consumed; distracting the attention of prey; illumination of
dark surroundings; production of a warning signal; defense of
territory; and sexual recognition.
The ability to produce bioluminescence can be deduced from
the names of luminescent organisms (Hader, 1987): Photo­
Pelagia noctiluca, Metridia
lucens, Watasenia scintillans, Symlectotheutis luminosa, Pyrophorus
(fig. 22.13).
22.4. CAMOUFLAGE
Animals have evolved a diverse array of camouflages, such as:
coloration that resembles their background; countershading,
which makes it possible to destroy the natural patterns of brightness;
disruptive coloration, which produces boundaries with the
highest contrast and generates impressions of false surface relief;
and shadow elimination, which modifies the form or orientation
of the organism.
Some animals can change their skin or cuticle color in a
manner that facilitates maintaining a constant body temperature.
Color alterations occur in a layer of iridophore cells within the
skin, where crystals cause interference reflection and hence iridescence.
In the skin of species adapted to dry environments,
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