Biology Reference
In-Depth Information
Light-Detection Structures
The biological world contains an amazing range of different types of light-detecting
structure. It is convenient to divide them into two broad categories, termed eyespots
and eyes. These categories are not sharply separated but grade into each other, as
would be expected if the more complex structures have evolved from simpler struc-
tures. Eyespots are defined as structures that detect the intensity and direction of
light, and so allow organisms to move towards or away from the light source, but
are unable to determine the light intensity from more than one direction at the same
time, so they cannot form an image. Eyes are defined as structures that can form
images, often by means of a lens or mirror, but not necessarily so - some eyes form
images using a small hole, in the manner of a pinhole camera.
About two-thirds of the thirty-three metazoan phyla possess light-detecting struc-
tures, but only six of these phyla possess eyes that form images. But these six phyla
contain about 96% of all known animal species alive today. These numbers indi-
cate the high value of possessing eyes. The first animal that developed eyes that
enabled it to catch prey and avoid predators would have a huge advantage over its
competitors. So the appearance of eyes is thought to have triggered an arms race, in
which competing animals were forced to evolve better and better eyes, as did their
prey and predators. The first eyes are suggested to have appeared before 540 mil-
lion years ago, when there is a sudden appearance of many animal phyla in the fossil
record, an event termed the “Cambrian explosion”. The word “Cambrian” is derived
from the Latin for the country of Wales, where the first evidence of this increase in
animal fossils was discovered. One reason for this explosion may be that the arms
race triggered by the appearance of eyes favoured the evolution of hard shells for
protection; such shelled animals fossilize much more readily than the soft-bodied
animals that preceded them.
It is plausible to think that before eyes appeared there were only eyespots, that
enabled organisms to move towards or away from the light, depending on their
life style. So we may gain clues about the likely series of evolutionary events by
comparing the structures and functions of eyespots and eyes in modern organ-
isms. Figure 5.2 illustrates some examples of eyespots. The simplest eyespot known
consists of a cluster of membrane-bound protein-pigment molecules found inside
unicellular eukaryotes such as
Euglena
and
Chlamydomonas
. These organisms are
photosynthetic and so need to find an illuminated environment in order to sur-
vive. The absorption of light by the eyespots triggers a metabolic pathway that
controls the rotation of the flagellum. In
Euglena
the eyespot lies in the cytosol
near the flagellum, but in
Chlamydomonas
it lies inside the single chloroplast. Both
the protein and the pigment in the eyespot of
Euglena
are different from those in
C
hlamydomonas
, suggesting that they evolved independently.
One way to improve the simple eyespot to make it better able to determine the
light direction would be to shield it from one side by means of light-absorbing pig-
ments, so that light can fall on the light-sensitive protein-pigment complex only
from the other side. Precisely this arrangement is found in the larva of the box
jellyfish,
Tripedalia,
where a cup of pigment granules embraces the membranes
