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of coral and algae. A full ten centimeters long, it displayed exuberant clusters
of spots that made it look like a glass of pink champagne.
Why are the nudibranchs so colorful? Like the colors of the fi re urchins
their bright colors have obviously not evolved for sexual attraction, since
the nudibranchs cannot see each other. They can only sense the presence
of potential mates chemically, by using a pair of feathery olfactory organs
called rhinophores. ( You can see the rhinophores at the nudibranch's head
near the bottom of Figure 17. At its rear, near the top of the picture, a cluster
of naked gills adorns the nudibranch's body like a fl ower.)
It is clear from many experiments that the colors and patterns of the
nudibranchs are actually sending warnings to other species that might
otherwise be tempted to eat them. These sea snails store toxins collected
from the small organisms that they eat, making them highly poisonous.
Like the showy colors and patterns of some butterfl ies, the colors of the
nudibranchs have evolved as a signal to predators that they are nasty-tast-
ing and dangerous. 7
Of course, this warning coloration will only succeed if the nudibranchs'
predators exhibit some degree of sophistication. The animals that prey on
the nudibranchs must see them clearly enough to detect their warning col-
ors and patterns, and must also be smart enough to be able to recall previ-
ous unpleasant encounters with similar nudibranchs. It is likely that in much
earlier times, when the world was patrolled by more stupid and forgetful
predators with poorer vision, animals like the nudibranchs that depended on
warning coloration would not have evolved. The evolution of nudibranchs
in their full glory depended on the emergence of smarter predators from that
early stupidworld. 8
Other groups of mollusks have moved far beyond the clams and nudi-
branchs in sophistication. They have been able to harness both sight and
movement to aid in their hunt for prey and for mates.
On a wide stretch of nearby sea bottom a tiny cuttlefi sh scoots along. It
raises its stumpy arms cautiously as it pauses on the sandy plain.
Like octopuses, cuttlefi sh have eight grasping arms, but they also have
two longer, extremely prehensile tentacles. Even schools of swift silvery fi sh
are not safe from a cuttlefi sh as it hunts. Such fi sh tend to be invisible to most
 
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