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Hammerhead
shark maneuvering to bite at an electric dipole.
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Hammerhead
shark biting at a dipole electric field; the voltage equipotentials
are shown in red.
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Enhanced electroreceptive
capabilities may have driven selection for the unique head morphology
of hammerhead sharks. The laterally expanded cephalofoil allows
hammerheads to possess longer ampullae of Lorenzini tubules which
might provide greater sensitivity to weak electric fields. In
addition, by spacing the electrosensory pores over a broader area,
the hammerhead sharks would sample a larger swath of the substratum
and thus increase the probability of prey encounter.
These hypotheses were
tested by comparing the behavioral response of juvenile scalloped
hammerhead sharks to similar sized sandbar sharks that possess
a typical requiem shark head morphology. Both species were exposed
to prey-simulating dipole electric fields and their responses
were recorded and quantified using digital video analysis. The
sharks oriented to, and bit at the electric field as if it were
a natural prey item. The hammerheads did not demonstrate greater
sensitivity to the dipole electric fields but would likely demonstrate
greater sensitivity if exposed to a large, uniform electric field
where the greater ampullary tubule length would provide an advantage.
Although they were not more sensitive, the hammerheads did sample
a much greater area than the sandbar sharks with equivalent spatial
resolution.
Thus, the enhanced
electrosensory hypothesis is supported as a selective advantage
for the evolution of the sphyrnid cephalofoil.
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