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Distal
tip of the chondrocranium and sagittal section from mid-way
along the length of the chondrocranium of a scalloped hammerhead
shark.
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Frontal
view of the head of a scalloped hammerhead shark.
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Although it has been
hypothesized that the sphyrnid cephalofoil provides lift and
enhanced
maneuverability, this hypothesis has not been empirically tested.
In sagittal section, the sphyrnid cephalofoil resembles an
airplane
wing with a flat ventral surface and a dorsal surface that is
rounded anteriorly and tapers posteriorly to a thin trailing
edge.
This broad planing surface should theoretically provide lift
as the shark swims. However, the lift generated by the cephalofoil
has not been quantified for any sphyrnid shark species. We are
currently testing the hydrodynamic function of the sphyrnid
cephalofoil
by comparing the lift generated by the sphyrnid head morphology
to the lift generated by a representative carcharhinid head
morphology.
In addition to testing
the hydodynamic lift, we are also examining the material properties
of the cephalofoil cartilage. We hypothesize that the chondrocranium
of a sphyrnid shark possesses greater stiffness and strength
than
a non-sphyrnid. A stiffer chondrocranium is necessary to withstand
the hydrodynamic stress forces exerted on the laterally expanded
blades of the cephalofoil when the shark turns at high speed
(when
orienting to prey for example). Because non-sphyrnid sharks are
not subjected to this selective pressure, they are not expected
to possess such a highly strengthened chondrocranium.
The final component
of this study is an examination of the biochemical composition
of the cartilage of various shark species. If the chondrocranium
does exhibit different material properties it will be reflected
in the concentrations of proteoglycan and collagen in the cartilage
matrix.
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