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Hydrodynamic Function of the Head in the Hammerhead Sharks (Elasmobranchii: Sphyrnidae)

Kazuhiro Nakaya
Copeia
Vol. 1995, No. 2 (May 3, 1995), pp. 330-336
DOI: 10.2307/1446895
Stable URL: http://www.jstor.org/stable/1446895
Page Count: 7
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Hydrodynamic Function of the Head in the Hammerhead Sharks (Elasmobranchii: Sphyrnidae)
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Abstract

The hammerhead sharks (family Sphyrnidae) have a unique head, where orbital and olfactory regions are laterally expanded. Hydrodynamic functions of this lateral expansion, or cephalofoil, are discussed herein from morphological/ anatomical and experimental aspects. Hammerhead and other carcharhiniform sharks have similar epaxial body musculature throughout head region, but their hypaxial musculature is quite different in size and position. Hypaxial musculature is well developed, occupying ventral side of the vertebral column in the hammerhead sharks; but in other carcharhiniform sharks, the hypaxial musculature is small, limited to the lateral aspect of the vertebral column. The vertebral column is completely surrounded by the epaxial and hypaxial musculature in the hammerhead sharks, but ventral side of the vertebral column in the other carcharhiniform sharks is free from muscle. Epaxial and hypaxial musculature were electrically stimulated to examine the resultant head movements. The inference from the morphology and these experiments revealed that the hammerhead sharks could both elevate and depress their head to a considerable degree, whereas other carcharhiniform sharks could only elevate their head. The presence of a fairly movable plate at the anteriormost part of the body was considered to create large hydrodynamic moments. The present results, together with observation of their swimming behavior, strongly suggest that the hammerhead sharks utilize their head for hydrodynamic purposes in various ways.

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