About the Cover
Cover
All hammerhead sharks (Elasmobranchii, Sphyrnidae) are characterized by a head morphology that is dorsoventrally compressed and laterally expanded. The resultant broad, winglike head is termed the cephalofoil. The bonnethead shark, Sphyrna tiburo, is an abundant small, coastal hammerhead species with the least lateral expansion of the cephalofoil relative to other members of the family. Moreover, it is unique among elasmobranchs in that its phenotype reveals a sexual dimorphism in head shape. A pronounced difference in head morphology develops between male and female bonnethead sharks when they reach sexual maturity. Although the anterior margin of the cephalofoil of female bonnethead sharks is broadly rounded, the adult male is characterized by a distinct bulge in the median anterior margin of the cephalofoil. This bulge is the result of the elongation of the rostral cartilages that occurs concomitantly with the elongation of the clasper cartilages (components of the male intromittent organs). Because both cartilaginous elements elongate simultaneously at the onset of sexual maturity, it is likely that the same mechanism is responsible. In addition, because all sharks retain an entirely cartilaginous skeleton throughout their ontogeny, it is relatively easy to restructure skeletal elements at any stage.
The cover photo shows the dorsal view of the head of an embryonic male bonnethead shark after clearing and staining. The tissue has been treated with trypsin and hydrogen peroxide to make it transparent, and the cartilaginous skeleton has been stained with alcian blue. This technique allows skeletal elements to be visualized in situ. Note in particular that the cleared and stained specimen facilitates examination of the rostral cartilages. Future investigations of the rostral cartilage elements may reveal that such cephalic sexual dimorphisms are in fact widespread among elasmobranchs.
Credits: Clear and stain preparation by Jennie L. Beltran and photo by Stephen M. Kajiura (Florida Atlantic University); cover layout by Beth Liles (Marine Biological Laboratory).
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Copyright © 2005 by the Marine Biological Laboratory.
