Hammerhead sharks (Elasmobranchii, Carcharhiniformes, Sphyrnidae) are a unique group of cartilaginous fishes that possess a dorso-ventrally compressed and laterally expanded region of the head known as the cephalofoil, formed by lateral expansion and modification of the rostral, olfactory, and optic regions of the chondrocranium (Compagno, 1984, 1988; Haenni, 2001). The degree of lateral expansion is variable generally ranging from 18% of shark total length (TL) in the bonnethead shark, Sphyrna tiburo, to 50% of TL in the winghead shark, Eusphyra blochii. The phylogenetic relationship of hammerhead sharks indicates that the species with the most extreme lateral expansion of the cephalofoil (Eusphyra blochii) is the most basal while the least laterally expanded species (Sphyrna tiburo) is the most derived (Martin, 1993; Lim et al., 2010).
A number of hypotheses have been put forth to explain the evolution of the cephalofoil. The hydrodynamic lift hypothesis states that the cephalofoil provides hydrodynamic lift at the anterior end of the animal, thereby increasing maneuverability (Nakaya, 1995; Driver, 1997). The cephalofoil may also function in prey manipulation (Strong et al., 1990; Chapman and Gruber, 2002). The greater olfactory gradient resolution hypothesis is based on the greater separation distance of the nares in sphyrnid sharks providing enhanced ability to spatially resolve odors on different sides of the head, increased olfactory acuity, and increased sampling area (Johnsen and Teeter, 1985; Kajiura et al., 2005; Gardiner and Atema, 2010). Furthermore, the cephalofoil provides for a greater sampling area than carcharhinid species (Kajiura et al., 2005). A second hypothesis based on sensory biology is the enhanced binocular vision hypothesis (Tester, 1963). This hypothesis states that the placement of the eyes on the laterally expanded cephalofoil enhances binocular vision anteriorly and increases the visual field of sphyrnids (Tester, 1963; Compagno, 1984, 1988). Recent work has shown support for enhanced binocular overlap and a decreased blind area in the most laterally expanded species E. blochii and S. lewini (McComb et al., 2009). The hypothesis that is most commonly proposed concerning the evolution of the sphyrnid cephalofoil is the enhanced electrosensory hypothesis (Compagno, 1984; Kajiura, 2001). The basis for this hypothesis is the idea that the larger the surface area of the cephalofoil is, the greater the surface area that is devoted to electroreception, providing the shark with increased ability to detect and spatially resolve the bioelectric fields of prey (Compagno, 1984, 1988; Kajiura, 2001; Brown, 2002; Kajiura and Holland, 2002). The laterally expanded head also enables sphyrnid sharks to possess ampullary tubules that are longer than those found in carcharhinid sharks (Chu and Wen, 1979) which may confer greater sensitivity to uniform electric fields than their sister taxa (Murray, 1974; Bennett and Clusin, 1978).
The most basal sphyrnid shark is the winghead shark Eusphyra blochii Cuvier, 1817 (Martin, 1993; Lim et al., 2010). Eusphyra blochii is a small, shallow water, viviparous, tropical shark found along the continental shelf from Pakistan to the Philippines and south to Australia. This species of hammerhead is characterized by a laterally expanded wing or arrow-shaped cephalofoil that ranges from 40 to 50% of its TL in width and has narrow, wing-like lateral blades (Compagno, 1984). Eusphyra blochii feed primarily on clupeid fish and penaeid shrimp (Stevens and Lyle, 1989). Young are born at 32 to 47 cm TL and size at maturity for males is 108 cm and size at maturity for females is 120 cm, with a maximum size of 152 cm (Compagno, 1984; Stevens and Lyle, 1989).
Links
Eusphyra blochii page on Wikipedia
Literature
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