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 Atlantic sharpnose shark Rhizoprionodon terraenovae (Richardson, 1836) is closely related to sphyrnid sharks (Compagno, 1984, 1988). Rhizoprionodon terraenovae is a euryhaline and viviparous species that inhabits coastal temperate and tropical waters. It can be found on continental shelves in the western North Atlantic from New Brunswick to Florida and the Gulf of Mexico. Rhizoprionodon terraenovae feed primarily on small bony fishes, shrimp, crabs, and mollusks. The teeth are typically serrated in adults and undifferentiated between sexes (Compagno, 1984). Males typically mature between 65 and 80 cm and females mature between 85 and 90 cm. The maximum size for R. terraenovae is 110 cm, with little disparity between sexes (Compagno, 1984).
Links
Rhizoprionodon terraenovae page from the Florida Museum of Natural History
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