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).
Rhizoprionodon terraenovae page from the Florida Museum of Natural History
Bennett MVL, Clusin WT. 1978. Physiology of the ampulla of Lorenzini, the electroreceptor of elasmobranchs. In: Hodgson ES, Mathewson RF, editors. Sensory Biology of Sharks, Skates, and Rays. Arlington, Virginia: Office of Naval Research. p. 483-505.
Brown BR. 2002. Modeling an electrosensory landscape: behavioral and morphological optimization in elasmobranch prey capture. Journal of Experimental Biology 205:999-1007.
Bush A, Holland K. 2002. Food limitation in a nursery area: estimates of daily ration in juvenile scalloped hammerheads, Sphyrna lewini (Griffith and Smith, 1834) in Kane'ohe Bay, O'ahu, Hawai'i. Journal of Experimental Marine Biology and Ecology 278:157-178.
Chapman DD, Gruber SH. 2002. A further observation of the prey-handling behavior of the great hammerhead shark, Sphyrna mokarran: predation upon the spotted eagle ray, Aetobatus narinari. Bulletin of Marine Science 70:947-952.
Chu YT, Wen MC. 1979. Monograph of fishes of China (No. 2): a study of the lateral-line canal system and that of Lorenzini ampulla and tubules of elasmobranchiate fishes of China. Shanghai: Science and Technology Press.
Clarke TA. 1971. The ecology of the scalloped hammerhead shark, Sphyrna lewini, in Hawaii. Pacific Science 25:133-144.
Compagno LJV. 1984. FAO species catalogue. Vol. 4. Sharks of the world. An annotated and illustrated catalogue of shark species known to data. Part 2. Carcharhiniformes. FAO Fish. Synop.: (125) Vol. 4, Pt. 2.
Compagno LJV. 1988. Sharks of the order Carcharhiniformes. Princeton: Princeton University Press.
Cortés E, Manire CA, Hueter RE. 1996. Diet, feeding habits, and diel feeding chronology of the bonnethead shark, Sphyrna tiburo, in southwest Florida. Bulletin of Marine Science 58:353-367.
Daniels CI. 1967. The distribution, morphology, and innervation of the ampullae of Lorenzini in the hammerhead shark and other species. MS Thesis, University of Hawaii, Honolulu. 42 pp.
Driver KH. 1997. Hydrodynamic properties and ecomorphology of the hammerhead shark (Family Sphyrnidae) cephalofoil. Dissertation, University of California Davis. 159 pp.
Gardiner JM, Atema J. 2010. The function of bilateral odor arrival time differences in olfactory orientation of sharks. Current Biology 20:1187-1191.
Haenni EG. 2001. On the growth, functional morphology, and embryological development of the cephalofoil in the bonnethead shark, Sphyrna tiburo. Dissertation, Clemson University. 253 pp.
Hazin F, Fischer A, Broadhurst M. 2001. Aspects of reproductive biology of the scalloped hammerhead shark, Sphyrna lewini, off northeastern Brazil. Environmental Biology of Fishes 61:151-159.
Johnsen PB, Teeter JH. 1985. Behavioral responses of bonnethead sharks (Sphyrna tiburo) to controlled olfactory stimulation. Marine Behaviour and Physiology 11:283-291.
Kajiura SM. 2001. Head morphology and electrosensory pore distribution of carcharhinid and sphyrnid sharks. Environmental Biology of Fishes 61:125-133.
Kajiura SM. 2003. Electroreception in neonatal bonnethead sharks, Sphyrna tiburo. Marine Biology 143:603-611.
Kajiura SM, Forni JB, Summers AP. 2005. Olfactory morphology of carcharhinid and sphyrnid sharks: does the cephalofoil confer a sensory advantage? Journal of Morphology 264:253-263.
Kajiura SM, Holland KN. 2002. Electroreception in juvenile scalloped hammerhead and sandbar sharks. Journal of Experimental Biology 205:2609-2621.
Klimley AP. 1985. Schooling in Sphyrna lewini, a species with low risk of predation: a non-egalitarian state. Zeitschrift für Tierpsychologie 70:297-319.
Klimley AP. 1987. The determinants of sexual segregation in the scalloped hammerhead shark, Sphyrna lewini. Environmental Biology of Fishes 18:27-40.
Klimley AP. 1993. Highly directional swimming by scalloped hammerhead sharks, Sphyrna lewini, and subsurface irradiance, temperature, bathymetry, and geomagnetic field. Marine Biology 117:1-22.
Lessa RP, Almeida Z. 1998. Feeding habits of the bonnethead shark, Sphyrna tiburo, from Northern Brazil. Cybium 22:383-394.
Lim DD, Motta P, Mara K, Martin AP. 2010. Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 55:572-579.
Lombardi-Carlson LA, Cortés E, Parsons GR, Manire CA. 2003. Latitudinal variation in life-history traits of bonnethead sharks, Sphyrna tiburo, (Carcharhiniformes: Sphyrnidae) from the eastern Gulf of Mexico. Marine and Freshwater Research 54:875-883.
Lowe CG. 1996. Kinematics and critical swimming speed of juvenile scalloped hammerhead sharks. Journal of Experimental Biology 199:2605-2610.
Lowe CG. 2001. Metabolic rates of juvenile scalloped hammerhead sharks (Sphyrna lewini). Marine Biology 139:447-453.
Lowe CG. 2002. Bioenergetics of free-ranging juvenile scalloped hammerhead sharks (Sphyrna lewini) in Kane'ohe Bay, O'ahu, HI. Journal of Experimental Marine Biology and Ecology 278:141-156.
Mara KR, Motta PJ, Huber DR. 2010. Bite force and performance in the durophagous bonnethead shark, Sphyrna tiburo. Journal of Experimental Zoology Part A Ecological Genetics and Physiology 313:95-105.
Martin A. 1993. Hammerhead shark origins. Nature 364:494.
McComb DM, Tricas TC, Kajiura SM. 2009. Enhanced visual fields in hammerhead sharks. Journal of Experimental Biology 212:4010-4018.
Murray RW. 1974. The ampulae of Lorenzini. In: Fessard A, editor. Handbook of sensory physiology. New York: Springer-Verlag.
Nakaya K. 1995. Hydrodynamic function of the head in the hammerhead sharks (Elasmobranchii: Sphyrnidae). Copeia 1995:330-336.
Stevens JD, Lyle JM. 1989. Biology of three hammerhead sharks (Eusphyra blochii, Sphyrna mokarran, and S. lewini) from northern Australia. Australian Journal of Marine and Freshwater Research 40:129-146.
Strong Jr. WR, Snelson FF, Gruber SH. 1990. Hammerhead shark predation on stingrays: an observation of prey handling by Sphyrna mokarran. Copeia 1990:836-840.
Tester AL. 1963. Olfaction, gestation and the common chemical sense in sharks. In: Gilbert PW, editor. Sharks and Survival. Boston: C.C. Heath and Company. p. 255-285.
Wilga CD, Motta PJ. 2000. Durophagy in sharks: feeding mechanics of the hammerhead Sphyrna tiburo. Journal of Experimental Biology 203:2781-2796.