The Digital Fish Library
The smooth hammerhead, Sphyrna zygaena (Linnaeus 1758), is part of a unique family of the Class Chondrichthyes (cartilaginous fishes) called Sphyrnidae, comprised of the Bonnethead, Hammerhead, and Scoophead sharks across 2 genera (Eusphyra and Sphyrna) and 8 species (Compagno 1999). As their common names suggest, members of Sphyrnidae are distinguishable by hammer-like, dorso-ventrally flattened lateral expansions of the head (termed “cephalofoil”) (Compagno 1984, 1998), with wide set eyes and elongated nasal openings. The hammer-shaped head of a smooth hammerhead is noticeably wide laterally (eye-to-eye) but relatively short longitudinally (26-29% of total width) in comparison to other members of Sphyrnidae (Compagno 1984). Its coloration is typically olive, light grey, or grey-brown above, with a white underside (Compagno 1984; Compagno et al. 1989).
Biology and Ecology
The smooth hammerhead shark, like other members of Sphyrnidae, reproduces via placental viviparity (Compagno 1984; Dulvy and Reynolds 1997), wherein embryos receive nutrients directly from the mother through a placental blood vessel link (Dulvy and Reynolds 1997). S. zygaena produces approximately 29-37 pups per litter (Compagno 1984; Muus and Nielsen 1999), which are between 50-61cm long at birth. These species reach sexual maturity between 210-240cm in length (Compagno 1984), but can grow to greater lengths, with the largest reported specimen at 500cm (♂) (Muus and Nielsen 1999). The smooth hammerhead is circumglobally distributed in amphitemperate and tropical waters, occurring in the Eastern Pacific (California to Chile), the Indo-Pacific (South Africa to Sri Lanka; southern Siberia to Vietnam), Western Atlantic (Canada to the Virgin Islands; Brazil to Argentina) and Eastern Atlantic (British Isles to Côte d'Ivoire, including the Mediterranean), as well as southern Australia, New Zealand, and Hawaii (Compagno 1984, 1998). They are often coastally associated but can occur well offshore to depths of up to 200m, migrating northwards in large aggregations in the summer months (Compagno 1998). These species are fairly versatile feeders, though it has been noted smaller sharks, skates, and rays are their preferred diet items (Compagno 1984). A recent study on gut contents in over 200 stomachs of S. zygaena,, however, indicated the primary components of their diet to be cephalopods (68.9%) and teleost (bony) fishes (29.8%), with smaller amounts of other chondrichthyans and crustaceans recorded (Cortés 1999).
Unique Features of the "Hammer"
The unique hammer-shaped cephalofoil (head) of the sphyrnids has raised several questions as to its function, and is still not fully understood. The evolutionary success of this distinguishable diagnostic feature implies a morphological or functional advantage, and several hypotheses have been posed to describe not only why the cephalofoil came to be, but why it has persisted and diversified for over 20 million years. The majority of tested hypotheses suggest a sensory and/or prey capture advantage, ranging from enhanced stereo-olfaction and olfactory acuity (Hasler 1957; Kajiura et al. 2005), maximization of the electroreceptive search area (Compagno 1984; Kajiura 2001; Kajiura and Holland 2002), provision of hydrodynamic lift (Aleev 1969; Nakaya 1995; Bertram et al. 2007) and enhanced maneuvering capabilities (Springer 1967; Kajiura et al. 2003). Further hypotheses have been proposed, though still remain untested, suggesting the laterally-expanded cephalofoil may enhance anterior binocular vision (Compagno 1984), provide for an increase in the sampling area for the lateral line canals (Compagno 1984), and may aid in prey handling capabilities (Strong et al. 1990). Though advances have been made towards elucidating the adaptive significance of this unique characteristic, the function of the cephalofoil cannot yet be fully explained.
Neuroanatomy of a Smooth Hammerhead Shark
In addition to the distinguishable head shape of these species, features of their brain organization stand out amongst other chondrichthyans as well. They have one of the largest brains of all sharks (Yopak et al. 2007), a trait previously correlated with habitat complexity (Bauchot et al. 1977; Northcutt 1978, 1989) as well as the complex social behaviors (i.e., schooling, dominance hierarchies, courtship behavior) often prevalent in species that live in complex habitats (Kotrschal et al. 1998). Sphyrnids, as well as species of the genus Carcharhinus, often live in coastal and reef-associated habitats and are considered to be social animals (Springer 1967; Myrberg and Gruber 1974; Klimley 1985), which both aggregate and form true schools. The relative size of the forebrain, or telencephalon, of the smooth hammerhead (and indeed other members of Sphyrna) is noticeably large in comparison to
other chondrichthyans, occupying over 58% of the total brain mass (Yopak et al. 2007). A relatively enlarged forebrain has also been associated with complex environments in sharks (Yopak et al. 2007), a situation found in many other vertebrates (Riddell and Corl 1977; Barton et al. 1995; Huber et al. 1997; Striedter 2005).
The cerebellum, which is a neural structure that evolved first in early elasmobranchs (Butler 2003) and has been carried through vertebrate evolution to humans, is also quite distinguishable in the smooth hammerhead shark. This structure occupies 22% of the brain of S. zygaena, which is relativelylarge when compared to other chondrichthyans (Yopak et al. 2007). This species also has a highly folded (or foliated) cerebellum, a part of the brain implicated in motor control and prey tracking. Surface complexity of the cerebellum is known to vary between species (Northcutt 1977, 1978; Yopak et al. 2007; Yopak and Montgomery in press), and one with high levels of folding, such as in S. zygaena, is a characteristic found in fast-moving or maneuverable sharks that live in 3D environments and hunt agile prey off the substrate.
Smooth Hammerhead Conservation
This species is of recreational and commercial importance: it is utilized fresh, smoked, and dried/salted for human consumption; hides are processed for leather products; liver oil is utilized for vitamins; carcasses are distributed in fishmeal; and fins are processed into soups (Compagno 1984). The finning industry, in particular, is a major cause of overexploitation in many large-bodied shark species, including the smooth hammerhead, though it is often difficult to assess the impact of bycatch and finning on a particular species due to inaccurate catch statistics, generic categorizations, and/or misidentification with other morphologically similar sharks (Abercrombie et al. 2005). The IUCN Red List categorizes the smooth hammerhead shark as being near-threatened and is considered to be at a low risk of extinction (IUCN); although they have a higher fecundity (with 30-40 pups per litter) than other, more threatened shark species (Compagno 1984; Muus and Nielsen 1999) and reproduce annually as opposed to biennially (Castro et al. 1999), they have slow growth rates and late sexual maturity, which contributes to their low resilience. In addition, this species is highly susceptible to overexploitation as they are often aggregating in predictable locations and fished in large numbers (Abercrombie et al. 2005), which emphasizes the need for better conservation management schemes for S. zygaena. Although this species is considered to be dangerous to humans, this is likely a case of misidentification due to their temperate habitats. They have reportedly stolen fish catches from spearfishermen, but are usually not aggressive when there is no bait present (Compagno 1984).