The smooth hammerhead shark was originally described by Swedish natural historian Karl Linnaeus in 1758 as Squalus zygaena. This name was later changed to Sphyrna zygaena (Linnaeus 1758) which is currently the valid name. The name Sphyrna translates from Greek to the English language "hammer", referring to the hammer-shaped head of this species. Linnaeus, considered the "Father of Taxonomy", created the system of naming species that is still in use today. Synonyms referring to this species include Zygaena malleus Valenciennes 1822, Zygaena vulgaris Cloquet 1830, and Zygaena subarcuata Storer 1848.
There are approximately 10 related species of hammerheads throughout tropical and temperate regions including the bonnethead (Sphyrna tiburo), great hammerhead (Sphyrna mokarran), and scalloped hammerhead (Sphyrna lewini).
English language common names for this shark include smooth hammerhead, common hammerhead, common hammerhead shark, common smooth hammerhead shark, hammerhead, hammerhead shark, round-headed hammerhead, and round-headed hammerhead shark. Other common names include abou bornita (Arabic), awal (Bikol), balagbagan (Tagalog), boat (Marathi), cabeza de martillo (Spanish), cachona (Spanish), cagnole (French), cambeba (Portuguese), carnuda (Spanish), corna (Spanish), cornuda (Portuguese), diarandoye (Wolof), Gemeiner Hammerfisch (German), gladde hammerhai (Dutch), gladde hamerkop (Afrikaans), hamerhaai (Dutch), hammerhaj (Swedish), iskandar (Arabic), jarjur (Arabic), kodosan (Visayan), kurazza (Maltese), leunada (Spanish), marteau (French), martell (Spanish), martelo (Portuguese), martillo (Spanish), pata (Portuguese), pateritza (Greek), pesce martello (Italian), pez martillo (Spanish), requin marteau (French), sarda de cachas (Spanish), shiro-shumokuzame (Japanese), tampugan (Bikol), variocha (Kannada), vasarahai (Finnish), yu palang (Malay), and zygaena (Greek).
Smooth hammerheads are found worldwide in temperate waters. In the western Atlantic Ocean, this species is found from Canada south to the Virgin Islands and from Brazil south to Argentina, while in the eastern Atlantic it occurs from the British Isles south to Côte d'Ivoire, including the Mediterranean Sea. In the western Indian Ocean, the smooth hammerhead occurs off the coasts of South Africa, India, and Sri Lanka. The distribution within the Pacific Ocean includes from Vietnam to Japan and Australia and New Zealand in the west, the Hawaiian Islands in the central region, and California (U.S.), Panama, Galapagos, Ecuador, and Chile in the east.
World distribution map for the smooth hammerhead
Preferring shallow waters less than 65 feet (20 m) in depth, the smooth hammerhead lives close to shore over continental shelves and in inshore waters including bays and estuaries. However this species has been reported in depths from 0-656 feet (0-200 m) including those reported from offshore locations. In some areas, juveniles may be abundant and form large aggregations. Adults occur either singly or in small groups.
During summer months, smooth hammerheads sometime form schools during migrations northward to cooler which is later followed by a return south in the winter. During hot summer weather, adults and juveniles can sometimes be seen swimming along the surface with their dorsal fins exposed.
Hammerhead swimming with dorsal fin exposed during a seasonal migration
On occasion, the smooth hammerhead has been known to enter freshwater habitats including the Indian River along the coast of Florida (US).
© George Burgess
- · Distinctive Features
The smooth hammerhead gets its common names from the large hammer-shaped head. This compressed head is also referred to as a "cephalophoil", allowing for easy distinction from other types of sharks. The cephalophoil is broad and flattened with a broadly rounded unnotched anterior margin. The eyes are located on the outer edges of the cephalophoil with the nostrils also spread far apart. The head is scalloped with a depression opposite each nostril. The ventrally-located mouth is strongly arched. It is thought that the head structure may give the shark some sensory advantages or that perhaps it improves maneuverability or increases lift.
Eye location on the cephalophoil
© George Burgess
View of head from below
source Bigelow and Schroeder (1948)
Within the hammerhead family, several species are differentiated from each other by variations within the cephalophoil. The great hammerhead (S. mokarran) is distinguished by the T-shaped head that has an almost straight front edge and a notch in the center. The scalloped hammerhead (S. lewini) is a smaller species with a rounded anterior margin and notch on the head. The bonnethead (S. tiburo) is much easier to identify with a shovel-shaped head.
Comparison of hammerhead sharks:
A. smooth hammerhead, B. scalloped hammerhead, C. great hammerhead, D. bonnethead
© George Burgess
The smooth hammerhead's back is smooth, lacking a mid-dorsal ridge. The moderately tall first dorsal fin has a rounded apex and is falcate in shape with a free rear tip in front of the origin of the pelvic fins. The origin of this first dorsal is located over the pectoral fin insertions. The low second dorsal fin is shorter than the anal fin, with the free rear tip not extending to the precaudal pit. Pelvic fins are not falcate with straight of slightly concave posterior margins. The pectoral fins have only slightly falcate posterior margins. The anal fin has a deeply notched posterior margin.
© George Burgess
- · Coloration
The body of the smooth hammerhead is dark olive to brownish-gray in color with a white underside. This is in contrast to the brown coloration that is common to many other species of hammerheads. Some individuals have dusky or black edged pectoral fins.
Dentition in the smooth hammerhead, A. upper tooth, B. Lower tooth
source Bigelow and Schroeder (1948) FNWA
There are 13-15 triangular, smooth-edged teeth in each side of the upper jaw sometimes along with a small symphyseal tooth. The lower jaw includes 12-14 smooth or weakly serrated teeth along with a single symphyseal tooth.
Dermal denticles from the smooth hammerhead
source Bigelow and Schroeder (1948) FNWA
Smooth hammerheads have denticles that are densely arranged with w-shaped posterior margins. Denticles are as broad as they are long with small specimens possessing three ridges extending from the center to the posterior edge while large individuals have five to seven ridges extending about halfway back from the anterior margin and three to five teeth along the posterior margin with the middle one the longer than the others.
Smooth hammerheads reach a maximum total length of approximately 13 feet (4 m)
- ·Size, Age, and Growth
The average size of the smooth hammerhead ranges between 8-12 feet (2.5-3.5 m) in length with a maximum total length of 16 feet (5 m) and maximum weight of 880 pounds (400 kg). Although maximum age has yet to be determined for this species, it is believed the smooth hammerhead may have a lifespan of 20 years or longer. At birth, smooth hammerheads measure approximately 20 inches (.5 m) in length. Females reach maturity at approximately 8.7 feet (2.7 m) and males at 7-8 feet (2.1-2.5 cm) in length, depending upon locality.
Smooth hammerheads feed on bony fish and elasmobranchs including stingrays
© George Ryschkewitsch
- · Food Habits
- Primarily a piscivore, the smooth hammerhead feeds on a variety of bony fishes including clupeids and small scombrids as well as elasmobranchs such as smaller sharks (as well as its own species) and stingrays. In fact skates and stingrays make up the majority of its diet in inshore locations. Invertebrate prey includes benthic crustaceans and cephalopods. In northern Europe, this shark feeds on herring and bass, while in North America Spanish mackerel and menhaden are commonly consumed fish. The smooth hammerhead has also been observed scavenging from surface longlines in the Mediterranean Sea.
- · Reproduction
- The smooth hammerhead is viviparous with the eggs hatching inside the body and the embryos nourished by a yolksac placenta. This placenta also tranports oxygen to the embryo and removes wastes. Birth occurs during summer months, resulting in a large litter of 20-40 pups following a 10-11 month gestation period. Pups measure approximately 20 inches (50 cm) in length at birth.
Dusky sharks (Carcharhinus obscurus) prey upon elasmobranchs including smaller sharks
courtesy National Marine Fisheries Service
- · Predators
- Larger sharks will prey on juvenile and subadult smooth hammerheads, with no major predators of the adults of this species.
- · Parasites
- The nematodes Parascarophis sphyrnae and Contracaecum sp. have been reported to parasitize the smooth hammerhead.
Importance to Humans
The smooth hammerhead is taken in the shark fisheries of south Florida and the West Indies and utilized fresh, dried-salted, and smoked for human consumption. However, in most markets, the meat is considered undesirable and not marketed. The liver oil is used in vitamins, fins for soup, hide for leather, and carcasses for fishmeal. This shark has the highest rated fins for the Asian market where sharkfin soup is a delicacy. However, shark finning is banned in U.S. waters.
Scalloped hammerheads (S. lewini) are the only hammerhead species known to school
courtesy U.S. Geological Survey
Danger to Humans
Hammerheads are considered potentially dangerous. According to the International Shark Attack File, there have been 21 unprovoked attacks with 2 resulting in fatalities for all species of the genus Sphyrna. Relatively few attacks have been attributed to this species due to its common occurrence in temperate rather than tropical waters where humans are more likely to enter the water.
Swimming hammerhead during a migration event
In the US, smooth hammerhead sharks are grouped with large coastal species, a group that biologist consider to be most vulnerable to overfishing. It is also taken by gillnet and longline and as bycatch in driftnet fisheries. Mortality is likely to be significant although little data is available on populations and fishing impact. The smooth hammerhead is difficult to identify in high seas fisheries where observers are often not present, resulting in insufficient bycatch data.
Currently, this species is considered to be "Lower Risk/Near Threatened" throughout its range. In waters off New Zealand, the smooth hammerhead is a prohibited target species and is the most abundant shark along the northwest coast. This hammerhead does not appear to be negatively impacted by fishing pressure off the southern coast of Australia. The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.