Scalloped Hammerhead, Sphyrna lewini (Griffin & Smith, 1834)
The Scalloped Hammerhead sometimes forms large migratory schools, dispersing at night to feed in deeper waters. The adult females are believed to live in deep water and only move onto the continental shelf to mate and when giving birth. After a period of 9-10 months they give birth to an average litter of 17 pups.
What do Scalloped Hammerheads look like?
3D interactive model of a Scalloped Hammerhead
The Scalloped Hammerhead has an undulating scalloped margin to the front of the head with an indentation medially. It has a low second dorsal fin and a relatively straight posterior pelvic fin margin.
The species is brownish-grey to olive or bronze above and pale below. Adults have dusky pectoral fin tips, but no other markings. Juveniles have dark pectoral, lower caudal and second dorsal fin tips.
Where do Scalloped Hammerheads live?
The species occurs in coastal and offshore waters down to 1000m Juveniles mainly in shallow inshore areas, subadults in deeper waters and adults further offshore and deep sea mounts.
Scalloped Hammerheads occur in most tropical and warm temperate waters worldwide.
In Australia it is known from about Geographe Bay in north-western Western Australia, around the tropical north and south to the Sydney, New South Wales.
The map below shows the Australian distribution of the species based on public sightings and specimens in Australian Museums. Source: Atlas of Living Australia.
What do Scalloped Hammerheads eat and how do mate?
Feeding and diet
Often seen in large schools that aggregate around seamounts and offshore islands during the day, then feed at night alone or in small groups. Females have been known to move up to 700km between islands in the eastern Pacific. Their diet consists mostly of teleost fishes and cephalopods.
Australian specimens of the Scalloped Hammerhead are born at approximately 45-50cm and males mature at ~140-160cm TL while females mature at ~200-220cm TL. They produce up to 17 pups per litter after a gestation period of up to 9-10 months.
How have Scalloped Hammerheads adapted and are they dangerous?
Behaviours and adaptations
The Scalloped Hammerhead sometimes forms large migratory schools, dispersing at night to feed in deeper waters.
Sharks can detect weak electrical fields using electrical sense organs called the ampullae of Lorenzini. These are connected to the exterior via pores in the shark's snout. This ability allows some species to detect the electrical cues that emanate from prey animals and even the weak electrical currents flowing through the earth's magnetic fields. Some species can detect voltages as low one millionth of a volt.
Hammerheads possess the largest and most complex brain of all sharks (Mantas and Devil rays have the largest brain of all Elasmobranchs). Large to process a diverse array of sensory information from the lateral line and electrical and magnetic fields.
Eye and optic nerve
Hammerhead Sharks have eyes on the ends of the hammer shaped head which give the shark excellent binocular vision. A Hammerhead can improve its stereoscopic vision even further by rotating its eyes and sweeping its head from side to side allowing for almost 360o vision.
Smell and taste are extremely important to a shark. The nostrils have highly sensitive membranes which can pick up even the trace of amino acids in the water. Large olfactory organs in the brain process this information and enable the shark to zero in on the source of a scent by swimming up current. They also use their olfactory system to find mates by detecting pheromones and it may even help guide transoceanic migrations and be a navigational aid.
Electrosensing is a most extraordinary shark sense and can detect tiny electric fields given off by living animals, inanimate objects and even water over the earths magnetic fields. Using the special jelly filled sensory organs known as ampullae of Lorenzini, sharks and Hammerheads especially, can detect prey at close quarters (up to 50cm) even when hidden.
Hammerheads, with their large broad wing-like heads, have more ampullae than any other group of sharks. This allows them to quickly and accurately locate, prey buried in the sand. A Hammerheads ability to detect electric fields is also used to orientate sharks within the earths magnetic fields, allowing for cross ocean migrations. Smooth Hammerheads, Sphyrna zygaena, use this sense to follow ‘magnetic highways’ along the ocean floor, linking their day time aggregation sites and night time nocturnal feeding sites.
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Danger to humans
It is considered a potentially dangerous species that has occasionally displayed its threat posture to scuba divers who have approached too closely (Thomson et al, 1979).
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- Allen, G.R. & R. Swainston. 1988. The Marine Fishes of North-Western Australia. A Field Guide for Anglers and Divers. Western Australian Museum. Pp. 201.
- Bray, D.J. 2017, Sphyrna lewini in Fishes of Australia, accessed 05 Aug 2022, https://fishesofaustralia.net.au/home/species/1968
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- Ebert, D.A., Fowler, S., Compagno, L., 2016. Sharks of the World, Wild Nature Press, pages 1-528Hutchins, B. & R. Swainston. 1986. Sea Fishes of Southern Australia. Complete Field Guide for Anglers and Divers. Swainston Publishing. Pp. 180.
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- Last, P.R. & J.D. Stevens. 1994 Sharks and Rays of Australia. CSIRO. Pp. 513.
- Randall, J.E., Allen, G.R. & R.C. Steene. 1997. Fishes of the Great Barrier Reef and Coral Sea. Crawford House Press. Pp. 557.
- Stevens, J.D. 1987. Sharks. Golden Press. Pp.240.
- Thompson, D.A., Findley, L.T. & A.N. Kerstitch. 1979. Reef Fishes of the Sea of Cortez. John Wiley & Sons. Pp. 302.