Smooth hammerhead (Sphyrna zygaena)

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Smooth hammerhead fact file

Smooth hammerhead description

KingdomAnimalia
PhylumChordata
ClassChondrichthyes
OrderCarcharhiniformes
FamilySphyrnidae
GenusSphyrna (1)

A member of a large and easily recognisable group of sharks, the smooth hammerhead can be distinguished from the other hammerhead species by the single notch in the centre of its hammer-shaped head. The eyes are located at either end of the hammer, and the particularly arched mouth is in line with the trail edge of the head (3). Olive-grey above and white below, the smooth hammerhead has a tall and sickle-shaped first dorsal fin and plain pectoral fins with black tips (4).

Also known as
common hammerhead, round-headed hammerhead.
Synonyms
Squalus zygaena.
Size
Length: 2.5 – 3.5 m (2)
Maximum length: 5 m (3)
Length at birth: 0.5 m (3)
Maximum weight: 400 kg (3)
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Smooth hammerhead biology

Known for their spectacular schooling behaviour, the smooth hammerhead gathers into these large groups as juveniles, but as adults they occur singly or in small groups. They hunt for bony fish, small sharks and stingrays and are known to be cannibalistic on occasion. When inshore, their diet consists mainly of skates and stingrays. Pinning their prey down with the hammer, the smooth hammerhead bites chunks from the wings of the stingray until it is immobilised. It is not unusual to see hammerheads with the barbs from stingrays implanted into their heads. In deeper waters they may also feed on crustaceans and cephalopods. In northern Europe, the smooth hammerhead eats mainly herring and bass, and in the Mediterranean, it is known to scavenge from longline fisheries. Other large shark species may prey on juvenile smooth hammerheads, but adults have no natural predators (2). The hammer-shaped head is thought to be a mechanism to spread out the ampullae of Lorenzini – sensory organs that detect electric currents, chemicals in the water, and temperature changes (6).

During an 11 – 12 month gestation, the eggs of the smooth hammerhead hatch inside the female’s body. The embryos are nourished by a yolk sac placenta and during the summer months, once the yolk sac has been used, between 20 and 40 young sharks hatch. Measuring just 0.5 m at birth, males and females reach maturity at 2.1 – 2.5 m and 2.7 m respectively (2).

The smooth hammerhead migrates northward during the summer to find cooler water. When overheating, it can be seen swimming slowly at the surface with the dorsal fin out of the water (5).

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Smooth hammerhead range

The smooth hammerhead is widespread in temperate and tropical waters, including the Atlantic, Pacific and Indian Oceans and the Mediterranean and Black Seas. It migrates northward in search of cooler waters for the summer months (3).

See this species on Google Earth.

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Smooth hammerhead habitat

Preferring shallow waters, the smooth hammerhead in usually found inshore over continental shelves, or in bays and estuaries. It may also be found well offshore in deeper waters, particularly when migrating. The smooth hammerhead has also been seen in freshwater habitats such as the Indian River, along the coast of Florida, USA (2).

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Smooth hammerhead status

The smooth hammerhead is classified as Vulnerable (VU) on the IUCN Red List (1).

IUCN Red List species status – Vulnerable

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Smooth hammerhead threats

The smooth hammerhead is rarely fished intentionally, but may be used for its liver oil in vitamins, its fins for shark fin soup in Asia, its hide for leather and the carcass is used as fishmeal (2). It forms part of accidental catch in gillnet, longline and driftnet fishing operations but, as the fins are prized in Asia, it is rarely released alive when caught (3).

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Smooth hammerhead conservation

Shark-fining is banned in US waters, and the smooth hammerhead is a prohibited target species in New Zealand waters. Despite relatively few attacks on humans, many people fear hammerhead sharks and so conservation efforts can be difficult. Currently this species is not immediately threatened with extinction, and whilst there is much research into its ecology and physiology, there is little targeted conservation action (2).

View information on this species at the UNEP World Conservation Monitoring Centre.
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Find out more

For further information on the conservation of sharks and rays see:

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

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Glossary

Cephalopoda
From the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
Crustacea
Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
Dorsal fin
The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
Pectoral fins
In fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
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References

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Image credit

Smooth hammerhead swimming  
Smooth hammerhead swimming

© Andy Murch / Elasmodiver.com

Elasmodiver.com
andymurch1@gmail.com
http://www.elasmodiver.com

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