Smalleye hammerhead  (Sphyrna tudes)

Classified as Vulnerable (VU) on the IUCN Red List (1).

The most prominent feature of the smalleye hammerhead, like other hammerhead species, is its peculiar mallet-shaped head structure, known as a cephalofoil. The cephalofoil of this species has a shallow, but distinct indentation at the front, and is straight at the back (2). The smalleye hammerhead gains its name from its relatively small eyes, situated at the ends of the cephalofoil. The other common name, golden hammerhead, arises from its distinctive golden colouring. However, juveniles are brighter, or more orange, whilst adults are paler; a difference caused by variations in the diet (3).

Occurs in the western Atlantic, from Venezuela to Uruguay. There are old records from the Mediterranean, and off the coast of West Africa and Mississippi, but these may be incorrect and require confirmation (2) (4).

The smalleye hammerhead inhabits waters on continental shelves, over muddy bottoms, at depths of 9 to 40 meters (3).

Relatively little is known about the biology of the smalleye hammerhead. What is known is that this is a viviparous species; embryos develop inside the mother, receiving nutrition via a yolk-sac placenta (2). After ten months of pregnancy, the mother gives birth to a litter of 5 to 12 pups, each measuring about 30 centimetres (3). Birthing takes place in shallow waters from late May to June (3). It is believed that females mate and are fertilised again shortly after giving birth, and thus they can reproduce annually (3). The difference in colour between adults and juveniles is thought to be due to differences in diets; juveniles feed primarily on shrimp, whilst the majority of the adult’s diet consists of small bony fishes and catfish eggs (3). This reportedly timid shark also feeds on swimming crabs, squid, and even preys on the newborn of the closely related scalloped hammerheads (2) (5).

The evolutionary purpose of the unique cephalofoil structure, formed from the front of the expanded skull, is not entirely clear, but several functions have been suggested. It may act to increase swimming efficiency, by adding lift; or it may allow the hammerhead to get a better appreciation of distance due to the widely spaced eyes. As the head swings from side to side in the water whilst swimming, the nostrils sample a greater volume of water than they would do otherwise, which may allow more opportunity to use its sense of smell. Its purpose could be to provide an increased surface area for the electrosensitive ampullae of Lorenzini, enhancing the shark’s ability to detect prey, or it could be a useful tool to pin slippery prey against the seabed (6) (7).

The smalleye hammerhead is caught in coastal fisheries, for human consumption and as by-catch, at levels which may be threatening certain populations. Catches of this shark around Trinidad were reported to have declined between 1985 and 1996, suggesting that the population has been depleted by the fisheries. Declines have also more recently been observed in northern Brazil. The fairly low reproductive rate of the smalleye hammerhead makes it particularly susceptible to over-fishing and it is possible that numbers have declined across its entire range (1)

Despite being considered to be vulnerable to extinction, there are no known conservation measures currently in place for the smalleye hammerhead. Actions such as the collection of catch data, and protection of shallow inshore habitat, have been recommended to protect this peculiar shark (1).

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

To learn more about shark conservation visit:

• Save Our Seas Foundation:
  www.saveourseas.com
• The Shark Trust:
  www.sharktrust.org
• Project Aware:
  www.projectaware.org