Hammerhead sharks

The Hammerhead shark, scientific name Sphyrna Zygaena, is named after the unusual and obvious structure of its head. Laterally extended and flat, the “hammer” section is called a cephalofoil. Proposed functions of the cephalofoil, are sensory reception, prey manipulation and manoeuvring. On the shark’s head, are electroreceptory sensory pores (Ampullae of Lorenzini). These pores lead to sensory tubes, which detect electricity given off by other living creatures. The eyes, mounted on the sides of the hammer-shaped head, allows for 360 degrees on the vertical plane. This means that Hammerheads can see above and below it at all times. The Hammerheads are light grey with a greenish tint, with white undersides. The mouth is disproportionately small, compared to its size. In the day, they form schools of up to 100 individuals, and at night, they become solitary hunters.

Warm tropical waters are the preferred habitat of the Hammerheads, all over the world. During the summer, mass migration occurs, in the search of cooler waters. These sharks may be found in waters ranging from less than 1 meter (3.3 ft), up to a depth of 80 meters (260 ft) offshore.

The Hammerhead’s diet consists of octopus, squid, many species of fish, including other sharks, and stingrays, which are a particular favourite. The sharks are often found swimming at the bottom of the ocean, in order to stalk their prey. The shark’s unique hammerhead, is used to pin the stingray down, and the prey is then eaten while it is weak and in shock. The largest of the Hammerhead group, the Great Hammerhead, is known to engage in cannibalism, eating other Hammerheads, including its own young. Another shark in this group, the Bonnethead, has been found to eat seagrass, along with the usual prey. The seagrass sometimes makes up for half of their stomach contents and they are able to partially digest it. This makes it the only shark that shows potentially omnivorous traits.

Hammerheads range from 0.9 to 6 meters (3.0 to 19.7 ft) in length. Weight ranges from 3 to 580 kg (6.6 to 1278.7 lb). These sharks only reproduce once a year, with the male biting the female violently until she agrees to mate with him. The male transfers sperm to the female with two organs called claspers. A yolk sac sustains the developing young. After depletion of yolk, the depleted yolk sac transforms into a yolk-sac placenta, through which sustenance is delivered by the mother until birth. The pups are born live, 12 – 15 pups at a time; The Great Hammerhead gives birth to 20 – 40 pups at a time. Until the baby sharks are older and large enough to fend for themselves, they huddle together and search out warmer waters. The Bonnethead is the only shark in this group able to reproduce asexually in the case of no males being available. The lifespan of a Hammerhead is between 20 – 30 years.

The status of the Hammerhead group is spread over vulnerable or endangered. This is due to overfishing, and a huge demand for their fins as a delicacy. The fins are cut off when the shark is caught, the shark, often alive, is then thrown back into the water, resulting in death. Fishing and commerce of these sharks are now under licensing and regulation. Hammerheads are documented as only being responsible for 17 unprovoked attacks on humans thus far. The relatively smaller Bonnethead has proven easier to keep in captivity than the larger Hammerheads. The larger sharks need specially adapted, very large tanks, due to injuries occurring to their heads too often. Breeding of the Bonnethead in captivity has been successful at only a handful of facilities.

Absolutely amazing diving trips to dive with Hammerheads are available from the coast of Kwazulu Natal. Protea Banks, the diving site, is situated about 8 km off Shelly Beach. In the warmer months, Hammerheads appear overhead of the divers, in their hundreds.