The Scalloped Hammerhead Shark

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by Marine Biologist, Daniele Zanoni

ID:
Size range :
Weight range :
Life span :
Status :
1.5 to 4.3 m
70 to 150 kg
20 to 30 years in the wild
endangered

There are nine known species of Hammerhead Sharks in the world and the most common one in the Red Sea is the Scalloped Hammerhead Shark (Sphyrna Lewini). They all belong to a family called Sphyrnidae (from Greek sphyra meaning hammer).

They are a coastal pelagic species occurring in continental and insular shelves, ranging from about 46° of latitude North to 36° of latitude South. During the day they can be found closer to the coast and in shallow water, while at night they hunt further off shore and deeper (up to about 300 m).
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This shark cannot pump water over the gills, so it needs to swim all its life in order to be able to breathe. Needless to say that if the shark is accidentally caught in fishing net, it will die by suffocation.

The main diet consists of fish such as herring, sardines, mackerels and sting rays (probably the most favourite food), and cephalopods (e.g. squids and octopi). Big adults have been reported to have eaten smaller sharks like the black tip shark (Carcharhinus melanopterus). The most common technique to find food is bottom hunting during the night.

Scalloped Hammerhead Sharks are not considered dangerous and are not normally aggressive towards humans.

Body Anatomy

This is probably the most curious shaped shark. Evolution made a great job with this creature. The hammer is supposed to have developed to increase the sight angle. With the eyes on the extremities of the hammer, the shark has a full 360° sight. Nostrils are also found near the eyes allowing the shark to detect chemical gradients and then find the source (which is normally a prey item).

Since this is one of the most negatively buoyant sharks, the hammer provides lift and stability during swimming, giving the shark great manoeuvrability. Special organs called ampullae of Lorenzini are located on the hammer and hence spread over a large area. These organs are electroreceptory sensory pores that will allow the shark to detect electrical impulses produced by the nervous system of its prey. It is one of the most sensitive receptors in the animal kingdom, able to detect half a billionth of a volt, meaning that the shark is able to detect prey even when the water visibility is highly reduced and even if the prey are under the sand or sediment.

The tail is highly asymmetrical with a terminal notch well marked on the dorsal lobe and a small but marked ventral lobe. Some hammerhead sharks have a darker dorsal skin than others due to shallow water swimming and therefore sun tanning. A nictitating membrane protects the shark's eyes during fights with prey or other animals.

These sharks can also feel the earth magnetic field and use it to navigate and/or to migrate.

Reproduction

Hammerhead Sharks are viviparous: young develop inside the mother's body in a yolk sac and then in a placental sac. Females give birth to live young. The litter varies from 4 to 40 young with a gestation period of 10 to 12 months. The young will stay together with some adults and they can form schools from 100 to more than a 1000 individuals.

All male sharks have a pair of claspers formed from the posterior of their pelvic fins. This organ is introduced into the female's cloaca to transfer the semen.

Some Scalloped Hammerhead Shark clans are matriarchal, which means led by females. The dominant ones will bite the younger ones in the nape region to show their dominance and hence getting the best males during mating.

In May 2007 a species of hammerhead shark reproduced in captivity by parthenogenesis, which is producing off spring without paternal genetic material.

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This page contains a single entry by Bryony published on May 15, 2010 1:52 PM.

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