>Newsletters>March 2005>Diving with Hammerhead Sharks

Layang Layang Diving Newsletter

Scalloped Hammer Head Sharks

One of the most wished-for underwater dive sightings has to be the unmistakable form of the mysterious hammerhead shark. Better still is the awe-inspiring moment when you realize you are staring at a vast school of hammerheads all weaving there way across your field of vision...

The hammer seems to be used for gathering data, hence the swinging of the sharks head as it swims, yet it prevents 3 dimensional vision. Whatever the pros and cons, the advantages of the hammer must outweigh the drawbacks, since hammerhead sharks are widespread and abundant - particularly where deep sea meets islands and sea-mounts, such as the Malaysia diving mecca of Borneo, Layang Layang, and the Komodo National Park in Indonesia. This abundance is proof of their success in the evolutionary arms race.

Scalloped Hammerheads Fact Sheet

Family name: Sphymidae
Order name: Lewini
Common name: Scalloped hammerhead shark
Scientific name: Sphymidae lewini

Scalloped hammerheads average about 10 feet (3 metres) in length for mature adults. Its name is derived from the notches or "Scallops" that appear along the front edge of the hammer. These are missing in other hammerheads. Another typical characteristic of the scalloped hammerhead shark is the free end tip of the second dorsal fin which nearly reaches the tail fin.

Distinguishing Features

Where you find scalloped hammerhead sharks you may also come across 3 other species of hammerheads which can be distinguished quite easily by appearance, mostly on the basis of the difference in their hammers.

Great hammerhead sharks are one of the largest flesh-eating fish in the world and can reach up to 7 metres in length. Its hammer lacks distinctive scallops on its T-shaped head which has a single central notch. The smooth hammerhead, has a smooth edge to its flat head and whitefin hammerheads, found only off the Ivory Coast have, believe it or not, white fins.

Feeding Habits

When the feeding habits of the scalloped hammerhead shark are examined then the uses for its hammer begin to emerge. The Hammer is dotted with small electrical sensors called "The Ampullae of Lorenzini" which, at close range, can detect the weak electrical fields generated by all animals. This helps greatly with feeding on fish such as sardines, herring and mackerels, occasionally also on invertebrates such as octopuses. Large scalloped hammerhead sharks also eat small-sized shark species such as the Atlantic sharpnose shark or the blacktip reef shark.

Great hammerhead sharks are one of the largest flesh-eating fish in the world and can reach up to 7 metres in length. Its hammer lacks distinctive scallops on its T-shaped head which has 1 central notch. The smooth hammerhead, has a smooth edge to its flat head and whitefin hammerheads, found only off the Ivory Coast have, believe it or not, white fins.

Incredibly, they also use this important sensory ability to detect magnetic fields underwater, both from the North and South Poles and those created by volcanic activity. The result of this is a type of 'highway' system on the ocean floor - a natural GPS, shark-style!

Generally speaking humans have nothing to fear from hammerheads although on rare occasions larger sharks have attacked people. While you shouldn't agitate any marine life, including sharks, it is some comfort that a hammerhead's uncommonly small mouth is more suited to eating fishes than people! The great hammerhead shark is the only one that is considered at all dangerous to humans.

Reproduction

Mating is one of the reasons that scalloped hammerhead sharks form such enormous shoals. Females position themselves in the middle of the shoal where males know that, size being important to scalloped hammerheads, the larger and more desirable females will be at the centre. Females outnumber males 6 to 1 so the organization of the shoal allows the males to find big healthy females in the centre of the shoal that can bear more young than the less robust females that keep to the shoal's fringes.

In the evening, amorous couples pair off as the shoals disperse and the mating begins. Tenderness is not really at the forefront of the males mind during the act. The male wraps himself around the female grasping hold of her with his teeth. The female sharks then bear these visible mementos of intimacy on their sides for years.

Pregnancy lasts between 9 and 10 months. Around the March to August period, females will travel to shallow, protected waters to give birth. The pups (normally between 15 and 30 of them) are born live which is unusual for sharks most of whom lay eggs.

Predation

Larger sharks will prey on small or injured scalloped hammerheads, while there are no major predators of the adults of this species.

Distribution

Hammerheads, particularly the scalloped variety are found in all tropical and warm temperate seas, from coastal areas near continents to oceanic islands far offshore.

As a general rule single sightings tend to occur more frequently near to the shore whereas oceanic islands and sea-mounts are really where you want to be to experience a school in all their glory. The coast of Malaysian Borneo which features oceanic islands such as Sipadan and deep water atolls such as Layang Layang are among the best locations for sightings. Komodo is also an area where nutrient-rich deep sea upwellings mean big pelagics such as scalloped hammerheads are regular visitors.

Dangers posed by man

Without much natural predation in the oceans, the greatest threat to hammerhead sharks comes from the fishing lines and nets dangled from above. The schooling pattern makes them easy prey for fishermen targeting large catches.

The practice of shark-finning to meet demand for shark fin soup in places like China, Singapore, Taiwan and Malaysia, accounts for most commercially caught hammerheads. You can do your bit by finding out more and signing up to join the Stop Shark-finning Campaign at www.sharktrust.org.

Are You Interested?

Want to be the diver in those photographs? You too can go diving with hammerhead sharks. Maybe you will be lucky enough to witness a huge shoal of them - generations of hammerheads cruising past you underwater!

Dive The World recommends Layang Layang, Borneo in Malaysia as a top spot for diving with these and other wonderful creatures of the sea. Scalloped hammerhead sharks are also regularly sighted in Komodo, where the roll-call of highlights is simply too long to list.