Hawaiian Marine Life Profiles: Fish

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Common Name:

Hammerhead Shark

Hawaiian Name:

mano kihikihi

Scientific Name:

Sphyrna lewini

Found in Exhibit:

Harbor Plaza, Open Ocean

Of all the sharks in the world, perhaps the easiest one to identify is the hammerhead shark, or mano kihikihi. Hammerheads are found in almost all of the tropical and warmer temperate waters of the world. Eight different species are known and three of these inhabit Hawaii's waters: the scalloped, the smooth, and the great hammerhead sharks. Hammerheads have intrigued the world with their unique body shape and unusual schooling behavior.

It is plain to see how the hammerhead sharks got their name; their body shape resembles a hammer when viewed from above. There are many theories as to why the heads are shaped as they are, but there are no definite answers yet. One theory is that the enlarged head allows the animal to have increased sensory abilities. Sharks rely on numerous senses to find food, navigate through the ocean, reproduce, and other activities. Electroreceptive organs located on their head and throughout their body enable sharks to sense weak electrical fields given off by potential prey. 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, producing a type of 'highway' navigation system on the ocean floor. When these receptors are spread over a greater area, it allows the shark to more accurately detect these electrical and magnetic pulses. Also, their olfactory organs are located farther apart on their head, giving them a greater volume of water to 'sniff' and make it easier to distinguish which direction a scent might be coming from. The actual shape of the head may aid in locomotion of hammerheads, providing lift or possibly a smaller turning radius. Scalloped hammerheads are always seen swinging their heads back and forth, which may maximize the amount of gathered information. The hammer-shaped head may also help the animal trap and hold its prey against the ocean floor.

The scalloped hammerhead, one of the most commonly seen hammerhead sharks in Hawaii, has four lobes on the leading edge of the "hammer", and generally reaches between 5 to 10 feet long. As adults, scalloped hammerheads are usually found in the open ocean, often around seamounts, or outer reef slopes. The smooth hammerhead, named for the lack of scallops, is slightly larger, averaging 13 feet in length. The great hammerhead, reaching 20 feet, is one of the largest flesh-eating fish in the world, but is rarely seen in Hawaiian waters. Though hammerheads are not usually aggressive, they should be considered potentially dangerous.

One of the most fascinating characteristics of scalloped hammerhead sharks is its frequent schooling behavior. It is the only known large shark to participate in this type of behavior. It is surprising that they do school, since the main reason why smaller fish school is to avoid predators by finding safety in numbers, and hammerheads do not have many predators. Another reason why fish would create a school is to surround prey, but most records show that hammerheads separate to feed alone at night. They eat a wide variety of food, including bony fish, rays, other sharks, squid, and octopus. Some fish form schools during breeding season and broadcast sperm and eggs into the water, but hammerheads mate away from the schooling groups.

The schools seem to be more social in composition, and may aid in mate selection. This may be important because scalloped hammerhead males are outnumbered six to one. Therefore, the majority of the sharks in the school are female, with the largest females located at the center. Since there is a direct relationship between the size of the female and the number of offspring, called pups, the males generally head right into the center for the largest female. The male will pick a mate and they will depart to breed in open water. A large female can bear up to 40 pups, while a shark in her first year of sexual maturity may have only 12 pups. Females will travel to shallow, protected waters in the spring and summer months to give birth. Before they are born, the embryos have flexible "hammers" that are bent towards the tails to facilitate birth. The pups are born live (most sharks lay eggs) and are generally about 17 inches long. Juveniles are not generally seen by divers, but are often caught in fishermen's nets, and have been found locally in Ma'alaea and Lahaina harbors.

Early Hawaiians highly revered sharks as both spiritually important and necessary for the health of the ocean. According to Kahu Charles Kauluwehi Maxwell Sr., Hawaiian Cultural Advisor, Hawaiians wisely understood the role of sharks, or mano, in keeping the ocean clean by primarily feeding on the diseased or injured, or recently deceased animals, and did not commonly catch or eat them. Also, many families, or ohana, considered the mano to be their 'aumakua, or sacred guardian, which would protect the members of the ohana. As all animals living in the oceans, sharks are considered the kino lau, the physical representation, of the god Kanaloa, and should be respected in that context.

These facts remain true today: sharks play a crucial role in the health of our oceans and planet, and are still considered the 'aumakua of many families in Hawaii. However their populations are dwindling rapidly all over the world due to the strong international market for shark fins. Hammerheads are especially vulnerable due to their schooling behavior and the predictability of the schools' locations. Shark hunting remains largely unregulated, and if something is not done soon to change this, we may lose important members of the ancient world, and drastically upset the balance of our ocean ecosystem.

PDF: hammerhead.pdf