TOKYO The movie "Jaws" left bite marks in the psyche of human society worldwide. But the ensuing 40-plus years of fear of the great white shark are only just now beginning to be filled out with details about the biology of this apex predator.
Although little is known about the great white shark, recent studies point to some interesting characteristics.
For one thing, the great white has a warmer internal body temperature than other sharks, which helps it swim faster.
Also, the sharks are born live, and the young are more than 1 meter long when they emerge, ready at the get-go.
Many aquariums around the world feature sharks swimming in captivity, but nowhere is the great white shark on display. In one case in the U.S., a great white was kept alive for several months, but usually these sharks quickly die. Earlier this year, the Okinawa Churaumi Aquarium kept a great white that was accidentally caught in a net, but the animal lived only four days in captivity.
Because the great white shark is so hard to keep alive, scientists have had little opportunity to observe the animal alive. The best they can do is examine sharks that have been caught in nets, and attach transmitters to specimens and track where they go.
The biology and ecology of the great white shark remain largely unknown.
But reproduction is one area where some light has been shed, and it turns out that the great white shark has evolved a unique method of propagation.
"The mother shark gives milk to the young in the womb," explained Keiichi Sato, senior researcher and manager of the Zoological Laboratory at the Okinawa Churashima Research Center.
LONG IN THE OVEN Sharks, like most fish, reproduce through eggs. Most large sharks, such as the great white shark and the whale shark, are ovoviviparous, meaning they incubate and hatch their eggs in the womb and give birth after the young have developed big enough to survive on their own.
That the great white shark is ovoviviparous has been known for a long time. But in the last few years, research has revealed details about how the sharks develop in the womb. For one thing, female great whites are impregnated by a number of males, and a number of eggs are fertilized.
Sato and fellow scientists determined that the mother secretes an oily milk from the walls of the uterus, and this is ingested by the babies in the womb after they have hatched. Some species of rays are known to supply their developing young with milk, but this is the first time a shark has been found to use this strategy.
After the babies have developed enough, the milk diet ends and the mother shark begins releasing unfertilized eggs from her ovaries as a food source. These unfertilized eggs are packed into around 1,000 capsules that float around in the womb for the baby sharks to eat. Their abdomens expand like balloons, and cutting one open reveals a stomach full of egg yolk.
Large carnivorous sharks like the sand tiger and the mako are also ovoviviparous, but in the womb the babies cannibalize their siblings, leaving just the winners of the survival game alive to be born.
The great white shark rears some five to 10 young, which develop in the womb without eating each other until they reach a length of 1.2-1.3m and a weight of 30kg before live birth.
"By the time they are born, they really have no enemies in the sea," Sato explained. The only fish that can eat the young sharks are larger sharks.
Fish born small are easy prey for other fish. That is why most fish adopt a shotgun strategy, laying massive numbers of eggs that hatch into vast clouds of fry. With this approach, some progeny will survive even if most become dinner. The great white shark has the opposite strategy -- it produces just a few eggs, and treats them with the utmost care, raising them internally for a prolonged period of time.
Researchers have captured video of great whites attacking even young seals, which are fatty, highly nutritious creatures. Great whites have also been observed eating large fish. In the waters off Japan, sharks appear to dine on large numbers of squid. "There are many giant squid, and they are rich in nutrients," noted Sato.
HETEROTHERMIC POIKILOTHERMS Scientists have also been learning more about the migratory patterns of the great white shark by attaching tiny cameras and sensors to the fish and studying their movements.
For the past two years, Yuuki Watanabe of the National Institute of Polar Research has been tracking the movements of seven great whites.
Sharks are poikilotherms, meaning they are "cold-blooded" animals whose internal body temperature varies widely. But great whites are unique in that they are also heterothermic, maintaining an internal body temperature that is around 5-15 degrees warmer than the surrounding water.
The warmer temperature enables the muscles to contract more quickly, giving the great white the ability to swim faster.
Recordings from sensors suggest that the sharks swim at an average speed of 7kph and can reach maximum speeds of 30kph. In comparison, fish usually swim at 1-2kph.
Because great white sharks can swim so fast, they can migrate over long distances. "That is useful because it gives them the ability to travel to where the food is during any given season," Watanabe noted.
Great white shark numbers have dwindled as they fall victim to habitat decline, official eradication efforts, sport fishing and policies adopted to prevent attacks on beach-goers.
To save these magnificent animals, the species was put under the protection of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), also known as the Washington Convention, in the 1990s. Little by little, the apex predator is making a comeback.