How are shark pups born?

After the brief courtship and one-night stand with the male, the female shark is left to carry the fertilized eggs in her oviduct, or womb. Gestation periods for sharks vary, from about five to six months to two years [sources: Carrier, Oregon Coast Aquarium]. The spiny dogfish shark has the longest gestation period, not just of sharks, but of all vertebrates, at 24 months [source: Bester].

Here's a rundown on the three different ways sharks swim into the world:

Oviparity: Oviparous sharks lay eggs, which are protected by an egg case. These egg cases are sometimes called "mermaid's purses." About one-third of shark species are oviparous [source: Australian Marine Conservation Society]. The female shark deposits the egg cases in the sea so that they'll be safe from predators. This represents her last real act as a parent. Horn sharks, for example, deposit their egg cases in rock crevices, where they harden into twisted spirals that are difficult to remove (although snails and seals have been known to break the shell). Port Jackson sharks do the same thing, carrying the egg cases in their mouth until they find a safe spot. That's about the extent of an oviparous shark's parental commitment, though. The embryo is nourished by the yolk in the egg sac and chews itself out when it is fully developed.

Viviparity: Viviparous sharks carry the embryos through the entire gestation period and give birth to live shark pups, similar to how mammals give birth. Embryos are attached to their mother with a yolk sac placenta, which is how they receive their nutrition. In some species, females also secrete uterine milk, which provides more nutrients to the yolk sac. Viviparity is considered the most advanced method of reproduction and is practiced by bigger species, such as blue and hammerhead sharks. About one-third of sharks are viviparous [source: Australian Marine Conservation Society].

Ovoviviparity: Most sharks are ovoviviparous; they combine the first two methods. They produce eggs, but instead of hatching outside the body as in oviparity, the eggs are carried within the female for the gestation period. When the egg hatches, the shark pup continues developing inside the female until it's born. For the first part of the gestation period, the embryos receive nourishment from their yolk sac, and once hatched, the lining of the uterus likely provides uterine milk or some other nutritious fluid. Because there is never a direct connection between the embryo and the mother, ovoviviparity is also sometimes known as aplacental viviparity.

In some ovoviviparous species, embryos get additional nutrition from eating their brothers and sisters in the womb. Learn about this shark-eat-shark world on the next page.

Ovoviviparous shark pups depend on yolk for nutrition, and when an embryo has expended its own yolk sac, it turns to the eggs around it. Some species practice intrauterine cannibalism, or eating the other fertilized or unfertilized eggs in the womb.

The best-known intrauterine cannibal is the sand tiger shark. Although the sand tiger shark has two uteri and produces many eggs, each litter yields just two pups -- one from each uterus. That's because as the sharks develop their embryonic teeth, they start to eat the other embryos, killing their unborn brothers and sisters, as well as the unfertilized eggs. It's survival of the fittest in the womb, until only one shark remains. Because of their pre-birth diet, sand tiger pups enter the world bigger than other pups; they measure approximately three feet (one meter) long [source: National Aquarium].

The sand tiger's cannibalism is known as adelphophagy, which literally means "eating one's brother" [source: Martin]. But other sharks practice cannibalism as well, albeit in a slightly more subdued form known as oophagy, which is the eating of eggs that haven't been fertilized. Approximately 14 species of sharks are thought to practice some form of intrauterine cannibalism [source: Martin].

Frank Greenaway/Dorling Kindersley/Getty Images

The swell shark as an embryo...and as a two-month-old pup.­

The number of shark pups in a litter varies among species; the sand tiger shark gives birth to one or two, while the viviparous blue shark has been known to give birth to 134 pups in one litter [source: Cooper]. The whale shark has given birth to 300, but such high numbers are rare [sources: Conrath, Greven]. But whether hatched from an egg or born live, shark pups emerge as miniature versions of the sharks they will become.

Shark pups are also very independent, and those that are born live swim away from their mothers as soon as they're born, perhaps to avoid being eaten. Even the tiniest sharks face the world on their own. Shark pups don't receive any further nourishment or support from their parents. It's up to the pup to find food and evade predators.

A shark pup's success in life is largely determined by its size at birth and whether the female shark has used a nursery area, or a shallow part of the sea with fewer predators than the open sea. Some shark species grow very slowly, putting them in danger of being eaten by bigger sharks for longer. If few pups survive to maturity and reproduce themselves, shark species could be in danger, particularly those subject to other pressures, such as fishing.

For these reasons, scientists keep trying to learn more about how shark pups are born. Knowing which species have infrequent mating periods, long gestation periods, low litter counts or slow growth will aid efforts in conservation and protection.

Scientists are currently working to develop an artificial shark womb. In the case of the sand tiger shark, a low birth rate in combination with overfishing is causing the sand tiger shark to disappear. When scientists determine exactly how to mimic the womb environment, the plan is to catch pregnant sand tiger sharks, remove embryos and insert them into the test tube wombs for the gestation period. Then, the shark pups would be released back into the wild [source: Bartlett].

I have this trick that I use with my pregnant friends ever since I wrote this article: If they're concerned about the birthing process, I explain how shark pups are born. The process is so violent from beginning to end that delivering a 9-pound baby suddenly doesn't seem very stressful.

Take, for example, conception. Sharks don't mate in pairs, so male sharks simply bite a female shark to hold her in place while they copulate, and then they disappear. A female shark may carry several dozen embryos or eggs within her uteri, but during gestation, the ones that get teeth first tend to eat the others. And then, after gestation, a period that can last up to two years for some species, mother sharks watch as their new pups swim away as soon as they're born -- perhaps because they're trying to avoid being eaten by their mother.

See what I mean? The next time a pregnant woman is worried about swollen feet or nighttime feedings, just remind her that at least human babies don't practice cannibalism in the womb or swim away from their mothers because they fear she's a predator.

Sources

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