Some shark species give birth to live young, while other species lay eggs. All shark eggs are fertilized internally with the male depositing sperm into the female's reproductive tract.
Approximately 40 percent of shark species, including bamboo sharks, horn sharks, swell sharks and carpet sharks, lay eggs. The female shark lays the eggs inside of a case called a mermaid's purse. The case secures itself to coral or seaweed and provides protection for the eggs until they hatch.
Sharks that give birth to live young nourish the embryos inside the uterus. In most cases, shark embryos remain in the egg and get nutrition from the egg yolk. The young hatch inside the uterus, and the female shark gives birth to live pups. Shark species that give birth in this way include tiger sharks, whale sharks, thresher sharks and dogfish sharks.
In approximately 10 percent of shark species, a placenta is formed from the egg yolk and attaches to the uterus. Similar to human embryos, the young receive nourishment from the female through the placenta. Bull sharks, lemon sharks and hammerhead sharks give birth in this way.
Embryos of some shark species, including white sharks and sand tiger sharks, receive nourishment in the uterus by eating the female's eggs. Usually the young eat unfertilized eggs, but the sand tiger shark eats other fertilized eggs until only one or two pups remain.