The egg sac of a Dogfish shark, commonly known as a "Mermaid's Purse."

Like most animals, sharks reproduce sexually. Shark mating behaviour can be very complex and pre-mating rituals vary enormously between species. Behaviour patterns, such as synchronised swimming, biting and colour changes, are common. Some species, such as the basking shark, engage in complex group circling behaviour, the purpose of which is not yet understood.

Once the sharks have successfully mated and the female’s eggs have been internally fertilised, the embryos develop in one of three ways, depending on the species:

Oviparous sharks lay eggs with thick cases that are resistant to predators, which they attach to rocks or seaweed. The eggs hatch days, or weeks, later, leaving the young to fend for themselves. Most egg cases are broadly rectangular and some, such as the dogfish's ‘mermaid’s purse’, are commonly washed up on seashores.

Viviparous sharks give birth to live young, which are nourished in the female shark’s uterus via a placenta, or a secretion known as uterine milk. Viviparity ensures that the young are very well nourished during development and thus fit to survive the rigours of the sea immediately after birth.

Ovoviviparous sharks also support the embryos internally and give birth to live young, but they do not provide any direct nourishment to their offspring. Instead, the developing sharks rely on the egg’s yolk sac for sustenance. Some species of shark, such as the mackerel sharks, practice oophagy, whereby the mother produces a stream of small, unfertilised eggs that are eaten by the developing embryos.