Shark eggs vary in appearance by species, although most have a leathery protective case, and they are often shaped like corkscrews. Tendrils or other attaching appendages are present on others to assist in their attachment to coral, seaweed or the ocean floor.
Approximately 40 percent of shark species lay eggs – or are oviparous – while the rest give birth to live young. In oviparous species, the developing embryo is nourished through a yolk sac. Eggs can take several months to hatch, but some species carry their eggs internally for a period before laying them. A shark egg that washes ashore is called a mermaid's purse.