Shark eggs are the reproductive structures of oviparous sharks, which are sharks that lay eggs. The eggs are leathery and come in various shapes, sizes, colors, and textures, depending on the species of shark. They are often referred to as "mermaids purses" and are sometimes covered in fibres or have curly tendrils at the ends. Some shark eggs are spiral- or corkscrew-shaped, such as those of bullhead sharks. The eggs contain developing embryos that receive nourishment and protection from the egg case. The egg cases are usually attached to seaweed or other underwater structures, where they remain until the young sharks, known as pups, are ready to hatch. The protective features of shark egg cases are vital adaptations that safeguard the embryos survival in the challenging marine environment. Some shark species, such as whale sharks, produce eggs but do not lay them. Instead, the young hatch while still in the females body and are born as miniature adults, a process known as ovoviviparity. Other shark species, such as the shortfin mako and bigeye thresher sharks, produce unfertilized eggs that are eaten by embryos, a process known as oophagy.