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A huge variety of animals produce eggs. These help to protect and provide for offspring as they develop.
There are over 500 species of shark living in waters around the world and the majority give birth to live young. The remainder are oviparous, meaning they lay eggs.
Around 40 to 50 different shark species live permanently in or regularly visit the waters surrounding Britain. Among them are egg layers such as the small-spotted catshark (Scyliorhinus canicula) and nursehound (Scyliorhinus stellaris).
Empty shark-egg cases occasionally wash up on coastlines around the world, including in Britain. They look quite different to the traditional idea of an egg.
Sharks and rays are fishes with skeletons made of cartilage, grouped together in the class Chondrichthyes. Their egg cases are sometimes referred to as mermaid's purses, and occasionally as Devil's purses.
The egg is a capsule that contains a developing animal and a yolk sac which the young gets its nutrition from. A capsule usually houses one embryo, but in some species there are multiple embryos per egg case.
Some cases have long tendrils that help them to attach to seaweed or rocky seafloors. This makes it less likely that the egg will be washed away by ocean currents.
If an egg case washes up on a beach, you can likely work out what species it's from based on its size and shape. Generally, shark eggs have curly tendrils at the ends or are covered in fibres, whereas ray eggs are usually squarer with horns protruding from the corners.
Bullhead sharks produce spiral- or corkscrew-shaped eggs.
Emma Bernard, a fossil fish expert at the Museum, explains the reasons behind the unusual shape.
There are more viviparous shark species - which bear live young - than sharks that lay eggs. But throughout Earth's oceans, viviparity occurs in a variety of forms.
Whale sharks (Rhincodon typus) are the largest species of shark. Although these animals produce eggs, they don't lay them. Instead, the young hatch while still in the female's body and are born as miniature adults. This is known as ovoviviparity.
In 1996, a paper documented the case of a whale shark that was dubbed a 'megamamma'. The animal, which was harpooned off the coast of Taiwan, was found to be carrying around 300 embryos. Many of the sharks had already hatched from their egg cases, ready to be released into the ocean.
In some species, the female will produce unfertilised eggs, which are eaten by embryos. This is known as oophagy ('egg eating') and occurs in species including the shortfin mako (Isurus oxyrinchus) and bigeye thresher sharks (Alopias superciliosus).
Embryos of other sharks survive by feeding on their smaller siblings. This is called intrauterine cannibalism or sometimes as embryophagy ('embryo eating'). This is known to occur in sand tiger sharks (Carcharias taurus).
Placental viviparity occurs in some species of shark - once the yolk sac has been depleted, it attaches to the uterine wall, acting as a pseudoplacenta.
The young get their nutrition through this link. Hammerhead (family Sphyrnidae), blue (Prionace glauca) and lemon sharks (Negaprion brevirostris) all produce offspring in this mammal-like way.