Did you guess correctly? It is a…………SHARK EGG!!!
Walking along the beach looking at the shells and seaweed washed in on the last tide can reveal many things, like this mermaid purse. It is not really a purse, that is just the common name given to all types of shark and skate eggs.
This is a Port Jackson shark egg found on one of Melbourne’s beaches. It was laid between August and November by a shark that was more than 11 years old. She would have produced up to 16 eggs, laying pairs of eggs every 10-14 days. When she laid this egg, it would have been soft and around 15cm long. She would have picked up the newly laid egg in her mouth and carefully wedged it into a rock crevice. Here it would remain for the next 11 months while the young shark developed inside protected by the leathery shell, feeding on the nutrient-rich yolk. When it hatches, the young Port Jackson shark will be between 18 to 22cm in length and is a miniature adult. The spiral nature of this egg case means that it is probably the best-known shark egg found on our beaches but there are others.
All sharks produce eggs, it is where they develop that differs. About 40% of shark species and all the of skates lay their eggs, the rest keep the eggs inside their body where the young develop. The story of shark reproduction could be a Creature Feature on its own, but we will leave it there for now.
A mermaid purse is food for many animals, from other sharks to carnivorous snails. It is estimated that predators eat around 89% of all Port Jackson eggs.
The egg case of the Draughtboard shark has long tendrils extending from each corner which wrap around seaweed or other structures anchoring the egg. This shark does not have a breeding season like the Port Jackson shark, instead, females lay pairs of eggs throughout the year. The young shark hatches after 11 to 12 months and is around 14 centimetres long.
Another type of mermaid purse you may find on the beach could come from a skate, an animal which looks like a stingray, but its tail lacks the stinging barb. All species of skates lay eggs while stingrays give birth to live young. Skate eggs are rectangular with a spike or horn at the corners which anchor the egg onto the seafloor where it is laid.
So next time you are wandering along the beach take a closer look as you may find a mermaid purse.
These YouTube videos show a shark and a skate emerging from their egg case.
This is a horn shark, a relative of the Port Jackson shark.
This is a skate emerging from its egg.
If you would like to learn more about shark eggs and the huge diversity of life that can be found in the shallows along our shorelines please contact Education Director Mandy Robertson on email@example.com