It shines in the sun at the water's edge — enigmatic, beautiful and oddly shaped. What is it? A mermaid's purse? Alien life form? Not quite.
Congratulations. You've just found a shark egg case or possibly the egg case of a skate or chimera, lesser-known relatives of sharks and rays.
"Just like a chicken egg, the eggs protect the embryo growing inside," says Helen O'Neill, a research technician at the CSIRO's National Fish Collection.
These strange and wonderful constructions of nature are a common find on Australian beaches and scientists at the CSIRO hope beachcombers will help them learn more.
In collaboration with UK-based Shark Trust, the CSIRO has launched a citizen science project called the Great Eggcase Hunt Australia, which is supported by the Shark Trust App.
There are at least 37 species of sharks and skate that lay eggs of many shapes and sizes to be found on Australian beaches.
The app helps finders identify the egg species and includes information about sharks, chimeras and rays. Finds logged in the app will be added to a global database to help scientists like Ms O'Neill with their research.
"It helps put together a better idea of where animals are found and what areas animals might be breeding in, which is a really important consideration for conservation," she said.
What species produce eggs?
Each egg case will have been home to a single shark, skate or chimera embryo.
Skate are pointy-snouted egg-laying rays that, unlike stingrays, do not have poisonous barbs although they do have sharp thorns in their tails, said Dr Will White, ichthyologist at the Australian National Fish Collection.
"A chimera has a skeleton of cartilage, which is what links them to sharks and rays," he said.
"They are very different in many ways."
While all chimera and skates lay eggs some sharks and rays, such as white sharks and manta rays, give birth to live young.
What do researchers hope to learn?
Many egg-laying sharks are under threat in Australian waters and researchers hope that, by logging their finds on the Shark Trust app, beachcombers can help build knowledge about their distribution and breeding habits.
The global database will also include scientists' advice on conservation actions, such as the creation of marine parks.
"We often know the range of the species [where they are found] but that doesn't let us know where they are breeding," Dr White said.
"If you are going to have marine parks you want them to be in areas where fish are breeding."
Eggs don't survive long buffeted by the ocean's currents, so most eggs found on a beach would have been laid close by.
Why are shark eggs such weird shapes?
The egg cases come in a striking array of shapes but each has evolved for the same reason — to maximise the chances of a healthy pup hatching.
"They're made from collagen or largely made out of collagen, so that is really strong, rubbery when they're wet texture but kind of crunchy when they are dry," Ms O'Neill said.
Draft board shark eggs have curly appendages at the ends that help them attach to kelp so they stay put and have a constant stream of oxygenating water going past them.
Port Jackson shark eggs are a really unusual spiral shape.
"This is because the mother will attach the egg into crevices between rocks and spiral it in so it can't come back out and get washed away to other areas," Ms O'Neill said.
"She picks it up with her mouth and just wedges it in.
"That helps protect the developing embryo inside from being eaten by predators."
Egg cases found on the beach often have a hole in them where either the pup has hatched or a predator has broken in.
