Is it a strange kind of seaweed? A new kind of fruit? The latest fashion accessory?

Don’t peek at the answer until you’ve given this a good try! Then, scroll down a bit and the truth will be revealed to you.

No, it’s an egg case from a port Jackson shark, found washed up on a beach in Australia. The corkscrew-shaped flanges help the shark mother secure the case in rock crevices. Egg cases like this were sometimes called “mermaid’s purses” when found on beaches.