If you've ever happened upon a corkscrew-shaped piece of seaweed on a beach, you may have actually been looking at a shark egg. While many sharks give birth to live babies, some do lay eggs for hatching later.
The bullhead shark species has a particularly intriguing egg shape.
Technically these corkscrew packages are egg casings (sort of like eggshells). The drill-bit shape in particular is perfect for warding off predators.
Mother sharks can literally screw them into rock crevices to keep them safe.
It's theorized that the gland itself is twisted — hence the casing shape. The eggs hatch in about seven to 10 months, and the empty casings later wash up on shore.
Without context, they can be mistaken for seaweed.
Bullhead sharks aren't the only sharks that lay unique eggs.
Most species have casings that are transparent when held up to the light.
This provides a rare view of a baby shark before it has hatched.
Shark eggs that wash up from the ocean are often dubbed "mermaid's purses." Each species gives its egg case its own unique flair. Though not technically a shark, skates are closely related to rays and sharks, and lay "ravioli" eggs.
Here's a skate egg casing with signature thin horns coming off the corners.
By the time they wash up, they're usually hardened and empty.
But sometimes they can be found slightly more pliable and with a live embryo still inside.
Nurse sharks also lay eggs. Theirs have a more distinct rounded middle.
Catshark eggs are longer and look kind of like tamales.
And the ghost shark egg case is pretty much as unique as the actual shark itself.
So, next time you're beach-combing, keep an eye out for these unique mermaid's purses.