Sea urchins are one of the ocean's most bizarre creatures.

In many species, their rotund bodies are covered in spikes, they exist at all sea depths, and their spines might even help treat bone fractures.

But few animals appreciate the spiny edges of sea urchins more than the carrier crabs that use them like a shield.

The carrier crab is uniquely built with two back legs that are especially adapted to grab anything from debris, to a neighbor. They're often found carrying sea urchins to use for protection, but for an urchin as mobile as the Astropyga radiata (also referred to as red urchin, fire urchin, or blue-spotted urchin), it's a bit unusual.

"I've never seen anything like this," says Harilaos Lessios, a