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.
WATCH: CARRIER CRAB USES SPINY URCHIN AS SHIELD Watch a carrier crab live up to its name by carrying a sea urchin on its back for protection.
The carrier crab is uniquely built with two back legs that are specially adapted to grab anything from debris to a neighbour. 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 National Geographic grantee and researcher at the Smithsonian Tropical Research Institute. Lessios notes that this species of crab will carry almost anything as a form of protection, but an Astropyga radiata is one of the most mobile urchins in the ocean. (It is distinguishable by its orangey star-fish-shaped top, marked with linear blue dots.)
WATCH A CRAB KIDNAP A SEA SLUG TO AVOID BEING EATEN The carrier crab puts other sea creatures on its back to deter predators.
They often form clusters, can move several meters per minute, and, according to Lessios, can migrate in small distances.
"It's quite amazing that the crab picked it up without the sea urchin moving," he says. The crab, unable to clamp down on the urchin, is simply propping it up, so the relationship, Lessios believes, is consensual.
Other species of sea urchin benefit from riding on carrier crabs because it allows them to find new feeding grounds. Carrier crabs are so often spotted carrying their urchin shields that they are also commonly referred to as "urchin crabs." When confronted with a predator, they've been known to wield urchins like a weapon.
In the video, cardinal fish can also be seen weaving in and out of the urchin's spines. In for the long haul, cardinal fish frequently use sea urchins and other bottom-dwelling creatures as a makeshift habitat. They burrow between the spines of urchins, hoping predators won't be able to pluck them from the sharp exterior.
While it might initially appear that other urchins are being carried against their will, the relationship between the two creatures is often mutually beneficial. They are one of nature's most common symbiotic relationships.
Unlike other species of urchin, Astropyga radiata aren't fatal, but their venom can inflict intense pain on whoever's skin it punctures. Luckily for divers, the urchin's bright, eye-catching colours ensure it is easily spotted.