Witness a carrier crab demonstrating its designated role as it transports a sea urchin on its shell for defensive purposes.

In the ocean, an intriguing and well-known symbiotic relationship exists between carrier crabs (Dorippe spp.) and mobile sea urchins, such as the Diadema setosum, or the Long-spined Sea Urchin. While carrier crabs typically carry the skeletons of sea urchins, there are documented cases of live, highly mobile sea urchins being carried by these crustaceans, a preference for both parties.

The Involved Species

Carrier Crabs (Dorippe spp.): Small crabs that inhabit the Indo-Pacific.
Mobile Sea Urchins (Diadema setosum): A well-known species observed in these symbiotic relationships. Normally, these sea urchins live on hard surfaces, reefs, or algal mats and move actively in their environment.

The Unusual Aspect

Sea urchins of the Diadema genus are known for their mobility and strong spines. It is atypical that they allow themselves to be passively carried. Normally, they cling to surfaces rather than being carried. In rare cases, however, particularly during low tide or on unfavourable surfaces, they allow themselves to be carried by carrier crabs.

Advantages of the Symbiosis

For the Carrier Crab

Protection: The long, sharp spines of the sea urchin deter predators.
Camouflage: The Diadema setosum is conspicuous, and potential predators associate the creature with pain and danger.
Mobility: Unlike a skeletonized sea urchin, a live sea urchin can overcome obstacles actively and move on the seabed without the crab having to exert itself.

For the Sea Urchin

Movement: The sea urchin can efficiently reach new, potentially nutrient-rich areas.
Protection from Predators: The crab can quickly escape from predators – an additional layer of protection for the otherwise slow-moving sea urchin.
Food: They may benefit from organic particles released during the crab's movement.

Summary

While carrier crabs typically carry empty sea urchin skeletons for camouflage, in rare cases, live, mobile Long-spined Sea Urchins (Diadema setosum) reside on the backs of crabs. This unusual partnership provides protection, camouflage, and mobility for both parties. The crab benefits from the deterrent effect of the spines, while the sea urchin benefits from the movement and additional protection.

Note: Such symbioses are rare and relatively understudied, but they are fascinating to observe. They demonstrate how adaptable animals can modify their strategies to survive together.

Recent research has also revealed striking similarities between the genome of sea urchins and humans, and some sea urchin species' spines may potentially aid in treating bone fractures. The Astropyga radiata, also known as the red urchin, fire urchin, or blue-spotted urchin, is another mobile sea urchin species that has been observed in symbiotic relationships. Harilaos Lessios, a researcher at the Smithsonian Tropical Research Institute, made a remark about the unusual behaviour of carrier crabs carrying the mobile Astropyga radiata. In a video, cardinal fish can be seen weaving in and out of the urchin's spines. The relationship between carrier crabs and sea urchins is often consensual and mutually beneficial. When moving, carrier crabs carrying sea urchins can move several meters per minute. When confronted with a predator, carrier crabs can use the sea urchins as a shield and even wield them like a weapon. Sea urchins, including the Astropyga radiata, can form clusters and migrate in small distances. Carrier crabs have two back legs adapted for grabbing objects.