The crab is so called urchin crab or carrier crab because of its habit of carrying a sea urchin on its carapace, usually:
the red urchin (Astropyga radiata)
the black long-spined urchin () the banded diadem urchin (Diadema savignyi).
All these urchins have long, hollow spines and may provide protection to the crab by reducing the risk of predation by fish while not being too heavy to carry. The urchin obtains benefit by being transported to new feeding grounds. Even large and mobile individuals submit passively to being carried in this way.
Text source: Wikipedia
Dorippe frascone has developed a method of kidnapping an upside-down jellyfish (Cassiopeia spp. like C. xamachana or C. andromeda) and carrying it as a sort of extra-thick protective cloak upon their back. The jellyfish belong to the diet of sea turtles, but getting too close can get a nasty sting, so that most creatures avoid them. That makes a safe passage for the crab
Cancer frascone Herbst, 1785
Cancer nodulosus Olivier, 1791