Animals That Borrow Other Species as Defenses, Disguises, and Shelters

Some carrier crabs respond to danger by putting another animal between themselves and whatever might attack. Species in groups commonly called carrier or porter crabs can hold objects and living organisms above their bodies using specially adapted rear legs. Sea urchins are particularly impressive choices because their dense covering of spines provides an obvious physical barrier against predators. A crab may position the urchin over its back like a mobile shield, carrying it while walking across the seafloor. Other individuals use shells, pieces of coral, sponges, or additional materials depending on what is available in their habitat. The behavior can provide camouflage as well as direct protection by breaking up the crab’s outline and making it harder for predators to identify. When the carried object is alive, the arrangement becomes even more remarkable because the crab has effectively recruited another species’ anatomy as defensive equipment. Researchers distinguish among numerous carrier-crab species and behaviors, so not every crab carries an urchin or uses the same object throughout its life.