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It’s not immediately obvious why certain species of sea urchins passively accept being picked up and carried by an animal called the carrier crab. It’s often the case in nature that when there is no objection to a physical interaction, that there is a benefit to it. So, the question then changes from, “Why doesn’t the urchin object?” to “What does the urchin get out of it?” A related question is naturally, “Why does this crab spend energy carrying sea urchins around on its back?” and the answer explains how the crab benefits from this behavior.

Carrier crabs have a mutualistic relationship with several species of sea urchins, which means that the interaction benefits both animals. The crabs gain protection from predators. While the crab moves along, it may use the sea urchin as shield. If a predator approaches, the crab will tip its back towards the predator which aims the urchin at the fish. It may even bury itself beneath the sand, leaving the urchin above ground. The urchins are moved to new feeding grounds in a more efficient and faster way than they are capable of on their own. It’s a beautiful friendship between carrier crabs and the urchins they carry.