Thornback crabs are occasionally found on the shore, but they prefer depths of 90 to 600 feet. They are solitary animals except during mating season, when they form large breeding mounds. In years when their numbers are particularly abundant, they can be a source of frustration for lobster fisherman, as they infest the lobster pots. Thornbacks are themselves fished for their delicious claw meat.

Male thornbacks are effective predators; their delicate-looking claws are actually quite powerful and can open small mussels to feed on them. Their claws are also double-jointed, so although it is generally safe for a person to hold crustaceans by each side of their shells, thornbacks are able to reach over their backs to pinch the offender. Females have smaller, less flexible claws and are thus more vulnerable to attack. To defend against their predators-which include lobsters, wrasses, and cuttlefish-many species of spider crabs decorate their spiny shells with seaweed, sponges, or aquatic debris to better blend in against the seabed.

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