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Other Crabs
Crabs are opportunistic scavengers and predators, and like other crustaceans, are perfectly willing to engage in cannibalism when the opportunity presents itself. Large crabs are capable of crushing smaller crabs' shells and molting crabs are especially in danger from their peers. Crabs can't grow larger without shedding their hard shell, or exoskeleton. During the two or three days it takes for the new shell to harden, these soft-shelled crabs are highly vulnerable.
Birds
Seagulls and other shore birds are a constant hazard to the crab species that live along the shore in the area of tidal activity. When the tide recedes, crabs are often left on shore, or in shallow pools among the rocks. Many species of birds take advantage of this exposure to pick up a crab and smash it on the rocks, either by gripping it in their bill or by dropping it from a height. Either method cracks the shell and makes the tender inner flesh available.
Fish
Many predatory fish consume crabs at each stage of their life cycle, from their minuscule infancy as plankton to adulthood. Eels, sea bass, halibut, cod, catfish and sea trout are just a few of the species known to prey on crabs. Octopus are another formidable predator. They have a beak powerful enough to bite through the crab's shell and the intelligence and dexterity to dismember one neatly.
Humans
One of the most successful predators of crab is humankind. Crab meat is valued for its rich, sweet flavor and is harvested in large quantities in most of the world's waters. A few species have large, meaty claws that yield high-quality meat, but most species are harvested for their leg or body meat. The world crab harvest remains high, although some species and some stocks have been pushed into decline by predation, habitat destruction and overfishing.