Saltwater crabs are primarily carnivorous, preying on animals that live on or near the ocean floor. The most common species of ocean-dwelling crabs feast largely on small fish, worms, squid, snails and animal matter found on the ocean floor. Most crabs also eat their own species, some going as far as to deliberately trap them, according to USA Today.
There are more than 4,000 species of crabs, and the majority live on the ocean floor where they find an environment rich in prey, such as smaller fish or crustaceans. Crabs may also supplement their diets with plankton, algae, gastropods and snails.
The most common types of saltwater crabs are king crabs, Dungeness crabs, snow crabs and blue crabs. The diets of each variety of crab vary slightly. While crabs are primarily carnivores, the blue crab feasts on live plants along with sea-dwelling animals. While all crabs eat other types of crabs, the snow crabs, blue crabs and Dungeness crabs actually trap and eat other crabs of their own specific type.
When fully grown, the largest species of crab, the king crab, relocates to deeper waters, where it hunts larger prey. These crabs, which can live up to 20 years, dine on larger crustaceans that include clams, mussels, barnacles and sea urchins.