Spider crabs derive their name from their apparent similarity to spiders. They have thick, rounded bodies with eight long legs that can grow to many times their body's length in some species. Spines, hairs and knobs cover their exoskeleton. Because they are rather sluggish creatures, spider crabs will often attach other organisms, like sea anemones and sponges, to themselves to protect themselves from predators. This is accomplished by using mucous secretions from their mouths as a glue. While spider crabs are known for their size, some species are less than half an inch in diameter.
Because of their size and slow-moving nature, spider crabs tend to stay in rocky areas, within a few hundred meters of the water's surface. Some also prefer muddy areas, but both locations offer protection and camouflage. However, some species like the European spider crab (Maja squinado) have been known to migrate as many as 100 miles in a season. Different species of spider crabs can be found in the Atlantic, Pacific and Indian oceans as well as the Mediterranean Sea.
In spite of their menacing look, most spider crabs are scavengers, eating whatever they can find in their environment. Because they are relatively slow-moving, they tend to consume animals that are already dead, including fish, starfish and mollusks. Algae is also a source of food for some species, like the European spider crab (Maja squinado). Spider crabs often become food for humans, as they are fished commercially in temperate waters. Some species are considered a delicacy, while others like the Japanese spider crab (Macrocheira kaempferi) are used as trophies because of their impressive appearance.
The most famous spider crab is the Japanese spider crab, which is the largest crustacean in the world and can live to be 100 years old. It is recognizable for its abnormally long legs, which can stretch up to 13 feet. However, its body is relatively small, measuring only about 14 inches in diameter. The male of the species has large pincers on the end of its feeding arms, adding to its menacing appearance. Because of their worth as food and trophies, they were threatened by overfishing, but recent conservation efforts have reduced their harvest.