Shinin-gani, the 10-legged deep-sea monster, has been hiding in dark kelp forests for 100 years, waiting for its next prey. If it catches you, it will drag you to the bottom of the sea with its giant legs, where it will feast on your rotting body for days.Or so Japanese folklore says.But the creature the story refers to is very real. The giant spider crab is the largest arthropod on the planet, with its legs spanning an astounding 12.5 feet (3.8 m) from claw to claw. That’s almost twice as big as basketball star LeBron James.Why would a crab need to be so huge? Is it really hunting humans? And can it live to be 100 years old?These giant spider crabs live along the coast of Japan. Their bodies are only 38.1 cm (15 in) wide, but their dramatically long legs give them most of their bulk and their name. This 16 to 20 kg (35 to 44 lb) critter likes depths of 150 to 300 m (492 ft to 984 ft), close to the seafloor, where temperatures can fall to 10 °C (50 °F).The Japanese spider crab’s monstrous size scares most predators away. But it also helps this giant survive its harsh living conditions.Food on the ocean floor is scarce, and the spider crab is not much of a swimmer or hunter. It uses its frail, oversized legs to move slowly, scavenging on dead or decaying plant and animal matter that falls from the surface.So don’t worry about those scary stories. These gentle giants won’t pull you overboard and eat you. You’d have to die an unfortunate death at sea and fall right into a spider crab’s dinner plate for it to bother eating you.Deep-sea gigantism helps the Japanese spider crab conserve energy. Larger cells keep it warm, even at depths of 500 m (1,640 ft), while its slow metabolism means it doesn’t need to move around often to find its next meal.But do they really live to age 100? Most deep-sea giants take a very long time to grow to their enormous size, and there’s still a lot to learn about the Japanese spider crab in the wild.So scientists can’t be sure of the exact number. But we are likely looking at a creature with a very long life span. And a life this long comes with its fair share of accidents. It’s rare to find a Japanese spider crab with all 10 legs intact. A close encounter with a predator or an unfortunate fishing net entanglement is all it takes for it to lose a spindly leg or three.Luckily, these crabs molt regularly to accommodate their growing bodies. Missing legs can grow back during these molting sessions.But as with everything else, these giant spider crabs can take a while to grow a new exoskeleton. During the few hours it takes to armor up again, the crab’s soft under tissue lies exposed, leaving it vulnerable to predators, including other Japanese spider crabs.But growing a new exoskeleton is well worth the risk, and not just for the crab. It is made of chitin, which has high structural integrity and is non-toxic to humans.Scientists are finding uses for chitin in nerve regeneration and in treating several debilitating disorders. And that’s not all.These awkward giants might also hold the key to fighting many fungal and bacterial infections. Like many other arthropods, Japanese spider crabs have pre-loaded immune systems that don’t need updating through regular bouts of illness.Scientists are studying ways to harness the natural antibiotics and antimicrobial peptides found in this crab’s immune system.