Imagine dropping a tape measure into the dark, freezing waters of the Pacific and watching it scroll out to twelve feet. That is the leg span of a full grown japanese spider crab. It’s massive. Honestly, if you saw one walking toward you on land, you’d probably think a sci-fi movie was leaking into reality. But these animals, known scientifically as Macrocheira kaempferi, aren't monsters; they are just really, really old survivors that have mastered the art of living where most things die.
Most people see a photo of a crab and think "dinner." With this species, dinner would require a sledgehammer and a lot of patience. They live deep. We’re talking 150 to 1,000 feet down in the ocean trenches around Japan. Down there, the pressure is immense and the light is basically non-existent.
The Absolute Size of a Full Grown Japanese Spider Crab
Size is the first thing everyone talks about. For good reason. While the body—the carapace—might only get to be about 16 inches wide, the legs are where the drama happens. A full grown japanese spider crab can reach a total leg span of 3.8 meters (roughly 12.5 feet). That’s wider than most cars are long.
Think about that.
If you stood one up on its back legs, it would dwarf a professional basketball player. But they aren't heavy in the way you’d expect. Despite the terrifying reach, they usually weigh in around 40 pounds. It’s all spindly, armored length. Marine biologist Dr. Mary Wicksten has noted in various decapod studies that the morphology of these crabs is perfectly suited for "stilt-walking" over the uneven, silty floor of the deep ocean. They don't swim. They trek.
Why the Legs Don't Just Snap
You’d think legs that thin and long would be fragile. They kind of are, but they are also incredibly flexible compared to the stubby legs of a Blue Crab or a Dungeness. The exoskeleton is reinforced with calcium carbonate. It's essentially bone-hard. Interestingly, you'll often find these crabs missing a limb. Because they live so long—some estimates suggest up to 100 years—they get into scrapes. Predators, fishing nets, or just getting stuck in a rock crevice can cost them a leg.
The good news? They can sometimes grow them back during a molt, though a full grown japanese spider crab molts much less frequently than a juvenile. Once they hit that massive terminal size, what they have is pretty much what they’re stuck with.
Where They Actually Live (And Why You Won't Step on One)
You aren't going to find these guys at the beach. They prefer the vents and holes of the Pacific Ocean floor, specifically around the southern coast of the Japanese island of Honshu. It’s cold there.
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The temperature stays around 10°C (50°F).
It’s a quiet, dark existence. They share this space with giant isopods and deep-sea eels. Because the environment is so stable and low-energy, the crabs have evolved to be slow. They aren't hunters in the traditional sense. They are scavengers. They eat whatever falls from above—dead fish, decaying plant matter, or the occasional slow-moving mollusk.
They are the janitors of the abyss.
The Molting Nightmare
Growing to be a full grown japanese spider crab is actually a life-threatening process. To get bigger, a crab has to shed its entire hard shell. This is called molting. For a creature with 12-foot legs, this is an Olympic-level feat of gymnastics. They have to wiggle out of their old armor, leaving them soft, squishy, and extremely vulnerable for several days while the new shell hardens.
During this window, they are basically a giant, protein-rich snack for anything with teeth. This is why many large specimens have scars or stunted limbs; if something bites them while they're soft, it leaves a permanent mark on their history.
Camouflage and the "Decorator" Habit
Younger spider crabs are actually "decorators." They don't just rely on their size. They pick up sponges, anemones, and bits of kelp and literally velcro them to their shells. They have tiny hooked hairs called setae that act like the rough side of a strip of Velcro.
Why do they do this?
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- To blend into the background.
- To use the stinging cells of anemones as a secondary defense.
- Because they are tasty, and everything wants to eat them when they’re small.
By the time they become a full grown japanese spider crab, they usually stop decorating. When you’re twelve feet wide, there isn't much point in pretending to be a rock. You’ve reached a size where only the largest sharks or humans pose a real threat.
The Fishing Industry and Conservation
In Japan, these crabs are called taka-ashi-gani, which literally means "tall-man crab." They are considered a delicacy in some regions, like the Izu Peninsula, but they aren't exactly easy to catch. You can't just throw a net in the surf. Fishermen have to use deep-sea trawls.
There's a catch, though. Their population has been dipping.
Because they take so long to reach maturity—potentially decades—overfishing hits them hard. If you catch a full grown japanese spider crab, you’re taking an animal out of the ecosystem that might have been alive during World War II. Japan has implemented laws that prohibit catching them during their mating season in the spring, when they move into slightly shallower waters (about 150 feet deep) to reproduce. This protection is vital. Without it, the species would have likely collapsed decades ago.
Misconceptions: Are They Dangerous?
Let’s be real: they look terrifying. Those claws (chelipeds) can be incredibly long, especially in males. If a full grown japanese spider crab pinched you, it would hurt. A lot. They have enough force to crack shells and give a nasty bruise or a deep cut.
But they aren't aggressive.
They are slow, methodical, and generally pretty chill. In public aquariums like the Georgia Aquarium or the Enoshima Aquarium in Japan, divers often share tanks with them. The crabs mostly just sit there, looking like giant underwater tripods. They don't hunt humans. They don't even really "attack" unless they are cornered and have no choice. They are the "gentle giants" of the arthropod world, even if they look like they belong in a horror flick.
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What it Takes to See One
If you want to see a full grown japanese spider crab in person, you have two real options: a high-end aquarium or a deep-sea submersible.
Aquariums struggle to keep them because of the pressure requirements. While they can survive at surface pressure, they need very specific, chilled water. They also need massive tanks. You can't just put a twelve-foot crab in a standard display. Seeing one move is a lesson in physics. Every step is calculated. Their legs bend in ways that seem impossible, pivoting on joints that look like they should snap under the weight of the water.
The Survival Strategy of Longevity
We often think of "survival of the fittest" as being the fastest or strongest. The Japanese spider crab proves that sometimes, survival is just about being "the biggest and the most patient." By living in the deep where competition is lower and growth is slow, they've carved out a niche that has remained unchanged for millions of years. They are living fossils in the truest sense.
Their life cycle is a mystery in many ways. We still don't know exactly how long they live in the wild—100 years is a common estimate, but some researchers think it could be even longer. We also don't fully understand their migration patterns. Do they stay in one trench for fifty years, or do they wander the coastline?
Actionable Insights for the Curious
If you're fascinated by these deep-sea giants, here is how you can actually engage with the topic or see them for yourself:
- Visit World-Class Aquariums: Check out the Georgia Aquarium in Atlanta or the Osaka Aquarium Kaiyukan in Japan. These facilities have dedicated cold-water tanks designed specifically to house these massive decapods.
- Support Sustainable Seafood: If you're traveling in Japan and see spider crab on the menu, ensure it is sourced outside of the spring mating season (January to April). Avoid eating them during this time to help maintain the wild population.
- Deep-Sea Research: Follow organizations like MBARI (Monterey Bay Aquarium Research Institute) or NOAA Ocean Exploration. While they focus on many species, their ROV (Remotely Operated Vehicle) feeds often capture footage of spider crabs in their natural, deep-sea habitats.
- Education: If you're a teacher or parent, use the Japanese spider crab as a case study for adaptation. It’s a perfect example of how animals evolve specific traits—like long legs and slow metabolisms—to survive in high-pressure, low-oxygen environments.
The full grown japanese spider crab remains one of the most striking examples of evolutionary engineering. It is a reminder that the ocean is still a place of giants and mysteries, tucked away in the dark where the sun never reaches. Whether you find them beautiful or nightmare-inducing, there is no denying they are one of the most impressive survivors on the planet.