You’ve seen them. Those viral photos. They look like something pulled straight out of a 1950s sci-fi flick where radiation makes bugs grow to the size of Buicks. Giant spindly legs. Orange, bumpy armor. Faces that only a mother—or a marine biologist—could love. Honestly, pictures of spider crabs are basically the internet’s favorite way to trigger collective arachnophobia without actually using spiders.
But here’s the thing. Most people looking at these images are getting the scale all wrong. Or they’re looking at two completely different animals and calling them the same thing. It’s confusing.
We need to talk about the Japanese Spider Crab (Macrocheira kaempferi). That’s the big boy. The undisputed heavyweight champion of the crustacean world. When you see a photo of a crab that looks like it could hug a Smart car, that’s the one. Then you have the European Spider Crab, which is much smaller, tastier (if you're into that), and way more common in tide pools. They aren't the same. Not even close.
The Visual Illusion of the Japanese Spider Crab
Most pictures of spider crabs that go viral rely on forced perspective. You know the trick. A diver swims three feet behind the crab, making the crab look like a Godzilla-sized monster. It’s a classic photography trope. Does that mean they’re small? No. They’re massive. We’re talking about a leg span that can reach 12.5 feet (3.8 meters).
Imagine standing in a room. Now imagine a crab that could touch two opposite walls at the same time. That’s the reality. Their bodies—the carapaces—are actually relatively small, usually only about 16 inches wide. It’s all in the legs. Those long, thin, hauntingly fragile-looking limbs are why they’re named after spiders. In the dark, murky waters off the Pacific side of the Japanese islands, they look like underwater ghosts.
They live deep. Very deep. We’re talking 150 to 600 meters down. Because the light is so low at those depths, their orange and white coloration actually acts as camouflage. In the deep sea, red light is filtered out first, so these bright orange giants effectively turn invisible to predators like large sharks.
Why Do They Look Like They’re Wearing Trash?
If you’ve been browsing pictures of spider crabs and noticed some of them look like walking compost heaps, you’ve likely found the "Decorator Crab." This is a fascinating subset of the spider crab family (Majoidea). They don’t just grow shells; they curate them.
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They have tiny hooked hairs on their backs called setae. It’s basically natural Velcro. They’ll grab a piece of seaweed, a sponge, or even a living anemone, chew the end to soften it, and then "Velcro" it to their back.
It’s brilliant.
Biologist Mary Wicksten has spent years documenting this behavior. She found that if you move a decorator crab to a new environment, it will literally "change clothes" to match the new background. This isn't just for fashion. It’s survival. An octopus has a much harder time spotting a pile of moving seaweed than a crunchy crab. Some species even use stinging anemones as a backpack. Imagine a predator trying to take a bite and getting a face full of stinging cells. It’s the ultimate "stay away" signal.
The Great Australian Migration
Every year, usually between May and July, something wild happens in Port Phillip Bay, Australia. Thousands—no, hundreds of thousands—of Great Spider Crabs (Leptomithrax gaimardii) march into the shallows.
If you want truly mind-blowing pictures of spider crabs, this is where you find them. They pile on top of each other. It’s a literal carpet of crabs, sometimes several meters deep.
Why? Safety in numbers.
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Crabs have a problem. To grow, they have to molt. They basically crawl out of their own skeletons. For a few days after molting, their new shell is as soft as a wet paper bag. They are incredibly vulnerable to stingrays and seals. By gathering in a massive "mosh pit," the individual's chance of being eaten drops significantly. It’s the "I don't have to be faster than the ray, I just have to be faster than my neighbor" strategy.
Spotting the Fakes and the Realities
Let’s get real about the "Crabzilla" photos. You’ve probably seen the satellite image of a 50-foot crab lurking in a British harbor.
It’s fake.
Biology has limits. The square-cube law basically dictates that if a crab were actually 50 feet wide, its legs would collapse under its own weight, even with the buoyancy of water. Plus, they’d need an insane amount of oxygen that simply isn't available in shallow harbor water.
When you look at authentic pictures of spider crabs, look for the joints. Real spider crab legs have very distinct, knobby "knees." They also have two rows of teeth on their claws, which they use more like tweezers than crushing pliers. They aren't hunters; they’re scavengers. They pick at dead fish and algae. They’re the janitors of the ocean floor, not the monsters of the deep.
Understanding the Life Cycle
These creatures are ancient. Not just as a species, but as individuals. It is widely believed by marine biologists that Japanese Spider Crabs can live to be 100 years old. Think about that. There are crabs crawling around right now that were alive during World War II.
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They grow slowly.
Because they live in such cold, high-pressure environments, their metabolism is sluggish. This is a common trait in deep-sea gigantism. When everything is cold, you grow big and you grow slow.
But their population is at risk. Overfishing in the 20th century hit them hard. Japan has since implemented laws banning the fishing of spider crabs during their mating season (typically January through April) to help the population recover. Most of the crabs you see in professional photography or in massive aquariums like the Georgia Aquarium or the Osaka Aquarium are there because they were accidentally caught in bottom-trawl nets or are part of specific conservation programs.
Identifying Spider Crabs vs. King Crabs
It’s a common mistake. People see a big crab and go "Spider Crab!"
Look at the legs.
- Spider Crabs have 10 legs (including the claws). All of them are usually visible.
- King Crabs appear to have only 8 legs. Their fifth pair is tiny and tucked away under the rear of their shell.
- The Shell: Spider crabs have a teardrop or triangular-shaped body. King crabs are more rounded or heart-shaped.
If the photo shows a crab with a pointy "nose" (called a rostrum), you’re almost certainly looking at a spider crab. That rostrum is a key diagnostic feature.
Actionable Tips for Crab Enthusiasts and Photographers
If you’re looking to find or photograph these alien-looking creatures yourself, you don't necessarily need a submarine.
- Visit a World-Class Aquarium: If you want to see the Japanese giants, the Osaka Aquarium Kaiyukan in Japan has one of the best displays. In the US, the Monterey Bay Aquarium often features deep-sea exhibits that include various species of spider crabs.
- Time the Australian Migration: If you’re a diver, head to Blairgowrie Pier in Victoria, Australia, during the winter months. The water is freezing, but the sight of a thousand-strong crab army is worth the hypothermia.
- Check Tide Pools: For the "Decorator" variety, you just need a pair of waterproof boots and a keen eye. Look for "rocks" that seem to be moving. In the UK and parts of the US East Coast, European spider crabs are common in the intertidal zone.
- Macro Photography: If you’re taking pictures of spider crabs, focus on the eyes. They are mounted on stalks and have a strange, iridescent quality that looks incredible with a macro lens.
- Support Conservation: Look for seafood guides like the Monterey Bay Aquarium’s "Seafood Watch." While spider crab isn't a common menu item in the West, supporting sustainable fishing helps protect the deep-sea ecosystems where the big ones live.
The next time a photo of a spindly, ten-foot monster pops up in your feed, don't just cringe. Look at the detail. Look at the way they’ve evolved to survive in a world of crushing pressure and eternal darkness. They aren't monsters. They’re just really, really old neighbors who’ve seen a lot more of the world than we ever will.