Walking the path at night, I hear near-constant rustling in the dried leaves. All along the route from my cottage to the shower block, I hear creatures scurrying away. Crabs. More crabs than I ever imagined possible.
Palmyra Atoll is a very remote island that is 1,000 miles south of Hawaii. I’m here for a fish research trip. Palmyra is a very crabby place. There are 12 kinds of land crabs here, and their numbers have been rising since rats were taken out.
Crabs also feature prominently in any Palmyra stay. Soon after landing on Palmyra, all researchers and visitors must attend an orientation to review safety rules. The remote island, administered by The Nature Conservancy and U. S. Fish and Wildlife Service, has no permanent inhabitants, so following rules and protocols is paramount. And one of the first rules you learn is a simple one: Watch your step. Watch every step.
This is in part for your safety; Palmyra is very, very far from medical assistance. But it’s also for the crabs. In the evenings, hermit crabs and other species come out en masse. The path is like an obstacle course with crustaceans as the theme. You need to wear a headlamp and be careful as you walk.
So I walk gingerly, shining my light on the path. Then I hear a much louder rustling. I swing my headlamp, and see a large form trundling through the leaf litter. This is no ordinary crab. It’s the one I’ve been looking for each evening: a coconut crab.
It’s a stunning creature, covered in blue-and-orange highlights, with an imposing claw waving in front. The crab is the size of a pie plate, and I know they can get much bigger.
It’s the biggest land crab and, really, the biggest land animal on Earth. It’s called the coconut crab. In many parts of the world, it faces a perilous future. On Palmyra, though, the situation is much brighter.
Crabs come in all shapes and sizes, from tiny pea crabs to giant Japanese spider crabs But what is the biggest crab in the world? As a crab enthusiast, I was curious to find out. So I did some research on the largest crab species on the planet
The Japanese Spider Crab
The largest crab in the world is the Japanese spider crab, scientific name Macrocheira kaempferi. These massive crustaceans have a leg span that can reach over 12 feet from claw to claw! Their bodies can grow to around 15 inches wide and they can weigh up to 42 pounds. That’s a big crab!
Japanese spider crabs live in the waters around Japan. They use their long spindly legs to walk along the seafloor instead of swimming. Their legs make them look spider-like (hence the name). These giant crabs also have a unique camouflage – their shell is covered in sponges algae and other marine organisms, allowing them to blend in with their surroundings.
At night, Japanese spider crabs scavenge for food like dead fish, shellfish and plants. During breeding season, they come together in large groups. While not the fastest growing crabs, Japanese spider crabs can live to be around 100 years old. Their meat is considered a delicacy in Japan.
The largest Japanese spider crab ever found was nicknamed “Big Daddy”. Its leg span measured an incredible 12 feet across and one of its legs was 4.8 feet long – the longest leg ever recorded on a crab!
Coconut Crab
The largest land dwelling crab is the coconut crab. These creatures live on various islands in the Indian and Pacific Oceans. They get their name from their ability to climb coconut trees.
Coconut crabs have a leg span reaching 3 feet across. They weigh around 9 pounds. Their big strong claws can crack open a coconut! They also use their sense of smell to find food like fruit, nuts and dead animals to scavenge.
The largest coconut crab ever found weighed around 9 pounds with a 3 foot leg span. These giant hermit crabs sure put other land crabs to shame in terms of size.
Giant Japanese Spider Crab vs. Coconut Crab
The Japanese spider crab lives in the ocean while the coconut crab lives on land. But how do they compare in size?
The Japanese spider crab is the larger species. Coconut crabs have a shorter leg span, reaching 3 feet across at max compared to 12 feet across for the Japanese spider crab.
Coconut crabs however have larger bodies than Japanese spider crabs. Coconut crabs can be 9 pounds in weight while Japanese spider crabs weigh around 42 pounds at largest.
So while the Japanese spider crab has much longer legs, the coconut crab has the larger main body size. Both are huge crabs compared to most other crab species on the planet!
Other Giant Crab Species
While the Japanese spider crab and coconut crab take top prize for size, there are some other giant crab species worthy of mention:
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Tasmanian Giant Crab – Found off Australia, these crabs have a body 18 inches wide and claws up to 17 inches long. They can weigh over 20 pounds!
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Alaskan King Crab – With a leg span over 5 feet across and weighing up to 28 pounds, these are monsters of cold Alaskan waters.
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Giant Japanese Spider Crab – The mud crabs found in Asia can have a shell over 11 inches wide and weigh more than 11 pounds.
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Dungeness Crab – Not a true giant but these Pacific crabs are quite large at 7-9 inches wide. They are a popular food crab.
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Snow Crab – Snow crabs grow to around 7 inches and 3.5 pounds but have long spindly legs like spider crabs.
While not record breakers, these plus other big crab species show how large some crabs can grow. They make living in the ocean seem like a dangerous proposition!
Crab Sizes vs. Other Animals
To give some perspective, I wanted to compare these giant crab sizes against other large animals:
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Coconut crabs have a similar leg span to an adult emperor penguin at 3 feet across.
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Japanese spider crabs have a leg span longer than a giraffe is tall, over 12 feet!
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The 18 inch wide shell of a Tasmanian giant crab is bigger than a volleyball or basketball in diameter.
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Alaskan king crabs at 5 feet leg tip to leg tip are wider than a concert piano’s length.
Looking at these comparisons shows just how incredibly gigantic some crabs can grow. They rival large marine mammals, tall mammals on land and even sports equipment in size.
Why Do Some Crabs Grow So Big?
What allows some crab species to reach such mammoth proportions compared to other crabs? A few key factors play a role:
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Long life span – Japanese spider crabs can live 100 years, allowing them to keep growing slowly over many decades.
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Thick exoskeleton – The strong shells of king and giant crabs give support for their heavy bodies.
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High protein diets – Scavenging dead animals provides essential nutrition for growth.
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Lack of predators – With few ocean predators to fear, giant crabs can devote energy to growth rather than defense.
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Cold water habitats – Colder environments like Alaska allow for bigger body sizes.
By living long lives with nutrient-rich diets and in cold, predator-free waters, certain crab species have been able to evolve into true ocean giants.
Giant Crab Conservation
While fascinating, some giant crab species like the Japanese spider crab and king crab face threats from overfishing. Their large size and tasty meat makes them prized catches.
Luckily, sustainable fishing limits, breeding programs and protected habitats help conserve these giants. Ecotourism gives local economies incentives to keep giant crab populations healthy.
So while giant crab species will hopefully continue thriving, more conservation efforts are still needed to ensure these amazing creatures fascinate sea lovers for generations to come. Their disappearance would be a loss for both ocean ecosystems and human wonder at nature.
In Summary
When it comes to the largest crabs in the world, a few key points:
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The Japanese spider crab is the biggest crab with 12 foot leg spans.
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Coconut crabs are the largest land crabs, reaching 3 feet across.
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Several crab species like king and Tasmanian crabs also grow quite huge.
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Crab size is enabled by long lives, thick shells and cold water.
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Conservation helps protect the most giant of the crab species.
Of Crabs and Rats
Palmyra Atoll is a naturalist’s wonderland, with so many creatures to capture your attention. There are manta rays and gray reef sharks, sea turtles and huge colonies of birds. And fish of every conceivable size and color. With all these charismatic species around, it might seem hard for an invertebrate to win anyone’s heart.
But the crabs hold their own.
In part, it’s because they’re everywhere. There are the strawberry hermit crabs patrolling the beaches, and little fiddlers darting into burrows. There are a crazy number of crabs at the food waste pit (any food waste is eaten within hours), outside your cottage, and near the docks.
The coconut crab is the star, and a night-time crab walk will usually yield at least a few. U. S. Fish and Wildlife staff lead a weekly “crab walk” for visiting researchers and Palmyra’s seasonal staff. It’s a popular activity as the group attempts to see all 12 land crab species in an evening. Still, the coconut crab steals the show.
Seeing all these crabs, amidst this biological richness, it’s easy to call Palmyra Atoll a “pristine” environment. But that may be a bit misleading. Palmyra’s known history of human habitation is indeed fairly recent. Notably, it was a U. S. Naval base during World War II. The human presence now seems minimal, but that’s not the same as non-existent. People have caused significant impacts. Chief among those: rats.
Rats arrived, as they often do, on ships. They quickly began eating their way through Palmyra’s terrestrial biodiversity. People who worked on the island ten or twenty years ago say that rats’ feet are always moving through the trees and on roofs.
Rats are bad for island wildlife, a fact well established in too many islands around the globe. Many conservationists know they are hell on birds. They eat eggs and nestlings, devastating seabird colonies.
Less known is that invasive rats also devastate crab populations. With its sharp claws and great strength, a coconut crab might look like a very dangerous enemy for a rat. That’s true. Rats mainly don’t bother with adult crabs. They focus on the young crabs. With adult crabs living for decades, they persisted on Palmyra, but few young crabs survived.
In 2011, The Nature Conservancy, U. S. Fish and Wildlife Service and Island Conservation partnered to rid Palmyra of rats. It’s a success story environmental writer Ted Williams has called “the miracle on Palmyra. “The latest science and technology made it possible to achieve a goal that no one had thought possible before: getting rid of every rat on the island.”
It worked. In 2015, Palmyra was proclaimed “rat free.”
Since then, several land crab populations have increased 450 percent. There are two species of tree crabs that researchers didn’t even know existed on Palmyra. They were first documented two years after rat eradication; now they are everywhere.
Coconut crab young are surviving. I often found coconut crabs scuttling away just a short distance from the research base.
Life as a Coconut Crab
A coconut crab will also run away when a person comes close, but if you move quietly toward it, it will stop and look at you for a long time. Up close, a coconut crab looks like a supersized version of a hermit crab, minus the shell.
That is actually the case: hermit crabs and coconut crabs are closely related. Like the hermit crabs you see in pet stores, coconut crabs live in their shells when they are young. As they get older, they abandon the shell and instead rely on their tough exterior for protection.
These are truly land crabs. Male coconut crabs only go into the water to lay eggs, which is the first step in the larval stage.
Unlike hermit crabs, they grow large. Very large. They can live for sixty years and grow to be three feet across and nine pounds heavy as adults.
Then there’s that claw. Stories have long been told about the crab’s finger-crushing abilities. Recent research quantified that power. A crab can squeeze with a crushing force of 3000 newtons, or about 675 pound per square inch. Compare that to a lobster, which has a crushing force of 250 newtons.
Coconut crabs can also lift more than 60 pounds, and yes, they can crush through coconuts. People think that this species likes to eat coconuts because of this, but that’s not really the case. Coconut crabs are scavengers and opportunists. They eat primarily fruits, nuts and the pith of fallen trees, but will also eat carrion. On Palmyra, they actively hunt prey, especially other crabs.
People also call them “robber crabs” because they will eat almost anything and take things people leave outside. Colorful stories abound, including one where a crab carried off a full whiskey bottle. Unlike, say, a pack rat, the coconut crab is not stealing shiny objects. Rather, it’s likely related to the crab’s excellent sense of smell. Animals that move around at night or in the dark rainforest need to be able to smell things very well to find food. Human objects with a trace of food are thus carried off.
Coconut crabs are found widely in coastal habitats around the Pacific and Indian oceans. In many parts of their range, populations are declining. As reported this year in Mongabay, in some areas, coconut crabs have disappeared completely.
The crabs face a predictable litany of threats. Coconut crabs always live near the coast, often in forest habitats – areas that are often developed. Additionally, people eat coconut crabs so they are vulnerable to overharvesting. On many Pacific islands, they’re a difficult species to see.
That’s not the case on Palmyra. It’s actually easier to see coconut crabs now than it was just a few years ago, thanks to a huge success in protecting them.
Top 10 Largest Crabs In The World
What is the biggest crab in the world?
The Tasmanian Giant Crab ( Pseudocarcinus genus) is one of the largest crabs in the world with a carapace width of up to 18 inches and a mass of up to 39 pounds. This giant lives in muddy bottoms off in the Southern Australian Ocean on the edge of the continental shelf.
How big can a crab get?
The crab’s body can potentially grow up to 16 inches (carapace width), and the entire crab can weigh a stunning 42 pounds. The Japanese spider crab is second only to the American lobster when it comes to sheer mass.
What is the largest spider crab?
Japanese Spider Crab The largest crab in the world is the Japanese spider crab (Macrocheira kaempferi). The crab’s body can potentially grow up to 16 inches (carapace width), and the entire crab can weigh a stunning 42 pounds.
How big do coconut crabs get?
The coconut crab ( Birgus latro) is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest terrestrial arthropod in the world, with a weight of up to 4.1 kg (9 lb). It can grow to up to 1 m (3 ft 3 in) in width from the tip of one leg to the tip of another.