9:20in productionCh. 1 · A lone survivor/ 9:20 · ceiling 15 min
Fish & sea life · Evolution
Japanese spider crab
The largest leg-span arthropod alive is a gentle, orange-and-white scavenger—deep-dwelling, overfished, and utterly singular.
The Japanese spider crab is the largest leg-span arthropod known, native only to deep waters off southern Honshū. It is the sole survivor of its genus, with three fossil relatives from Japan’s Miocene and Oligocene. Its size is verified: up to 3.7 m leg span, 40 cm carapace width, 19 kg mass. Males show sexual dimorphism in cheliped length and twisted pleopods; larvae are described as primitive. It is omnivorous and scavenging. Overfishing has driven population decline, prompting seasonal bans and restocking—but no IUCN status is assigned. Its 100-minute molting rhythm is unique among crabs. It is not shallow, not common, not unmanaged, and not generalist in distribution.
It is the only living crab in its genus—and its fossil cousins lived in Japan 23–34 million years ago.
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Size, measured
Its 3.7-m leg span is the greatest of any arthropod; its body reaches 40 cm wide and 19 kg—second only to the American lobster by mass.
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Sex and ancestry
Males have longer chelipeds and twisted first pleopods; larvae are primitive—a sign of deep evolutionary age.
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Where and how it feeds
It lives only in deep ocean vents and holes off southern Honshū—and it is omnivorous and scavenging.
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Managed, not assessed
Overfishing has caused population decline—prompting seasonal bans and restocking, not IUCN listing.
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Behaviour, observed
It is gentle, orange-and-white, and molts on a strict 100-minute cycle—unlike any other crab.
Worth your time?
Yes. See the whole thing.
4.5/ 5
What works
leg span is the largest of any arthropod
sole extant species of Macrocheira
primitive larvae and twisted male pleopods observed
seasonal fishing bans and restocking implemented due to overfishing
What does not
has an IUCN conservation category
lives in shallow water
avoids human management
See it if
deep-sea biologists
crustacean taxonomists
conservation practitioners managing fisheries
Skip it if
tide-pool visitors
aquarium hobbyists (no mention of captivity)
general wildlife tourists
The written brief1 min read
What the species is and where it came from
It is a marine crab native to Japanese waters. It is the only living species of Macrocheira. Its range is restricted to deep ocean vents and holes off southern Honshū.
How it works, in terms someone would actually use
It lives in deep ocean vents and holes off southern Honshū. It is omnivorous and scavenging. It molts every 100 minutes during its growth phase.
What it gets right
It gets right extreme size—3.7 m leg span, 40 cm carapace, 19 kg mass—and evolutionary distinctness as the sole extant Macrocheira, with three fossil relatives from Japan’s Miocene and Oligocene.
What it does not
It does not have an IUCN conservation category assigned. It does not avoid human pressure: overfishing has driven population decline and forced management interventions.
What it changed
Nothing is stated in the sources about what it changed.
Who it is for, and who it is not
It is for divers, deep-sea biologists, and students of crustacean evolution. It is not for casual tide-poolers or shallow-water snorkelers—it lives too deep, moves too slowly, and is too heavily regulated to encounter without purpose.
Is it worth your time
Yes—if you want to see the largest leg-span arthropod alive, a living fossil with primitive larvae and twisted male pleopods, and a species actively managed through seasonal fishing bans and restocking.