Reading path: Unusual marine body plans →
Questions to keep separate
| Question or feature | Explanation | What it does not establish |
|---|---|---|
| Carapace width | About 30 cm is the reported maximum width of the central shell. [1] | Not the crab’s full leg span or a typical size for all individuals. |
| Claw-to-claw span | Reported maximum reach is 3.8 m; especially long male chelipeds contribute to span. [1] [2] | Not body width, standing height or the length of one leg. |
| Depth and habitat | Seabed habitat off Japan’s Pacific coast; usual range about 200–550 m, with shallower spawning movements. [1] | A usual range is not an absolute depth boundary. |
| Longevity | The Aquarium of the Pacific account describes lifespan as unknown. [2] | Do not state an estimate as a measured lifespan. |
Measure the shell and the reach separately
The Japanese spider crab, Macrocheira kaempferi, has a central carapace—the hard upper shell over the body—and exceptionally long appendages. Aquarium accounts give a maximum carapace width of about 30 cm and a maximum claw-to-claw span of 3.8 m. Those figures describe different dimensions, not a body nearly four metres wide. [1] [2]
A useful scale exercise is to imagine a 30 cm shell centered inside a 3.8 m outline from claw tip to claw tip. Most of the outline comes from the extended limbs. The span is a reported maximum, not an average for every crab or a measurement that applies equally to both sexes. [1] [2]
Ten appendages, with different proportions
The crab has ten appendages counted as legs: eight walking legs and two claw-bearing chelipeds. Adult males have especially elongated claw-bearing legs, while female proportions differ. This sex-related anatomy helps explain why a single headline span should not be treated as a standard size for every individual. [1] [2]
A span also does not tell you the length of one leg, the crab’s height when standing, or its shell width. These measurements answer different questions: shell width describes the central body; claw-to-claw span describes the reach across the animal’s outstretched appendages.
A deep-sea crab on Japan’s Pacific margin
Japanese spider crabs live on the seabed off Japan’s Pacific coast, including sandy and rocky shelf and slope habitats. One aquarium account gives a usual depth range of about 200–550 m. Shallower movements occur for spawning, so that usual range should not be mistaken for a depth limit or a year-round location. [1] [2]
The bottom habitat and slow walking fit an animal that searches along the seafloor rather than pursuing fast-moving prey through open water. The supplied accounts describe these crabs as scavengers, but that label is incomplete: they may also take small live animals or open molluscs. [1] [2]
Feeding, growth and the vulnerable molt
Reported foods include dead fish, other invertebrates and algae. The ability to take live small animals and open molluscs broadens that picture: scavenging is part of the diet, not its whole definition. [1] [2]
Like other crabs, a spider crab grows by molting its external shell. A damaged limb may regrow at a later molt, but the process leaves the animal particularly vulnerable just after it sheds its old shell, before the replacement hardens. This is a temporary vulnerability tied to the mechanics of growth, not evidence that every damaged limb will recover on a fixed schedule. [1]
Read maximums and lifespan claims carefully
A maximum measurement describes the largest reported size in an account; it does not describe a typical crab. Nor should an estimated lifespan be presented as a settled fact: one aquarium account says longevity is unknown, so a 50–100-year figure should be treated as an estimate rather than a measured universal lifespan. [2]
These natural-history accounts also do not establish a current population trend or specific fishery rules. They support a clear description of anatomy, habitat and feeding, but those broader claims require evidence beyond general aquarium pages.
Sources & further reading
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