Kingdom Animalia
Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Decapoda
Infraorder Brachyura
Superfamily Majoidea
Family Majidae
Genus Macrocheira
Species Macrocheira kaempferi

A crab the size of a station wagon.

Japanese spider crabs live a hundred years.  Specimens eating biotic refuse on the Pacific seafloor near Japan have been bypassed by the echoes of Kitty Hawk, both World Wars, satellites, the fragmentation of culture.

Fossils of Japanese spider crabs are nearly identical to extant animals.  Evolution is a function of competition.  We should hope to attain such elegance.

Think — intuitively — spider.  Think of what a creature thirteen feet across would look like if it weighed 40 pounds.  Think lines, function, economy of expression.  That the Japanese culture blossomed in the same place is beautiful symmetry.

Taxonomy streamlines the animal kingdom into groups of creatures with shared characteristics convenient for scientific study.  Crabs and spiders are defined by Phylum Arthropoda — jointed limbs, cuticles.  Their size is curtailed by gravity, which is diminished in water.  Put the fearsome spider crab on land and it collapses under its own weight. 

Folklore says that the crabs appeared in the jungles of Japan; physics says otherwise.  Folklore also says that it feeds on the bodies of shipwreck victims; the relative density of the human body to water agrees.  The crab is omnivorous, subsisting partially on sunken carcasses.

I used the word "fearsome" incorrectly.  The crab places sponges on its body to distract predators (this does not work on the fishermen).  It's gentle enough to be raised communally in aquaria.

It's a delicacy.  It becomes such an easy target when it wanders to shallow waters to lay eggs in spring that netting it thence is illegal.  Like any good meal, Japanese spider crab is simple: steam & add salt.

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sources

Buzzle - Intelligent Life on the Web.  "Giant Japanese Spider Crabs." http://www.buzzle.com/articles/giant-japanese-spider-crabs.html.

Wikipedia.  "Japanese spider crab."  http://en.wikipedia.org/wiki/Japanese_spider_crab.

Alfonso Cagnelli.  "The Giant Japanese Spider Crab." http://ezinearticles.com/?The-Giant-Japanese-Spider-Crab&id=1465248.