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220px-Japanese spider crab

Japanese Spider Crab

Japanese soldiers in World War II encountered curiosities in isolated areas of their own homeland as well.

Soldiers stationed on remote islands of the Japanese archipelago, particularly in the Ryukyu island chain, were occasionally startled by giant crabs roaming around on beaches and in coastal forests. The crabs were described as being like large spiders, with long, spindly legs and small bodies.

One horrifying account allegedly occurred in the aftermath of The Battle of Okinawa, a battle that was fought over 82 days in 1945 and is considered to be the largest amphibious assault in the Pacific War. A Japanese unit came across a large number of what they described as huge, spider-like crabs feeding on the dead bodies of fallen soldiers on the beach. The crabs in this case were said to have leg spans ranging from 2 to 4 meters.

It is generally thought that these accounts are the result of sighting giant Japanese spider crabs that had somehow wandered up onto land. Some of the soldiers themselves described them as such. Japanese spider crabs inhabit the waters of Japan and have the longest leg spans of any arthropod, up to 3.8 meters (12 feet). As their name suggests, they look very much like large, long legged spiders. These creatures would certainly be a chilling and surprising sight on land.

There are several problems with this species as an explanation for accounts of giant land crabs, though. First, although spider crabs have been known to come into shallower waters from time to time, adult crabs are usually found at depths of 50 to 600 meters (160 to 2,000 feet). In addition, it seems unlikely that these crabs would be able to function on land to any appreciable degree. Arthropods have exoskeletons, skeletons on the outside of their bodies as opposed to humans, with endoskeletons.

This creates the problem of the weight of this exoskeleton when scaled to large sizes. Muscle strength is largely a factor of the muscle at its widest point, so as the crab grows, its muscle with regards to strength grows in two dimensions as the exoskeleton grows in three dimensions. Simply put, at such large sizes, the exoskeleton would simply become too heavy for the crab to carry on land. These crabs can get so large precisely because they live underwater, where the water can support the weight of their exoskeletons.

To be sure, there are large species of land crab, such as the coconut crab which is the largest terrestrial arthropod. However, these crabs are more robust than spider crabs and have adapted to life on land such as evolving an organ called a branchiostegal lung, which is a sort of cross between gills and lungs.

Japanese spider crabs have no such adaptations, and with such long legs spans, it seems unlikely that they would be able to effectively function on land, if at all. I remain doubtful that Japanese soldiers would have seen spider crabs lurking and scurrying about on land in coastal areas. Perhaps something more like a coconut crab or some unknown terrestrial arthropod was behind the reports.

Whatever they were seeing remains mysterious.

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