Ever wonder how big the biggest arthropod in the world is? I have, so I figured I would do a bit of poking around and see what I could find. Turns out, the world’s biggest arthropod is really big indeed! It is a crab called the Japanese Spider Crab (Macrocheira kaempferi).
These enormous crabs live in the coastal waters of Japan, mostly around the southern margin of Honshu, the main island of Japan. Adult crabs (males get a bit bigger than females) can get to be 3.8 meters across which about the same length as my car! And they can weight up to 19 kg (42 lbs) which is actually lighter than I would have guessed. To get a better sense of just how big one of these crabs is, check out these silhouettes comparing the Japanese Spider Crab to other, more familiar species.
Japanese Spider Crabs generally are found around depths of 150-300 meters, and have occasionally been found as deep as 600 meter. They are migratory, coming up to as shallow as 50 meters in the spring to breed. Fertilized eggs are carried by the female until they hatch at which point the planktonic larva float to the surface until they mature.
Although, these crabs are not harvested for food all that heavily, some commercial fishing for Japanese Spider Crabs does occur, and they are even considered a delicacy by some. The shallowness of their breeding range makes them especially vulnerable to fishing during this time, but after observing population decline and a decline in the size of crabs being caught, the Japanese government has banned fishing for spider crabs during spring.
Unlike some of what you may read on the internet about these crabs, they are nothing to be afraid of. First off, if left in their natural habitat, they will pretty much never cross paths with a human. Secondly, they are omnivores and scavengers that eat small animal like molluscs or even scape rocks to eat algae, so even if they did cross your path they would have no interest in trying to eat you. And thirdly, people who work with Japanese Spider Crabs report that they are generally very gentle animals that are mostly just curious about their environment.