SeaWorld put out a stunning video last week of a Japanese spider crab (Macrocheira kaempferi) cracking open its outer shell and seeming to clamber out of itself.

The molting process captured in the video out of San Diego is natural. As NOAA Fisheries explains on its website, crabs cannot grow linearly like humans and other soft-skinned creatures. Instead, they periodically bust out of their confining shells and undergo growth spurts, before settling into their new shells. [A Gallery of Creatures That Molt]

To pull off this trick, the crab secretes enzymes that separate its inner body from the hard shell, and then it develops a "paper-thin" new shell underneath. This can take weeks, NOAA Fisheries writes. And when it's done, the crab gets ready to move out of its now too-small shell. To pull off that trick, the crab absorbs seawater to swell the spaces in its shell, causing it to start to fall apart. Eventually, the old shell opens up at a seam around the main body, and the crab pulls itself free. That final step is what's seen in this video.

Spider crabs have the longest leg-spans of any crab, making their moltings particularly dramatic. According to the Oregon Coast Aquarium website, they can measure 13 feet (4 meters) across, with carapaces (main bodies) that are 15 inches (38 centimeters) across. They can weigh 44 pounds (20 kilograms).

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