Folks, your biology teacher was right: Science is cool. This realization is as much a part of growing up as having fewer friends, but who needs friends when you can watch hundreds of thousands of spider crabs marching together under the sea.
If that sounds appealing (welcome to your new life), check out the above clip. It's part of BBC's new Blue Planet II, a sequel to the wildly popular 2001 series. Make sure to watch it with audio, because David Attenborough's British accent trumpeting phrases like, "Around the first full moon of winter an army materializes: spider crabs," really puts the wonder of the ocean in perspective.
The synchronized spider crab shuffle at 0:50 is good comedy, but things get terrifying when they start climbing on top of one another and become a living mound that's "nearly 100 meters long." At times the footage looks more alien invasion than nature doc.
As to why in the heck these spineless crustaceans are trooping around and piling on each other instead of occupying a circle of hell: They're growing together. In order to do that, they must break out of their "hard, unexpandable shell," a process you can witness (to your horror) at 1:55. Under the old shell is a new softer one, which takes a few days to harden, a period during which their legs hardly work. This makes them vulnerable to predators like stingrays, so the spider crabs band together for safety. The whole process is called mounting.
As you can tell by the footage, the crew got pretty intimate with the spider crabs. "The crabs didn’t seem concerned about our presence and I would often find one or two clambering over my legs while filming!" camera operator Alex Vail wrote in a behind-the-scenes article.
Blue Planet II is still ongoing, so it's safe to assume there are plenty more aquatic oddities and terrors coming our way.