Don't worry, they aren't assembling into an army to go to war against humans and take over the world. They couldn't even if they wanted to. They move too slowly for one, and their long thin pincers are too weak to do us any real damage, for another.
While the exact reason is not known, scientists say that the sea creatures are coming together in their droves to moult – shed their outer shells which have become too small for them. These incredible scenes in the video above are from Australia. Since the crabs' soft bodies are more vulnerable to natural predators like the octopus, larger fish and seagulls, they are hoping for safety in numbers. Normally, they camouflage themselves by covering their shells with seaweed and algae.
Aquatic scientist Sheree Marris said, "I've seen the aggregation so many times but it never ceases to amaze me. This was by far the largest I have ever seen and it's going going to get bigger and better as the crabs are still on the march."
The video below shows incredibly detailed time lapse footage of one of the crabs moulting. Don't forget to breathe, though.
Here's a live example of the famous adage, "Don't judge a book by its cover." Even potentially dangerous creatures like giving out hugs. Albeit slightly creepy ones.