This incredible crustacean pile-up was filmed in Australia's Port Phillip Bay, where spider crabs (Libinia emarginata) are gathering by the thousands...

The animals colonise these waters – a safe haven in which to moult their hard exteriors – each spring. They typically start moving up from the depths in March, and this bunch of early "risers" put on quite a show when they tore apart a small octopus. 

Common spider crabs will eat just about anything they can overpower (typically algae, small fish and sea stars), but they're also quite sluggish. A jet-powered cephalopod like this would usually have no trouble avoiding a single crab – a mass of pincers, however, is a trickier threat. It's also possible that the octopus was already dead when the crabs turned up, or that it tried to land a crab dinner when things backfired. 

"[The crabs] get together to be safe in numbers for shedding — they're so edible when they're soft," Parks Victoria chief conservation scientist Dr Mark Norman told The Herald Sun. 

Eventually, this leggy congregation will grow to be the size of a football field, ten cran layers deep! In the coming months, local divers should expect to see "crab pyramids" forming in the area: 

"Film crews come from around the world to film [the migration] but most people have no idea it’s going on," says Norman.

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Top header image: Saspotato/Flickr