MELBOURNE, Australia (AFP) - A 50,000-strong swarm of spider crabs gathered off a Melbourne beach this week, covering a stretch of seafloor the size of a football field and likely "scaring the hell" out of local fish, marine scientists said.

Museum Victoria scientists Mark Norman and Julian Finn said they filmed the mass gathering, which was up to a meter (3.3 feet) deep in places, with crabs piled 10 high.

Spider crabs are about 20 centimeters (nine inches) long with legs extending up to 40 centimeters.

Norman told The Age newspaper on Thursday the reason for the mass swarming was unknown, but it was likely related to breeding.

The crabs are normally found in small groups in Melbourne's Port Phillip Bay and the Bass Strait, but few people have encountered an invasion of this scale, he said.

"We got some earlier warning from divers that they were here en masse, but we had no idea it was going to be so big," he said.

"It was like something out of a science-fiction movie... as far as you could see there were crabs," he said, adding that the crustaceans were "scaring the hell out of local fish".

The swarm began gathering early this week but by Thursday was thinning out, he said.

"This has been observed elsewhere in the world, like the Mediterranean ... it's not a rare event, but it's rare that you encounter it."

Norman would not say exactly where the swarm was located, only that it was in four meters of water off a beach in Melbourne's Port Phillip Bay.