• Every year, millions of Christmas Island red crabs make their way to the Indian Ocean to lay eggs.
  • David Attenborough's Netflix series "Our Planet II," shows an adult crab feasting on baby crablets. 
  • "I have never seen this behavior before," marine biologist Lucy Turner said. 

Each year, billions of baby crabs attempt a treacherous journey from the Indian Ocean to the forests of Australia's Christmas Island. The dangers include manta rays, whale sharks, and yellow crazy ants. 

But rare footage in David Attenborough's new Netflix series, "Our Planet II," shows an unusual source of peril: an adult Christmas Island red crab (Gecarcoidea natalis) using its pincers to pick up a few crablets and stuff them in its mouth. 

You can watch the unusual, cannibalistic behavior in the video below at about 1:22 minutes in:

"These crabs are opportunistic scavengers so will feed on anything," Lucy Turner, a marine biologist at the University of Plymouth, U.K., told Live Science. "I have never seen this behavior before, though, where they are cannibalistic of juveniles. I've seen them eat other dead adults."

The Christmas Island crabs are terrestrial, inhabiting the island's rainforests during most of the year. But sexually mature crabs leave their forest burrows in October or November to mate.

The timing is dependent on the season's first rainfall and the lunar cycle. A few weeks later, the females release their eggs – up to 100,000 each, according to Parks Australia – into the ocean.

When the eggs hit the water, the red crab larvae hatch. It takes about a month and several larval stages for the crabs to develop into crablets, measuring about 0.19 inches (5 millimeters).

Most years, the crabs won't reach the beach and are consumed as larvae by the fish and other predators. Conditions are right for the tides and currents to help sweep them towards the shore usually only once a decade. On these occasions, the masses of crablets scrambling over rocks and each other make the shore look red. 

Once on land, the crablets start their trek toward the forest. It can take nine days for the tiny crabs to reach the rainforest.

Along the way, the crablets may be attacked by yellow crazy ants, an invasive species that uses formic acid to blind and immobilize red crabs before consuming their corpses, according to The Conversation. And now, it seems the crablets have to contend with hungry adult crabs that usually only scavenge those that are already dead. 

You can watch "Our Planet II" on Netflix.