If you tuned in to BBC's Planet Earth II this month, you got a refresher on Christmas Island's famous red crabs. Each year, tens of millions of these distinctive crustaceans stage a colourful invasion on the small island, which lies just south of Java. The red crabs traverse the Australian territory to mate on its sandy shores, and this year's group just completed the journey!

Red crab eggs hatch as soon as they come into contact with sea water, and for the first time ever, the spawning event was broadcast in real-time by Parks Australia!

A female red crab can lay 100,000 eggs per brood, but the number that actually develop depends on a variety of factors. How many hungry predators are in the area? How rough are the waters? For now, these tiny plankton are entirely at the mercy of their environment. 

If conditions are right, the larvae will morph into prawn-like animals called megalopae over the coming weeks. After moulting, they will finally return to land as fully formed crablings. "Some years we see very few, or none," says Australia's Department of Environment and Energy. "The successful emergence of baby crabs is unpredictable but is incredible when large numbers emerge."

Although they measure just five millimetres across, the baby crabs will begin the long march back to the forest as soon as they leave the water. 

"It takes about nine days to reach the plateau," adds the team at the Christmas Island Tourism Association, who hit the beach to watch the spawning event. "[Once there], they seem to disappear and are rarely seen, living in rocky outcrops and under fallen tree branches and debris on the forest floor for the first three years of their lives."

crab babies_2016_11_24.jpg
Red crab babies along the east coast of Christmas Island in 2015. Image: Parks Australia/Facebook

This year's brood is of particular ecological importance as an infestation of yellow crazy ants has been killing red crabs at an alarming rate. The invasive insects perceive the scuttling crowd of crabs as a threat, and spray passersby with formic acid in defence. The ants have wiped out over ten million red crabs to date – and it's humans who brought them to the island. Until just recently, Christmas Island had few biosecurity measures in place, so unwanted passengers like the ants snuck in with tourists.

While scientists work to mitigate the problem, every surviving crab is precious. By feeding on leaf litter, the crustaceans keep the forest floor in good health, and their abandoned burrows provide critical habitat for other animals. If their numbers continue to drop, we could see unwanted ripple effects throughout the island ecosystem.

Protecting red crabs takes many helping hands, and in recent years, the clawed creatures have earned their own commuter lanes, road signs and crossing guards. Here's hoping this year's babies scuttle their way to safety! 

Top header image: Geraint Draheim, Flickr