As the first full moon of winter arrives, hundreds of thousands of them emerge from the darkness and march across the seabed like an army.
This is the extraordinary moment the spider crabs' annual invasion of the Mornington Peninsula in south east Australia begins
Once a year, the crustaceans move en-masse across the bay's sandy bottom, clambering over one another to create mounds nearly 100 metres long in order to moult so they can grow.
The stunning images are captured in the next instalment of the BBC's Blue Planet II , which airs on Sunday at 9pm.
Sir David Attenborough reveals: "Like all crabs, their bodies are enclosed in a hard unexpandable shell. So to grow they have to break out of it, and that allows the soft one that has developed beneath to expand. It will take days for the new shell to harden."