Hundreds of crown-of-thorns starfish found on a beach in southern Japan in January stranded themselves because they were starving, say researchers.
More than 800 were discovered on a 300m stretch of sand on Ishigaki island.
The starfish population “outbreak” was first identified in 2009, when masses of juveniles were seen feeding on the island’s outer coral reef.
The coral-eating starfish then took three years to move onto the beach where they perished.
The reason for the starfish population boom is not clear, but the strange behaviour has shown marine scientists what can happen when these slow-moving creatures completely deplete their food source.
“The shortage of food, corals, is a probable cause of the stranding,” said Go Suzuki from the Fisheries Research Agency, who witnessed the phenomenon from his research station.
In a paper, published in the journal Coral Reefs, Mr Suzuki and colleagues described how an area once covered with up to 60% coral was reduced to 1% by the voracious starfish.
The marine scientists described how the starfish gradually moved closer to the beach, possibly in search of more coral to feed on.
Mr Suzuki suggested that the current may have helped the starfish along their doomed path, pushing the animals towards the shore.
Noting that they died on the beach, rather than in the water, the team concluded that when the starving starfish were eventually washed up they were too weak to return to the sea.