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The mysterious culprit behind a deadly sea star disease is not an infection, as scientists once thought.

Instead, multiple types of bacteria living within millimeters of sea stars’ skin deplete oxygen from the water and effectively suffocate the animals, researchers report January 6 in Frontiers in Microbiology. Such microbes thrive when there are high levels of organic matter in warm water and create a low oxygen environment that can make sea stars melt in a puddle of slime.

Sea star wasting disease — which causes lethal symptoms like decaying tissue and loss of limbs — first gained notoriety in 2013 when sea stars living off the U.S. Pacific Coast died in massive numbers. Outbreaks of the disease had also occurred before 2013, but never at such a large scale.

Scientists suspected that a virus or bacterium might be making sea stars sick. That hypothesis was supported in a 2014 study that found unhealthy animals may have been infected by a virus. But the link vanished when subsequent studies found no relationship between the virus and dying sea stars, leaving researchers perplexed.