Sea stars, also known as starfish, have a reputation for being resilient animals that can regenerate lost limbs. However, in 2013, sea stars off of the Pacific Coast began rapidly dying in huge numbers, and no one knew why.

Similar sea star mass die-offs have been recorded for decades, but this event was one of the largest wildlife mass-mortality events ever recorded. Sick sea stars become covered in lesions, permanently losing their limbs and melting into blobs of decayed tissue. This illness, termed sea star wasting syndrome (SSWS), was widely thought to be caused by a viral infection. However, this could not be replicated in the lab and was ultimately disproven.

Now, in a new study published in Frontiers in Microbiology, researchers have found the mysterious illness was caused by microorganisms sucking up oxygen from the water around infected sea stars, essentially suffocating them. 

The researchers had previously investigated and ruled out other factors, such as water temperature, as the cause for SSWS. However, when they examined the water immediately surrounding sick sea stars to the environments around healthy sea stars, they found that nutrient-loving bacteria living on the sea stars had used up all the oxygen that they need to breathe. 

Although SSWS is caused by an ecological interaction rather than an infection, it can still be transmitted between sea stars. As dying sea stars decay, they generate organic matter that can promote bacterial growth on nearby sea stars in a dangerous feedback loop.

Sea stars play essential roles in many ecosystems and help maintain local biodiversity. Knowing how this disease develops can help researchers treat sick sea stars in the lab, helping to preserve delicate ecological relationships.