On the west coast of North America ranging from Alaska down to California, starfish are dying at an alarming rate. Beginning in June of 2013, millions of these echinoderms have died due to a disease that causes their legs to fall off and their internal organs to fall into the water. This disease is affecting wild starfish as well as those kept in captivity, and the worst part is that nobody knows what it causing it. The disease is tentatively called “Starfish Wasting Syndrome.” This announcement comes from Jonathan Sleeman, director of the US Geological Survey’s National Wildlife Health Center.

Starfish first emerged 450 million years ago in the Ordovician, around the same time that sharks first appeared. They have a large impact on marine ecosystems ranging from icy polar waters to warm tropical reefs by keeping populations of mussels, snails, and barnacles in check. The starfish has been called a “keystone species” because it only takes a few starfish to make an incredible impact on their environment.

Though there are about 1,500 known species of starfish alive today, there are two species particularly impacted by this deadly disease: the purple sea star (Pisaster ochraceus) and the sunflower sea star (Pycnopodia helianthoides). Though these species are typically able to regenerate arms that are lost due to trauma, this disease works much too quickly for the starfish to recover.

At the onset of infection, the arms break out in white lesions. In very short order, the lesions spread and causes the arms to deform and ultimately break off. Not only do the limbs fall away, but the body of the starfish also loses its integrity and releases its inner organs, killing the organism. It only takes a few days for the animal to die after the first sign of disease. There is a 95% mortality rate for those who get infected.

Though starfish are extremely susceptible to changes in water temperature and warming waters due to climate fluctuations can kill small populations, nothing has ever been seen on this scale before. Previous die-offs had an obvious cause and affected only a fraction of what has been lost since last summer.

The same disease that is stumping marine biologists on the west coast also affected a small population of starfish on the east coast last year. Thankfully, the impact was minimal and did not spread up and down the coastline.

Currently, researchers are analyzing samples in the lab in order to determine the cause, including if it is caused by pollutants in the water or an infection from a microorganism. Some speculate that the latter is most likely, and this could be the handiwork of a parasite or virus.