Scientists say about a dozen different species of starfish are afflicted with the disease, which they're calling "sea star wasting syndrome." Cases have been reported on both the east and west coasts.
Sick starfish develop lesions on their bodies and begin to twist their arms into knots. The arms then crawl in opposite directions until they rip off. The starfish are unable to regenerate new limbs and die within 24 hours.
The disease is spreading up and down the coast, but seems centered around the Pacific from Alaska as far as San Diego. The prevailing theory right now is that a boat carried a pathogen from the other side of the world. Scientists say the theory is supported by the fact that the so-called hotspots generally line up with major shipping routes.
The starfish deaths also have a major impact on the ocean ecosystem.
Sea stars are voracious predators, like lions on the seafloor. They gobble up mussels, clams, sea cucumbers, crab and even other starfish. That's why they're called a keystone species, meaning they have a disproportionate impact on an ecosystem, shaping the biodiversity of the seascape.
Laura James, a diver who first noticed the dying starfish, is trying to track the disease in real time through social media with the tag #sickstarfish.