This video, filmed at the Shannon Point Marine Center in Washington, shows you the contrast between normal, healthy sea stars and those suffering from wasting disease. (Note: Some parts have been sped up, so you can healthy see sea stars moving and diseased limbs falling off.)
First documented in 2011, the disease really took off in 2013, and it's still a big scientific mystery as researchers try to figure out what is causing mass starfish die-offs and whether there's anything that can be done to stop it.
So far, we know enough to say that Sea Star Wasting Disease most likely has nothing to do with radiation leaked from Fukushima. Instead, the best available evidence suggests that it's a real disease — caused by a virus, bacterium, fungus, or protist infecting the starfish — and that it might have some important connections to water temperature and what foods the starfish are eating. PBS Newshour has a nice piece summarizing where this science currently stands:
At the University of California Santa Barbara Aquarium, captive sea stars started showing signs of the syndrome at the same time as their wild counterparts who live on the rocks several hundred feet from the tanks. The captive sea stars are kept in tanks of filtered seawater. In one tank they were fed mussels harvested from the rocks outside. In another tank the sea stars were fed frozen squid.
The animals that ate frozen squid stayed healthy, while the sea stars that ate the wild-harvested mussels contracted the syndrome. Blanchette cautions that these observations are purely anecdotal and the sample size is very small, but she believes this hypothesis merits further study.
Scientists are particularly worried about the disease because of its potential to cause localized extinctions of various starfish species — deaths that would have big impacts on regional food chains where the starfish are major predators of shellfish and other creatures. Because of that, it's interesting to learn that, in some of the areas first affected by the disease, scientists are now finding swarms of baby starfish. Apparently born in a last-ditch effort to continue the species, the baby starfish are only now being noticed because it can take six months before they're big enough to be spotted with the naked eye. However, it's still unclear whether the new babies represent a sea star rebound — as happened in some earlier outbreaks — or whether they, too, will be killed off.