WASHINGTON — Starfish have been mysteriously dying by the millions in recent months along the US west coast, worrying biologists who say the sea creatures are key to the marine ecosystem.

Scientists first started noticing the mass deaths in June 2013. Different types of starfish, also known as sea stars, were affected, from wild ones along the coast to those in captivity, according to Jonathan Sleeman, director of the US Geological Survey’s National Wildlife Health Center.

“The two species affected most are Pisaster ochraceus (purple sea star or ochre starfish) and Pycnopodia helianthoides (sunflower sea star),” he wrote in a statement in December.

The sunflower sea star is considered among the largest starfish and can span more than a meter in diameter.

The most commonly observed symptoms are white lesions on the arms of the sea star. The lesions spread rapidly, resulting in the loss of the arm. Within days, the infection consumes the creature’s entire body, and it dies.

Entire populations have been wiped out in Puget Sound off the coast of Washington state, in the Salish Sea off Canada’s British Columbia as well as along the coast of California. The mortality rate is estimated at 95 percent.

Scientists who have spent decades studying the […]