In recent months, millions of starfish have been mysteriously dying along the U.S. west coast, worrying biologists who say the sea creatures are key to the marine ecosystem, Agence France-Presse reported.

The alarming mass deaths were first noticed by scientists 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 U.S. 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.

Spanning more than a meter in diameter, the sunflower sea star is considered among the largest starfish, AFP reported.

"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," AFP reported. "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.

After spending decades studying the local ecosystem, scientists have yet to identify the cause.

"What we currently think is likely happening is that there is a pathogen, like a parasite or a virus or a bacteria, that is infecting the sea stars and that compromises in some way their immune system," Pete Raimondi, chair of the department of ecology and evolutionary biology, at the University of California, Santa Cruz, told AFP.

Then, the creatures become more susceptible to bacteria which is "causing a secondary infection that causes most of the damages that you see," AFP reported.

According to AFP, the 2013 phenomenon has not been observed solely along the West Coast. Last year, a smaller outbreak also killed East Coast sea stars.

"Previous cases were believed to be associated with warmer waters -- sea stars have sensitive skin and prefer cooler water -- but this was not the case in 2013," according to AFP. "And when the die-offs happened previously, the geographic span of the infections was much smaller, and far fewer sea stars were affected."

Along the southern coast of California, an epidemic nearly wiped out the Pisaster ochraceus from tidal pools in 1983.

In 1997, scientists said another smaller die-off may have been caused by warmer waters in an El Nino year, AFP reported.

Sea stars are important because "they play a key role in this ecosystem on the West Coast," Raimondi said.

Sea stars eat mussels, barnacles, snails, mollusks and other smaller sea life, so their health is considered a measure of marine life on the whole in a given area.

When sea stars decline in number, "the mussel population has the potential to dramatically increase, which could significantly alter the rocky intertidal zone," according to Sleeman.

"While sea stars make up an important component of the base of the ocean food chain and are considered a top predator, they are in turn eaten by other starfish, shorebirds, gulls, and sometimes sea otters," AFP reported.

Scientists are collecting reports from the public and taking specimens to the lab for analysis and doing genetic sequencing in an effort to find out what is causing the mass deaths and whether a toxin or an infection may be to blame.