Sea star study to examine coastwide impacts of wasting disease outbreak

September 9, 2017

After a massive wasting disease outbreak caused sea stars across the western North American shoreline to literally dissolve into goo in 2013 and 2014, researchers are beginning a three-year study to determine what the consequences are.

UC Santa Cruz’s Institute of Marine Science Director Peter Raimondi said the study will specifically examine the evolutionary and ecological consequences of the die-offs, such as whether the sea stars have grown resistant to the disease and whether the absence of sea stars has allowed other species like mussels to overrun tideland areas.

What’s still currently unknown though is why the wasting disease became so widespread.

“To be completely honest, we don’t know why it occurred in the first place,” Raimondi said. “It’s pretty mysterious why it was so bad and prevalent and why it stopped.”

While smaller outbreaks of the wasting disease have been recorded in the past, the disease devastated sea star populations from Alaska to Mexico from 2013 through 2014. In some areas, up to 95 percent of the sea stars were killed. The North Coast saw about an 80 percent reduction in purple ochre sea stars and six-armed sun stars.

The disease initially causes white lesions to show up on the arms of affected sea stars and eventually, the organisms start to disintegrate or turn into goo in a period lasting a week or less.

Following the outbreak, droves of baby sea stars began showing up on the shores of Oregon and California during the summer of 2015. In some areas in southern Oregon, baby sea star abundance was more than 300 times the normal amount.

The baby sea star boom also came to the Humboldt County shoreline. But Humboldt State University Marine Biology graduate student Georgia Bennett, who has been studying sea stars in the Trinidad area for nearly four years, said that they also observed a high death rate of baby sea stars that same year.

“The juveniles would contract the disease and they would immediately succumb to it,” Bennett said.

While recent surveys on the Trinidad coastline between May and August have found fewer sea stars overall, Bennett said the good news is that they are finding fewer infected stars as well.

Bennett said purple ochre sea stars are a keystone species in the region as they feed on California mussels, which can dominate intertidal habitat if unchecked by predators. As more sea stars die, mussels are able to expand their reach, taking over surfaces that algae would normally grow on and thereby reducing available food and habitat for other marine critters. The population of larger six-armed sun stars has also reduced over the years. Bennett said these stars feed on sea urchins, which can mow down whole kelp forests if left unchecked.

Bennett said a major questions now are whether the sea star populations will recover, and what will happen if they don’t.

The three-year study led by UC Santa Cruz, Oregon State University and UC Merced will attempt to show whether the loss of sea stars has shifted the diversity of species in intertidal areas throughout the western U.S.

“A lot of people get a great deal of pleasure of visiting the rocky shores and seeing all the colorful organisms,” Oregon State University Marine Biology Professor Bruce Menge said. “I know there is a lot of disappointment in the public that it’s really no longer the way it was. It’s now quite different. In the future it could become even more different if mussels take over.”

Menge has been studying sea star populations for several decades, but said the wasting disease outbreak that occurred in recent years was unprecedented in time and scale. Whole populations of sea stars in some areas of southern California have been wiped out, he said.

In his study areas in southern and central Oregon, Menge said recent surveys show that sea star abundance varies from place to place, with some areas have very few sea stars and others showing high abundance. However, the wasting disease has not been as prevalent, though baby sea stars are still dying off.

“It’s a mixed bag,” Menge said. “At some locations that first flush, the ones that came in the first year, hasn’t been repeated and most of those little ones that came in seemed to have died. ... At other locations they did well and are actually growing toward adulthood.”

Raimondi said another aspect of their study will use genetic mapping techniques to determine how certain sea star populations fared during the wasting disease outbreaks.

Raimondi said survivors of the outbreak may have genes that make them more resistant to the wasting disease, which can be passed on to future generations. Another outcome could be a genetic bottleneck, Raimondi and Bennett said, which could result in less genetic variation and make the sea stars more susceptible to future die-offs.

“Having babies is one thing, but they need to survive to rebuild the population,” Raimondi said.

While there have been hopeful signs that the wasting disease has lessened in intensity, Raimondi said he recently returned from a trip to Alaska where there was evidence that a massive outbreak was occurring once again.

“There may be some good signs, but it’s definitely not very good,” Raimondi said.