An “underwater zombie apocalypse.” That’s how wildlife veterinarian Joe Gaydos of the University of California (UC), Davis, describes “sea star wasting disease,” a blight that has decimated more than 20 species of sea stars from Mexico to Alaska since 2013. Now, a new study by Gaydos and colleagues has more bad news: The disease has hit the sunflower star (Pycnopodia helianthoides)—a key predator within kelp forests—hardest of all. This once-common species has vanished from the majority of its range, sending shock waves through the ecosystems it once called home. The team also found a worrying association between warmer ocean temperatures and the severity of the outbreak, suggesting climate change could exacerbate future marine epidemics.
“This is shocking,” says marine ecologist Mark Carr of UC Santa Cruz, who was not involved in the study. “This is not just a population reduction, this is virtually the loss of a key species over thousands of miles. We’ve never seen anything like this before.”
Sea star wasting disease progresses from “that looks weird,” to “horror movie,” over a few days. White lesions appear, then expand into fissures of melting tissue. Limbs fall off and crawl away. And finally, the sea star disintegrates into a pale mound of decaying flesh.
Scientists still haven’t identified the pathogen responsible for the disease. Research suggests the culprit is a virus, but which one remains unknown. Similar die-offs have struck the West Coast in previous decades, but none has been so deadly over such a large area. Of the 20 species affected by the outbreak, lab tests showed the sunflower star to be among the most susceptible.
The meter-wide, 24-armed sunflower star stalks the kelp forest swallowing prey like kelp-munching sea urchins whole. As one of the top predators of invertebrates these supersize stars help maintain balance in the kelp forest ecosystem. Left unchecked, sea urchins can mow down kelp forests, leaving behind a denuded and depauperate undersea landscape. The sunflower star used to be a common sight underwater, but since its disappearance and the subsequent boom of urchins, northern California has lost more than 90% of its kelp forests, according to the California Department of Fish and Wildlife.
The loss of those kelp forests has left the other species that depend on them hungry, homeless, or dead. In December 2018, California moved to extend a ban on recreational fishing for red abalone (Haliotis rufescens) after surveys showed the mollusks, which feed on kelp, were starving to death in huge numbers. Impacts to fish species are more challenging to quantify, but Carr says kelp forests are of vital importance not just as food, but as habitat, especially for young fish hoping to evade predators.
To gauge the impact of sea star wasting disease on the sunflower star, Gaydos’s colleague Drew Harvell, a Cornell University marine ecologist based in Friday Harbor, Washington, and other team members analyzed counts of the sunflower stars from nearly 11,000 shallow water scuba dives and close to 9000 bottom trawling surveys in deeper water. Hundreds of citizen scientists trained to identify and record the presence of the sunflower star conducted the shallow water surveys, and the National Oceanic and Atmospheric Administration (NOAA) conducted the bottom trawls, which consist of systematically dragging a net along the sea floor to sample marine biodiversity.
These data sets spanned nearly a decade prior to the collapse of sea stars and covered more than 3000 kilometers of coastline. Shallow and deep-water surveys showed stable populations followed by steep declines of the sunflower star ranging from a 60% population reduction up to 100% in some areas after the onset of the wasting disease in 2013, the researchers report today in Science Advances.
“Many people expected the sunflower stars to be taking refuge in the deep water where we couldn’t count them,” says Steve Lonhart, a kelp forest ecologist with the NOAA based in Monterey, California, who was not involved in the study. “We hoped they were hiding down there—this research shows that hope was naïve.”
The onset of sea star wasting disease also coincided with the warmest 3-year period on record for California’s coastal waters—2014, 2015, and 2016—according to NOAA climate researcher Nate Mantua in Santa Cruz, who was not involved in the study. To see whether there was a connection between water temperature and the disease, the study authors compared sea surface temperatures from the times and locations of each survey with the decline in sunflower stars. Their analysis found that the times and locations of the biggest death tolls coincided with the presence of abnormally warm water.
Mantua is the co-author of a 2018 paper in the Bulletin of the American Meteorological Society showing that climate change played a large role in the warming of California’s coastal waters from 2014 to 2016. Climate projections indicate those temperatures will become commonplace by the 2050s, he says.
“Many of these outbreaks are heat sensitive. In the lab, sea stars got sick sooner and died faster in warmer water,” Harvell says. “A warming ocean could increase the impact of infectious diseases like this one.”
The declining kelp forests of northern California are unlikely to recover unless sea urchins succumb to a pestilence of their own or their natural predators are restored. Harvell thinks the imperiled sunflower star should get strong consideration for being added to the U.S. Endangered Species List, and that a formal recovery plan may be necessary.
“I’m more worried now than I was before I read this paper,” Lonhart says. “We could be watching the extinction of what was a common species just 5 years ago.”