A mystery disease outbreak that has devastated more than 20 species of starfish along the western coast of North America since 2013 has claimed yet another victim: the sunflower sea star (Pycnopodia helianthoides), a major predator within kelp forests in the Northeast Pacific.
The “sea star wasting disease,” as the infectious disease is called, kills off starfish rapidly: diseased sea stars develop skin lesions, their arms detach from their central disc, their internal organs spill out, and individuals die, leaving behind bits and pieces of limbs and bodies. The disease epidemic isn’t new and has previously killed starfish. But the latest epidemic event between 2013 and 2015 has been particularly disastrous, researchers say, both in magnitude and geographic extent. And this time, warming oceans may have increased the deadliness of the disease, the researchers report in a new study published in Science Advances.
To see the extent to which sunflower sea stars had been affected by the disease, trained recreational scuba divers searched for the sunflower sea star off the coast of Washington, Oregon, California and northern British Columbia (Canada) over nearly 11,000 dive surveys conducted between 2006 and 2017. Scientists from the U.S. National Oceanic and Atmospheric Administration also counted the sea star in deeper waters by conducting around 9,000 bottom trawling surveys between 2004 and 2016. In these surveys, the researchers would drag a large net on the ocean floor for a set time or distance, and record the kinds of species they managed to sample.
Both surveys showed that the sea stars had relatively stable populations until 2012. After the onset of the sea star wasting disease, however, the starfish’s population declined by 80 to 100 percent across its 3,000-kilometer (1,860-mile) range.
“At one time plentiful in nearshore waters, the sunflower sea stars right now cannot be found off the California coast and are rare into Alaska,” Drew Harvell, a professor of ecology and evolutionary biology at Cornell University and co-lead author of the paper, said in a statement. “Numbers of the sea stars have stayed so low in the past three years, we consider them endangered in the southern part of their range, and we don’t have data for northern Alaska.”
The widespread decline of the starfish, especially in deeper waters, has been particularly shocking, researchers say.
“Many people expected the sunflower stars to be taking refuge in the deep water where we couldn’t count them,” Steve Lonhart, a researcher with the NOAA, told Science news. “We hoped they were hiding down there — this research shows that hope was naïve.”
The study’s authors say that warming oceans due to climate change could have exacerbated the disease’s impact. They found that the occurrence of the largest declines in the sunflower sea star numbers coincided with abnormally high sea surface temperatures.
“The heat wave in the oceans — a product of increasing atmospheric temperatures — is exacerbating the sea star wasting disease,” Harvell said. “It’s a lethal disease, and when you add a higher temperature to that, it kills faster, causing a bigger impact.”
The collapse of the sunflower sea stars could have cascading effects on the ecosystem. The sea star is a major predator of sea urchins, and without the sea stars to keep a check on the urchin population, the latter would feast on the kelp forests, leaving behind a barren seafloor. Researchers have already seen large declines in kelp forests where sunflower sea star numbers have dropped drastically.
“In California, Washington and parts of British Columbia, sunflower sea stars keep urchins under control,” said co-author Joseph Gaydos, a wildlife veterinarian at the University of California, Davis, and science director of the UC Davis SeaDoc Society program. “Without sunflower stars, urchin populations expand and threaten kelp forests and biodiversity. This cascading effect has a really big impact.”
Banner image: Thousands of sunflower sea stars swarm Croker Rock near Croker Island in the Indian Arm fjord, north of Vancouver, British Columbia, on Oct. 9, 2013. Three weeks later, the sea stars had vanished. Image by Neil McDaniel.
C. D. Harvell et al. (2019) Disease epidemic and a marine heat wave are associated with the continental-scale collapse of a pivotal predator (Pycnopodia helianthoides). Science Advances. DOI: 10.1126/sciadv.aau7042
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