Why Almost All of the West Coast’s Sunflower Sea Stars Have Wilted Away
A new study suggests most of the keystone predators have died off due to an unknown pathogen and increasing ocean temperatures
Visitors to the United States’ Pacific coast over the last six years may have noticed that something is missing. A massive number of sea stars that used to dot the coastline are gone. And it’s not just tidepools and coastal inlets that are missing their sea stars either—they've vanished from the deeper ocean as well, new research shows.
Research shows that one species in particular, the huge sunflower sea star, has been hit especially hard, leading to dire consequences for kelp forests where it is a top predator, according to a new study in the journal Science Advances.
In the fall of 2013, ecologists and divers began to see something terrifying along the west coast of North America, from Alaska to Mexico. The seafloor was littered with severed sea star arms, and sea stars of every species clinging to coastal rocks were covered in sores and disintegrating into white mush, reports Ed Yong at The Atlantic.
The apocalyptic scenario has continued off and on to this day, with sea stars disappearing from entire swathes of coastline. While researchers have determined some of the 20 or so species impacted by the die-off were infected with a virus that causes sea star wasting disease, SSWD, not all species were susceptible to the disease, meaning there are likely other broader, overlapping causes.
Researchers sought to quantify the impact of the disease and understand the cause of the die-off in Pycnopodia helianthoides, or sunflower sea star. These predators can grow up to roughly three feet in diameter, munch on sea urchins and mussels, and, until recently, could be commonly found all the way from Alaskan waters to the coasts of Mexico.
Since SSWD first appeared, sunflower stars have more or less completely disappeared from their 2,000-mile range and are gone from the California coast. Some ecologists thought that the stars may have migrated into deeper water to avoid the factors leading to the sea star apocalypse. But according to the new study, that's not the case.
Deep water trawls and surveys by recreational divers confirm that the sea sunflowers have vanished down to about 3,000 feet. Trawls by NOAA in California and Oregon between 2013 and 2015 found that 100 percent of the stars were gone from deep water, and in Washington state they had declined by 99.2 percent. Yong reports that in 2016, over 700 trawls NOAA not find a single star, and last summer they found just one.
“This thing was as common as a robin,” study author Drew Harvell of Cornell University tells Yong. “You would go on a dive and always see sunflower stars.”
The revelation that the stars are dead and not sitting in deep water, waiting out the epidemic is a bad omen for many marine scientists.
“This is shocking,” Mark Carr, a University of California, Santa Cruz marine ecologist who was not involved in the study, tells Alex Fox at Science. “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.”
The loss of the sunflower star is already having major effects on coastal ecosystems. The manhole-sized, 24-armed star is a highly-tuned keystone predator and kept kelp-munching urchins and coast-clogging mussels in check. Fox reports that without the sunflower star doing its job, northern California has already lost 90 percent of its kelp forests, which are one of the most biodiverse and important coastal ecosystems.
That, in turn has led to a ban on red abalone fishing, since the mollusk relies on kelp and are now dying off at a high rate. Whales, sea otters, seals and many species of birds rely on the kelp forests for food and protection from the elements too, but many former kelp forests have already been transformed into urchin barrens, with nothing but the spiny black creatures covering the seafloor.
So why have the sunflower stars been hit so hard? The researchers believe that the sunflower is particularly susceptible to whatever pathogen is causing the wasting disease, and that other creatures more tolerant of the virus keep transmitting it to the vulnerable stars.
But the severity of the die-off is likely caused by a one-two punch. There have been smaller-scale die-off in the past, but the researchers believe large increases in ocean temperatures caused by strong El Nino years in the past decade and climate change are likely stressing the stars, making them more susceptible to the pathogens and causing a much broader pandemic.
“The heat wave in the oceans—a product of increasing atmospheric temperatures—is exacerbating the sea star wasting disease,” Harvell says in a press release. “It’s a lethal disease, and when you add a higher temperature to that, it kills faster, causing a bigger impact.”
Research published last summer showed that one species hit by the wasting disease, the ochre sea star, seems to be recovering and its genome has even changed as a result of the disease. It’s yet to be seen whether the sunflower star will also have the genetic resources to weather the storm. As Yong at The Atlantic points out, these types of disease outbreaks compounded by high temperatures are becoming more common and have impacted other marine species as well and have even begun to take toll on land mammals as well.