The shellfish pathogen that hit California’s Channel Islands in the 1980s began to quickly kill one of the tideland’s most important animals — black abalone.
But what unnerved scientists was what they learned next: Whenever ocean waters grew warmer, the deadly infection known as withering syndrome spread and killed even more abalone.
By the 2000s, this phenomenon had helped transform black abalone into an endangered species — and a symbol of how much climate change may one day influence the spread of marine diseases.
Long before a virus would kill West Coast sea stars by the millions, scientists had begun to wonder when a major human-caused marine-disease outbreak would strike. Now they’re wondering if so-called sea-star wasting disease is an example of the threat they predicted — or just part of a natural cycle they don’t yet understand.
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This month, scientists announced they’d identified the culprit responsible for a mass die-off of 20 species of starfish that started in Washington last year, and then spread to Southern California and north to Alaska. The cause was a virus that had been found in sea stars since at least the 1940s. But it had never killed anywhere near as many creatures or across so vast an area.
It’s too soon to say whether the sudden explosion of this starfish disease is linked to environmental changes wrought by humans, such as global warming or ocean acidification, which is the souring of seas by carbon-dioxide emissions.
But scientists say the die-off may be the most extensive marine-disease event ever documented. Few experts believe it will be the last.
“The most dire interpretation of the sea-star event is that it could just be the first in a wave of similar events,” said Bruce Menge, a marine-biology professor at Oregon State University.
In fact, from work with corals and eelgrass, dolphins, seals and fish, researchers increasingly are finding that climate change is likely to affect disease susceptibility and transmission in a host of important ways.
“A warmer world is a sicker world,” said C. Drew Harvell, a marine epidemiologist and coral-disease expert from Cornell University who has played a key role in studying the sea-star die-off. “A warming world can cause disease to increase, both by compromising the host and because a lot of microorganisms become more virulent or are happier at warmer temperatures.”
A parasite that affects East Coast oysters offers one of the clearest examples of how a warming world may change ocean diseases.
The pathogen can kill masses of oysters but was rarely seen north of the Chesapeake Bay. Then, in the late 1980s and early 1990s, a warm-water system allowed the parasite to quickly spread from Maryland to Cape Cod, Mass., and beyond. There, it infected more oysters faster than before and killed them far quicker.
Many pathogens tend to get knocked back by cold winters. But as the marine world continues to warm, their survivability and suitable habitat just expands.
“In my opinion, we’re going to see more infectious-disease outbreaks in the ocean,” said Colleen Burge, a research associate at the University of Washington and lead author of a of ocean pathogens published last year.
And as climate change and ocean acidification alter the food chain, the growth of plants and animals, and the timing of their interactions, those factors also will affect how diseases spread.
“Any organism that is going to be under more stress or has fewer numbers or lower genetic diversity is going to be more highly susceptible,” Burge said.
One place where that’s already true is the Caribbean, where disease has hit more than two-thirds of coral reefs.
“There’s been a big uptick in these infectious diseases that are affecting coral, and the impacts are really huge,” said Harvell. “Partly that’s because corals live so close to their upper thermal limits that any kind of warming puts them over the edge.”
So when corals experience warm waters, sometimes that causes them to jettison the algae that lends them color, turning them white in a process known as bleaching. That in turn, can make them more vulnerable to disease.
During one warm-water period off the Florida Keys in 2005, corals that experienced bleaching were more likely to soon become infected with deadly white-plague disease.
Meanwhile, ocean acidification is expected to make it harder for corals to grow, which may reduce some corals’ ability to fight off infection.
Sometimes it isn’t a weakened host that’s the problem, but a more virulent bug.
Many coral diseases, parasites and fungal infections also spread more rapidly when waters warm, sometimes up to 14 times more rapidly.
In fact, bacterial infections also have been known to cause coral bleaching. During that same warm period in Florida in 2005, corals and sea fans already suffering from splotchy purple or gray patches known as dark-spot disease were found to be more likely to bleach than healthy corals.
And sometimes the spread is more complicated as the fabric of an ecosystem changes. Thirty years ago a disease wiped out black sea urchins in an area of the Caribbean. Without the urchins to chomp down on plant life, algae took over 90 percent of leaflike elkhorn corals and branching staghorn corals. Warming temperatures have so beaten back remaining corals that the environment and all that lives there has been completely transformed.
But predicting how and where these issues might surface is inherently difficult. Just as climate change alters the environment in complex but fundamental ways, diseases, too, are influenced by many factors. And it can take years before the important ones are understood.
No simple explanation
The reason scientists worry about the sea-star die-off is that starfish play an important role in tidal environments. Their absence will fundamentally change how those nearshore areas function.
But the difficulty in understanding what triggered the sea-star virus to suddenly become so deadly this time is that little about the outbreak has been consistent.
In some areas in British Columbia, sea stars had become so abundant that researchers wondered if the virus somehow was just working as a natural check to keep sea-star numbers down. But in other regions, starfish numbers were already way down.
“We certainly have some populations where they were arm to arm to arm, but if you go back and look, it wasn’t necessarily those high-density sites that were hit first,” said Ben Miner, a Western Washington University professor who has tracked the sea-star epidemic.
In addition, some sites with few sea stars still were hit again.
Researchers have seen some relationship between the starfish outbreak and warm waters. But the disease hit Oregon when waters were cool and spread into Alaska just as summer turned to fall.
Menge, for one, suspects acidification may have played a role, because the death of sea stars in Oregon often corresponded to times when winds drew waters to shore from down deep. Those waters — already naturally rich in CO2 — now are more sour than they used to be, thanks to the absorption of fossil-fuel emissions from the atmosphere.
Other scientists believe it could be a combination of several factors at once.
The one thing most agree on now is that it will take time to figure out, and may never be entirely understood. And the same could prove true with future marine outbreaks.
“We all want it to be something that’s really clear,” said Burge, the UW researcher, “but with diseases, it often just is not.”