The largest-ever marine epidemic has been wiping out billions of starfish in a particularly unpleasant way for more than a decade – but now researchers have finally succeeded in pinpointing the cause of sea star wasting disease (SSWD), bringing hope that a cure can be found.
Eastern Pacific marine ecosystems from Mexico to Alaska have been devastated by a mysterious killer that has now been identified as a strain of the Vibrio pectenicida bacterium, FHCF-3.
Vibrio species proliferate in warm water and have infected not only coral and shellfish in the past but also humans, because Vibrio cholerae is what causes cholera.
The first symptoms of the sea star wasting disease are lesions that rapidly cause the animal to become contorted and lose its arms as the tissues melt. Within about a fortnight it is dead.
The scientific team that has solved the riddle was made up of researchers from the University of British Columbia (UBC); the Hakai Institute, also in BC; and the University of Washington. Their major challenge in trying to identify the disease was that other stressors such as temperature change can also cause sea stars to contort and lose their arms.
The epidemic has affected 20 sea star species but the team concentrated on sunflower sea stars (Pycnopodia helianthoides), which have been so badly affected that more than 90% of the population is reckoned to have been lost to SSWD.
When healthy stars were exposed to others that had already contracted the disease through contaminated water, infected tissue or coelomic fluid (starfish ‘blood’), more than 90% became infected within a week.
Vibrio was detected in the coelomic fluid of sick sea stars. “Using DNA sequencing, we saw there was a huge signal of a particular bacteria,” says UBC research scientist and study co-author Amy M Chan. “This was our prime suspect to isolate.
“When I did, I saw basically only one kind of bacteria growing on the plates and thought: ‘This has got to be it’.”
When pure cultures of V pectenicida strains including FHCF-3 were injected into healthy sea stars, all were dead within days of showing symptoms.
The researchers believe that the spread of SSWD since 2013 has been related to ocean-warming. “We see the disease occurring earlier and more rapidly in warmer waters,” says first author of the study Dr Melanie Prentice, a UBC and Hakai Institute research associate.
“Sea stars may already be impacted by climate change, so introducing a pathogen that does well in the same circumstances could be a double whammy for some species.”
The loss of billions of sunflower sea stars has caused widespread, lasting effects on marine ecosystems, say the researchers. Without the predator that kept sea urchin numbers in check, populations have increased and devoured the kelp-forest habitats on many other marine creatures depend.
“These forests also contribute millions of dollars through fisheries and tourism, sequester carbon dioxide, protect coastlines and are culturally significant for coastal First Nations,” says Prentice. “Now that we’ve identified the disease-causing agent, we can start looking at how to mitigate the impacts of this epidemic.”
The Nature Conservancy and Washington Department of Fish & Wildlife also supported the research.
“We are actively pursuing studies looking at genetic associations with disease resistance, captive breeding and experimental introduction of captively raised stars back into the wild,” says The Nature Conservancy California chapter director of ocean science Jono Wilson.
These studies will, he says, indicate the most effective strategies and locations needed to reintroduce sunflower sea stars into the wild. The study has been published in Nature Ecology & Evolution.
