Billions of starfish died of rotting and melting skin disease. Scientists now know who did it

Billions of sea stars have decayed and melted over the past decade, and scientists have been trying to figure out what killed them. Now, a team of researchers has solved the murder mystery that unfolded deep in the ocean. A study carried out into the deaths revealed that the bacterial cousin of cholera triggered the mass deaths that have stumped marine researchers for 10 years. The team of scientists from the US and Canada studied wild and captive starfish, and zeroed in on the culprit - Vibrio pectenicida. This bacterium caused the sea star wasting disease, leading the creatures to reach a point of decay before dying off. The disease in sea stars was first recorded in 2013. A massive outbreak of SSWD struck the North American Pacific coast. It spread from Alaska to Mexico and killed more than 20 different species of sea stars. Within days, starfish in the affected region developed lesions, with the decay setting in soon, killing them within days. This 12-year-old incident is believed to be the largest marine disease epidemic ever recorded in the wild. Also Read: Fearing being killed by sexually aggressive females, male tarantulas evolve extra-long genitals

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A starfish species lost 90 per cent of its population

Besides the 2013 mass die-off, several more instances of sea star wasting disease have been recorded in the past decade. The population of one particular sea star species, Pycnopodia helianthoides, has been reduced by a whopping 90 per cent because of SSWD. This triggered an imbalance in the ecology of regions where the starfish lived. As their numbers reduced, sea urchins started to thrive and destroyed kelp forests. Scientists have tried to solve the mystery of the mass deaths ever since the disease first emerged. In 2014, a study noted that a sea star-associated densovirus led to the skin-melting disease in sea stars. However, it was quickly dismissed as any pathogenic virus could only affect a minority, ruling out that it caused the mass die-off. Also Read: Mysterious symbols believed to be 'ancient warning from ancestors' surface on Hawaii beach

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Sea star wasting disease

The researchers compared the samples of sea stars with SSWD and healthy specimens. They discovered high levels of the bacteria in the coelomic fluid of diseased sea stars. The same was not found in the healthy starfish. They isolated the bacteria and grew them, and then exposed healthy sea stars to them. The creatures quickly developed SSWD and died. Further analysis revealed that SSWD was caused by a specific strain of the bacteria, called FHCF-3. The methodology used to test the bacteria and their effects is the same as that used to check which germ is causing a particular disease in humans. This convinced the scientists that Vibrio pectenicida had killed the sea stars. However, the study authors are still working to understand what causes the disease. Possible reasons include their shared food, physical contact or a low-lying presence in their surroundings.