In 2013, the lives of millions of starfish were mysteriously destroyed. Once-strong limbs groping arms for food, shriveled and tore away from the rest of their bodies, melting into a sickly blob.
“Weapons were everywhere,” said ecologist Drew Harvell The Atlantic Ocean‘s Ed Yong last year. “It looked like an explosion zone.”
The gloomy remains of these animals, usually capable of regenerating their own limbs, were scattered all along the west coast of North America, in one of the largest mass deaths in the wild ever recorded. More than 20 species of starfish perished.
In some areas, sunflower star (Pycnopodia helianthoides) populations fell by an average of about 90 percent in weeks, a loss that caused this once common and abundant species to disappear from most of its range in just a few years.
The culprit who caused this Sea Star Wasting (SSW) even got starfish in captivity and killed individual animals within days.
Leg of it Pisaster ochraceus the breakup of Sea Star Wasting syndrome. (Elizabeth Cerny-Chipman / Oregon State University / CC BY-SA 2.0)
This led scientists to suspect that some type of pathogen, such as a virus or bacteria, was infecting these amazing marine creatures. However, in later investigations, the prime suspect was acquitted.
Meanwhile, more starfish deaths followed around the world, including half a world away in Port Phillip Bay, Australia.
Now, San Francisco State University marine biologist Citlalli Aquino and colleagues have finally unraveled the mystery and showed that something much more complicated was going on.
By comparing the types of bacteria in healthy starfish to those suffering from the devastating disease, the researchers found that bacteria that thrive in low-oxygen environments were abundant in the sick animals, as were copiotrophs – bacteria that love nutrient-dense environments.
Laboratory experiments confirmed that depleting oxygenated water caused tissue melting in 75 percent of starfish. Adding excess nutrients or phytoplankton to the water also deteriorated the starfish’s health.
By re-analyzing tissue samples from the 2013 event, the researchers discovered excess nitrogen – a sign that these animals are choking.
“Starfish spread oxygen across their outer surface through tiny structures called papulae or skin gills,” explains Cornell University marine microbiologist Ian Hewson. “If there is not enough oxygen around the papulae, the starfish cannot breathe.”
These microorganisms do not directly cause disease, but steal the starfish’s oxygen supply when increased levels of organic matter prompt the microbes to bloom. As a result, the starfish literally drown in their own environment. Then their decaying bodies further increase nutrients for the microbes, creating a terrible feedback loop from starfish death.
Aquino and team noted that most SSW events are reported in the late fall or summer, when phytoplankton that increases the levels of nutrients in the water via photosynthesis is more abundant.
Warmer temperatures are known causes of phytoplankton blooms, and sea star wastage in Australia followed the longest and most intense heat wave on record. Starfish wasting events elsewhere also followed the elevated sea temperatures.
“Warmer waters cannot hold as much oxygen [compared with colder water] only through physics, ”Hewson told Erin Garcia de Jesus Science News.
None of this bodes well for our future on a warming planet.
University of Vermont biologist Melissa Pespeni, who was not involved in the study, said Science News this intricate maze of biological and environmental factors is “a new kind of idea for that [disease] transfer.”
Devastating consequences of the loss of these precious stars from the sea have already echoed across ecosystems. The sunflower star is a voracious predator with up to 24 arms that span up to 1 meter and make its way across the sea floor to devour sea urchins, snails and other invertebrates.
Without the sunflower and other starfish controlling the sea urchins, these herbivores eat their way through giant kelp forests. In 2016, sea urchins had already reduced the kelp population by 80 percent in some areas, decimating these once thriving underwater forests.
“This is a very clear example of a trophic cascade, an ecological domino effect caused by changes at the end of a food chain,” said Isabelle Côté, a marine ecologist from Simon Fraser University, who researched the consequences for the health environment.
“It reminds us that everything is connected with everything else.”
This research is published in Limits in Microbiologand.