In recent years, billions of sea stars have perished in the . Their hard, spiny bodies have turned into a gooey mass due to an unknown disease.
Scientists have finally identified the culprit. Four years of gene sequencing and laboratory experiments helped researchers pinpoint the cause of the devastating sea star disease: the bacterium Vibrio pectenicida.
What is known about the deadly sea star disease?
The outbreak of the disease, first reported by biologists in November 2013, spread across ecosystems along the entire west coast of North America. Soon after, similar reports of sea star die-offs emerged worldwide, affecting over 40 species.
The disease begins with lesions on the bumpy surface. Then, the muscles of the sea stars start to break down, and their arms become distorted and fall off. Within a few days, the animal dies.
The sunflower sea stars (Pycnopodia helianthoides) were particularly hard hit in the early years, with their population declining by more than 90 percent. These , adorned in shades of orange and purple, can have up to 24 arms. However, the disease, combined with the devastating marine heatwaves caused by human-induced climate change, has wiped out a significant portion of these vibrant beings.
By 2015, they had virtually disappeared from the shores of the west coast, stretching from Alaska to Mexico. Consequently, in 2020, the International Union for Conservation of Nature declared this species endangered.
Key inhabitants of the ecosystem
These beautiful creatures play a crucial role in the marine ecosystem. They feed on sea urchins, which graze on . Without sea stars, urchin populations would explode, decimating kelp forests and leaving barren wastelands.
This alarming scenario has prompted researchers to seek rescue solutions. Kelp forests are vital habitats for ecologically, culturally, and commercially important species such as sea otters, seals, sea lions, fish, lobsters, crabs, and shrimp. These forests absorb carbon dioxide and protect coastlines from the most severe storm impacts, as noted by Science Alert.
What have scientists learned?
The bacterium Vibrio pectenicida belongs to the same genus that causes cholera in humans and bleaches corals. However, as marine ecologist Drew Harvell from the University of Washington pointed out, “this vibrio is a cunning organism, as it does not manifest in histological studies like other bacteria.” This may be due to its ability to produce an immune-inhibitory toxin.
Initially, Harvell and her colleagues suspected that the culprit was a virus. Therefore, they were surprised to discover the pathogen within a more common group of bacteria. Several obstacles stood in the way of identifying this bacterium: the difficulty of finding healthy sea stars for comparison, the absence of visible pathogens in affected tissues, and a lack of knowledge about marine infectious diseases.
Evolutionary ecologist Melanie Prentiss, the lead researcher, conducted seven controlled impact experiments with sunflower sea stars raised in quarantine conditions.
Upon contact with infected tissue or fluid, healthy sea stars soon began to show symptoms, their limbs curling up, and the animals ultimately died.
Researchers knew that if the pathogen was viral, it would be able to pass through a filter with a pore size of 0.22 microns or withstand heat treatment. However, sea stars exposed to infected material that had been filtered or heated survived unscathed. This confirmed that the disease’s origin was bacterial.
RNA sequencing of infected sea star samples, obtained from both the laboratory and the ocean, revealed the presence of Vibrio pectenicida, a known pathogen of scallop and oyster larvae. This strain of bacteria, FHCF-3, was isolated from infected sea stars and used to infect healthy ones. Soon, their arms began to curl and “melt,” leading the team to realize they had found the pathogen.
Researchers believe that climate change is implicated in the outbreak, as Vibrio pectenicida is known to thrive in warm waters. Moreover, some populations of sunflower sea stars cling to life in the cold-water fjords of British Columbia, where it may be too cold for the bacteria to reproduce.
The findings of the study were published in the journal Nature Ecology & Evolution.
