Stars of the Seashore Under Threat
UVM biologists study spread of sea star wasting disease
- By Lauren Milideo
Scattered in tide pools, sea stars draw us out to the coastline to glimpse the life thriving just along the edge of our terrestrial world — their distinctive profiles, iconic symbols of the ocean beyond.
But on the North American west coast, they don’t offer such a pretty picture. Many of these familiar creatures have gotten so sick that they will “turn to goo and die.” That blunt prognosis is in the words of Melissa Pespeni, UVM assistant professor of biology. In her lab in the Marsh Life Sciences Building, Pespeni, postdoc Melanie Lloyd ’08, and a team of undergraduate students are studying the effects of the sea star wasting disease, which has devastated regional sea star populations. Their work is funded by a National Science Foundation RAPID grant.
This threat is remarkably widespread, notes Pespeni. “There have been outbreaks of this disease in the past, but never as extreme and as geographically extensive and as lethal as the outbreak that started back in 2013, going all the way from Alaska down to Baja California, Mexico, and affecting so many species.” The likely culprit is a virus, described in a 2014 publication, that causes sea stars to develop characteristic symptoms including lesions. Eventually, says Pespeni, the stars will lose their arms, become soft and gelatinous, and expire.
There is still much to learn about this disease, including why some individuals get sick from the pathogen and others do not. “That virus is also found in museum specimens that are seventy to one hundred years old,” notes Pespeni. “So it’s been around, and it can be found in most individuals and even in individuals that aren’t necessarily sick or showing major symptoms.”
So, says Pespeni, “the big unanswered question is, why now?”
Increasing environmental stressors, including pollutants and extreme temperatures, may affect the sea stars’ susceptibility, as well as their microbiomes, so to begin answering this question, the Pespeni lab is performing experiments to see what role the sea stars’ genes and microbiomes — the community of bacteria, viruses, and other microorganisms that live in and on the sea stars — might play in determining which individuals get sick.
Pespeni and her team have about 20 bat stars, shipped from California. The species is known to show resistance to the pathogen. Into half of these sea stars’ individual tanks, a bit of wasting disease-infected tissue is introduced, and the sea stars consume it. Next, the team observes the sea stars over two weeks, noting the development of wasting disease symptoms and taking four or five tissue samples from each infected individual as the pathogen takes its toll. These samples provide three key pieces of the infection puzzle over the two-week timespan: RNA sequences from the sea stars and their microbial communities, and from the virus. The healthy bat stars are also sampled.
Each sequence provides different information, notes Pespeni. Microbial metabolic processes, reflected in their RNA, indicate which microbes are most active at the time each sample is taken; viral RNA shows which viruses are active at different points in the advancing disease, and the sea star’s own RNA sequences indicate which genes are being activated at these times — they “tell us what the sea star is doing to respond to this experimental infection,” says Pespeni.
“Are they launching an immune response? Are they turning on genes that will help them fight this infection? When did they start doing that?” The experiment will be repeated with ochre sea stars, a species that is more susceptible to sea star wasting disease.
The importance of this method lies in the time series, Pespeni says, a novel approach that allows her to see not just a momentary snapshot of the disease but a dynamic story of its progression, from several interrelated angles. “It’s like looking at a concert, a symphony — who’s playing more loudly when symptoms become more extreme? What are the shifts in composition once the sea stars become infected, and start showing symptoms?” The goal is to correlate the interactions of multiple different players in the sea stars’ infection and immune response.
Such research provides important insight into a crucial member of coastal marine ecosystems. “If you remove sea stars, then the sea urchins can get out of control. They can mow down kelp forests, because their favorite food is kelp,” Pespeni says. And kelp forests provide a safe place for young fish to hide, avoiding danger, until they grow big enough to take on the open ocean. Mussels, too, can take over without sea stars to keep them in check. “Sea stars in the intertidal zone really help make it a species-rich complex community, so without sea stars then you just get a huge mussel bed and you don’t have chitin or limpets or barnacles or all these other amazing things,” notes Pespeni.
Disease outbreaks such as the sea star maladay are part of a larger picture of a changing environment. “We’re having these massive effects on the marine environment,” notes Pespeni, and marine animals face stressors in addition to disease, including extreme temperatures, CO2 and acidification. This type of study allows us to understand the “mechanisms of resilience,” notes Pespeni. “How are species and populations going to be able to survive in these future environmental conditions?”