By REEM KHONDAKAR
Since July of last year, sea stars across the East and West Coast have been dying by the hundreds of thousands, according to Prof. Ian Hewson, microbiology. As a result he is investigating whether a water-borne microorganism is behind what has now been coined “sea star wasting disease.”
Commonly known as starfish, different species of sea stars are part of the class Asteroidia. Hewson said the most diverse and abundant populations of sea stars are found along the Pacific Coast of the United States. According to Hewson, researchers realized that sea stars were dying beginning in the summer of 2013, when lesions appeared on sea stars at roughly the same time in different coastal area of the country, from Southern California to Alaska.
“Initially it seemed like [the disease] was restricted to one or two species,” Hewson said. “However, it has since progressed pretty rapidly into other species of sea stars, to the point where hundreds of thousands of sea stars are completely disappearing.”
Previous mass sea star deaths have been correlated to dramatic climatological events, such as El Nino. El Nino is a periodic warming of a band of water in the Pacific Ocean that develops off the coast of South America. In the 1970s El Nino caused a massive upwelling of nutrient rich water which created ideal conditions for several pathogens that killed off sea stars.But, according to Hewson, no climate-related link has been found with the current sea star wasting disease.
Hewson said he began working on the project in October.
“What really triggered my activity was when, within the span of a day or two, sea stars in the Vancouver and Seattle aquariums — which had housed sea stars for the past 40 years — all died,” Hewson said. “That was an indication to me that there was something perhaps water-borne, something capable of making it into the aquariums and wiping out their sea stars.”
Research on sea star wasting disease was funded by a National Sciences Foundation Rapid Response Grant, which is awarded based on the urgency of the problem.
“This is by far the most extensive event today that’s happened, and also the most wide ranging as far as the number of species affected by the disease,” Hewson said.
Multiple institutions across North America are collaborating to find the cause of sea star deaths, each approaching the problem from different angles. According to Hewson, the University of South Florida and Cornell are investigating potential water-borne disease agents, while Western Washington University and the University of California Santa Cruz are investigating potential environmental causes of sea stars death and the impact of sea star deaths on the ecosystem.
To find what disease agents might be behind the sea-star deaths, Hewson said he compared the abundance of different parasites and viruses in healthy and diseased sea stars. Hewson said he extracts the pathogens from the sea stars, sequences their DNA and compares them to the known DNA sequences of all organisms collected in GenBank, a nationally-funded genome database.
This comparison tells Hewson what pathogens are actually present in the sea star, he said. Finally, using a technique called quantitative PCR, he can find the abundance of each pathogen in sea stars by measuring the relative amounts of their DNA present.
“This can tell us whether there is more of organism X associated with the diseased [sea stars] than with the healthy,” Hewson said.
According to Hewson, the approach does not rule out the possibility that some of the pathogens might be opportunists which colonize a sea star that has already been compromised. To overcome these issues, Hewson said he uses an approach where potential pathogens are isolated from the diseased sea star and exposed to a healthy sea star.
After observing the progression of the disease in the inoculated sea star, he re-isolates the pathogen from the sea star, thus providing a direct connection between the pathogen and the disease. But finding microorganisms in diseased sea stars does not necessarily mean those pathogens are causing the disease, Hewson said.
One of the major hurdles in studying sea stars from a microbiological standpoint, according to Hewson, is that sea stars constantly take up sea water, and therefore also constantly take in the bacteria that live in sea water.
“Essentially they are sea water, and it is difficult to exclude a lot of bacteria from the analysis because they have it running through their veins, so to speak,” Hewson said.
Another issue is that pathogens may later infect the sea star after it has been compromised by the original disease agent. Hewson compared the problem to that of when HIV was yet to be determined as the causative agent behind severely compromised immune systems.
“When HIV first started, the main symptoms that people were turning up with were all manners of weird infections, which were not actually the root of the disease,” Hewson said. “It took some time to find what the cause was, the HIV virus. We’re dealing with a situation not unlike that.”
While the research is still ongoing and candidate pathogens are still being identified, Hewson said he believes that as of now, it is possible that multiple disease agents are behind the sea star deaths.