Scientists seek to learn why a parovirus present in sea stars since 1942 has become so virulent in recent years.
In 1969, ecologist Robert Paine coined the term “keystone species,” in reference to the sea star species Pisaster ochraceus. His experiments, which involved removing coastal sea stars from the ecosystem, caused die-offs, overgrowth, or emigration in surrounding species such as algae, kelp, urchins, mussels, and various species of fish, causing him to theorize the importance of key species whose role in the environment serves to keep a given ecosystem healthy.
Now the very keystone species Paine studied is dying off, and scientists are struggling to discover the cause.
The disease causes white lesions along the arms of afflicted animals, leading to arm loss and eventually, total disintegration. The first to succumb were sunflower stars, mottled stars, and ochre stars, with many others following behind.
Benjamin Miner, an associate professor of biology at Western Washington University, heard about the ailing stars from his wife, Melissa Miner, a member of the group that had made the initial observations. He jumped on the bandwagon, making observations, collecting specimens, and studying them in his lab. The disease ÛÏkind of happened in pulses,Û he said, so that by the time he realized how heavily impacted the populations were, most locations had already seen significant population reductions.
The spread of the disease was so extensive that Miner found evidence of it in almost every site he studied.
As the bodies piled up, the Multi-Agency Rocky Intertidal Network (MARINe), a consortium of research groups established to monitor more than 200 rocky intertidal sites on the Pacific coast, provided a central database to collect observations. As more data came in, one indicator stuck out: sea stars living in aquariums which sterilize seawater with ultraviolet (UV) light did not contract the disease, while sea stars in unsterilized water aquariums did, including those that filtered incoming seawater through sand.
On the basis of this evidence, Ian Hewson, a biological oceanographer and microbial ecologist at Cornell University, reasoned that a microbial agent might be to blame. Hewson and his lab had recently finished a study looking at urchin viruses in Hawaii, so they were well-poised to get to the bottom of the sea star mystery.
Institutions along the Western coast collected samples of ill sea stars and sent them to Hewson’s lab, which he has named ÛÏTeam Aquatic VirusÛ because of his specialty in marine viruses. With help from a $22,000 rapid-response grant from the National Science Foundation (NSF), Hewson ruled out bacteria, fungi, and protozoans, narrowing the culprit down to a parvovirus that he named sea star-associated densovirus, or SSaDV.
The densovirus discovery was announced in a paper published in the Proceedings of the National Academy of Sciences in November, but many questions still remain. Why did this virus, which has been recorded in sea star samples from as far back as 1942, suddenly become so virulent? How does the virus move from host to host? What will be the consequences for the affected coastal ecosystems? To answer these questions, more work is yet to be done.
ÛÏWe’ve recently completed a survey of sea stars from around the world,Û Hewson said, ÛÏand I can tell you that we’ve recovered the same sort of densoviruses from many different locations, not clearly associated with disease.Û
This is the first time that the virus has been connected to disease, and this is by far the most severe episode of sea star disease in recorded observations. Hewson, who has been studying aquatic viruses for more than a decade, said: ÛÏIn my work I’ve never seen a virus which is associated with disease, at least not on this level.Û
Hewson would like to continue to study the virus, if he can get funding to keep at it, noting that ÛÏthe sort of work that we do is pretty horribly expensive.Û If he gets the chance, Hewson would like to follow his hunch that the virus is affecting the sea star immune system and preventing the sea star from fending off bacterial attacks.
ÛÏThe virus is not what causes the lesions or the melting,Û Hewson explained. ÛÏWhat we have found is that the number of bacterial communities is higher in diseased than in healthy stars, and so we think something is going on with the sea star immune system.Û
There are long-term consequences of the loss of these keystone species, as Hewson went on to explain: ÛÏthe loss of so many sea stars will result in profound changes in the diversity structure on the West Coast.Û
The population of juvenile sea stars shows promise of being unaffected, and could repopulate the millions of stars lost to SSWD. ÛÏWe do know that viruses never wipe out their hosts,Û said Hewson. ÛÏBut if the hosts become rare enough, the virus could go into hiding.Û
Ben Miner, for one, is circumspect. He cites reports that sea urchins are more numerous at a few sites in the Salish Sea, where lack of the predator sea stars has made room for urchins to move in. He also notes the healthy population of lingcod this year, whose egg nests had formerly been pilfered by sea stars.
ÛÏQuantifying whether it’s a good change or a bad change, it’s not really for me to make that call,Û said Miner. ÛÏChanges that might be bad for some might be great for others.Û