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Biological oceanography

Wasting Away in Virus-ville

Article: Kohl WT, McClure TI, Miner BG (2016) Decreased Temperature Facilitates Short-Term Sea Star Wasting Disease Survival in the Keystone Intertidal Sea Star Pisaster ochraceus. PLoS ONE 11(4): e0153670. doi:10.1371/journal.pone.0153670

Background

In 2013, a mysterious illness began spreading through sea star populations up and down the west coast of the United States. Termed sea star wasting disease, it has caused massive die offs in several sea star species. Symptoms of the disease include decreased appetite, lesions, uncharacteristic arm twisting, arm loss, and eventually death (Fig. 1). The disease is thought by many scientists to be caused by a virus, as previously covered by Oceanbites writer Sarah Giltz here. However, further research has discovered that the virus was present in individuals that were not showing symptoms, as well as museum specimens up to 72 years old. This raised the question of what had changed recently in outbreak areas to start making the sea stars succumb to the virus, instead of apparently living with it?

Symptoms of sea star wasting disease: A) Arm twisting, B) Corckscrew arm twisting and arm detachment, C) Lesions and arm loss, D) Close up of detached arm wound (After Kohl et al.)

Symptoms of sea star wasting disease: A) Arm twisting, B) Corckscrew arm twisting and arm detachment, C) Lesions and arm loss, D) Close up of detached arm wound (After Kohl et al.)

Some recent studies hoping to help answer this question found that water temperatures were warmer during or preceding wasting disease outbreaks, and found smaller scale wasting events throughout recent history were associated with warmer water temps. A controlled laboratory experiment (not the main feature of today’s article) investigating warmer waters as a trigger for the outbreaks placed some sea stars in tanks with water at the same temperature they were collected at and some into tanks several degrees warmer.

Sea stars in the warmer tanks did die of the wasting disease faster than those in the cool tanks, but there was a catch in the design of the experiment. The organisms were collected from the cool ocean and placed into the ‘warm’ tanks without any acclimation, meaning the shock of such a sudden temperature change may have affected their survival rate and their ability to fight the virus.

Methods

Fig 2: Average time-to-death for the sea stars in each temperature tank, with error bars. (After Kohl et al.)

Fig 2: Average time-to-death for the sea stars in each temperature tank, with error bars. (After Kohl et al.)

Kohl et al., today’s feature article, aimed to offer evidence that the change in temperature wasn’t to blame; it was the warmer waters that accelerated the wasting disease. To show this, sea stars were collected from warm summer waters and placed some into tanks that were similarly warm and some into cooler, winter temperature waters. This was essentially the opposite of the previous experiment. This way, if the stars placed into cooler waters (experiencing a similar change in temperature as the previous experiment) fared worse than those kept at a constant temperature, the researchers would know that the thermal stress was to blame, not the virus. Alternatively, if the cold sea stars survived longer, the researchers could be reasonably sure that cooler waters somehow suppressed the virus or at least slowed its effects.

All in all, 17 sea stars were collected from two different sites in Washington and randomly placed into the ‘winter’ or ‘summer’ tanks. All the sea stars were assumed to be infected, as there were signs of wasting disease at both sites, although not all stars were showing signs. None of the stars taken were showing more than moderate signs of wasting disease, in other words, they had a decent amount of time remaining to be studied. The stars were kept well supplied with food and checked on about a daily basis, with their symptoms recorded.

Results

The study found that cooler temperatures did seem to help out the sea stars, with stars living around twice as long as those in the warm water (Fig 2). Although the cold water sea stars still eventually died of the wasting disease, some were observed to have temporarily healed lesions. Symptoms in the warmer water sea stars progressed quicker and showed no signs of healing (Fig 3).

Significance

These results suggest that cooler water temperatures do indeed slow the progression of wasting disease, and show that it was not temperature change shock that affected sea stars in previous experiments. It is still unclear exactly how temperature is affecting the virus that causes the disease, but warming ocean temperatures now seem like a plausible trigger for the sudden spread of the outbreaks. Studies like this are important for narrowing down the causes of such unexpected events that may become more common in a changing climate.

Fig. 3: A description of the symptoms denoting each 'morbidity level' and a plot of the average morbidity level of the sea stars in each tank.

Fig. 3: A description of the symptoms denoting each ‘morbidity level’ and a plot of the average morbidity level of the sea stars in each tank.

P.S. Shout out to Kari for the title help!

Discussion

5 Responses to “Wasting Away in Virus-ville”

  1. This article “Wasting Away in Virus-ville” about sea stars is very interesting. We read about how the sea stars have this disease called the sea star wasting disease. What it does is it starts by twisting the arms until they tear off. It punctures the organs with large lesions and it doesn’t stop until the sea star is dead. It intrigues us knowing that in cold water it lives around two times longer than ones in warm water. Cold water delays sea star wasting disease, but the sea stars will still die. I like how you found out that the earlier study had a confounding variable-the temperature shock caused by moving the sea stars into the warm water.
    We have one question- What are the possible reasons that the wasting disease has started killing sea stars if the virus has been in healthy organisms for a long time?

    Posted by Elias Gold | May 24, 2016, 1:47 pm
    • Thanks for reading!

      There are many reasons the virus could be having a greater impact now than in the past. As this article pointed out, warmer water makes the sea stars more susceptible to the disease. That warmer water may be coming from a normal shift in ocean temperatures, like the El Nino cycle, or it could be from global climate change caused by human activities.

      It is sometimes tricky for scientists studying one small piece of the world, like these sea stars, to link what they’re observing to changes in temperature and climate that are happening on a global scale. While outbreaks of this disease have happened before when local water temperatures got a little warmer, they’ve never spread this far or lasted this long, possibly because the region cooled down again. Several other studies have indicated that higher water temperatures are the likely cause for greater disease impacts, and the scale of this outbreak suggests that larger scale processes may be influencing the water temperature.

      In terms of the exact mechanics of why the virus is more effective at higher temperatures, much remains unknown. It may be that warmer temperatures allow bacterial species to grow faster on the sea star, weakening the immune system and giving the virus enough leverage to become effective. Or it could be the reverse, that the virus is always weakening the sea star, but not enough to kill it, and then warmer temperatures produce enough other infections to kill the organism. This is still a very active area of research.

      Posted by Austen Blair | May 25, 2016, 4:53 pm
    • At this point we’re still working on those very questions. Why here? Why now?

      The current hypothesis is that Sea Star-Associated Densovirus (SSaDV), the virus identified by our collaborators at Cornell University (also open access, citation below), may weaken stars immune response (though there is some evidence of an immune response being mounted, see reference below), allowing opportunistic bacterial infections to overcome stars’ immune systems, eventually leading to wasting and death. Our work relates in that there is evidence of accelerated growth of several species of pathogenic microbes at higher temperatures, these same microbes have been identified in the remains of wasted stars. This is significant as P. ochraceus, and other sea star species are ectothermic, so they rely on behavior and the environment to control their body temperature.

      So the hypothesis goes something like this: stars infected with SSaDV encounter warmer water temperatures in the spring and summer (especially lately), as their immune response may be somehow compromised or ‘busy’ (this remains unconfirmed) due to infection with SSaDV, this warmer temperature both further stresses the star and increases pathogenic microbial growth rates, and this is what produces the appearance of symptoms and eventually, death.

      Hewson, I., Button, J. B., Gudenkauf, B. M., Miner, B., Newton, A. L., Gaydos, J. K., … & Fradkin, S. (2014). Densovirus associated with sea-star wasting disease and mass mortality. Proceedings of the National Academy of Sciences, 111(48), 17278-17283. http://www.pnas.org/content/111/48/17278.full

      Fuess LE, Eisenlord ME, Closek CJ, Tracy AM, Mauntz R, Gignoux-Wolfsohn S, et al. (2015) Up in Arms: Immune and Nervous System Response to Sea Star Wasting Disease. PLoS ONE 10(7): e0133053. doi:10.1371/journal.pone.0133053

      Posted by Warren Kohl | May 25, 2016, 6:07 pm
  2. Very interesting!
    What species of stars did you use in the study? Were they wild caught or captive? If wild caught, did they all reside in the same area?

    Posted by Brittany | May 15, 2016, 11:43 pm
    • Thanks for reading, and sorry this reply is a bit late!

      The sea stars in this study were all wild caught ochre sea stars (Pisaster ochraceus). They were caught from two seperate sites in northern Washington State, then randomly assigned to the different tanks for the experiment.

      Posted by Austen Blair | May 25, 2016, 2:26 pm

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