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