Kohl, W. T., McClure, T. I., & Miner, B. G. (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
Illness in the Intertidal
We all get sick from time to time; catch colds or get the sniffles. One of the best ways your body knows to start healing is to raise its core temperature. Congratulations, you have a fever! Many doctors advocate bringing high fevers down, especially to avoid further damage to one’s internal organs or brain. However, in cases of bacterial or viral-induced fevers, lowering your temperature could lead to added recovery time since not all the nasty bugs are killed off quickly. Now you might be thinking what this concept of temperatures, fever, and progressions of illness has to do with anything marine-related.
Thanks to a new study by Warren Kohl and colleagues, it has been determined that sea stars infected with Sea Star Wasting Disease (SSWD), experience a slower progression of the illness when living in cooler waters.
SSWD has been around for a long time—nearly three quarters of a century, at least since humans have known about it. In recent years, it’s become a large concern on both the eastern and western coasts of the USA with mass mortality events in 1972, 1978, and 2013. Approximately forty distinct species of sea star can succumb to the disease. The disease itself is relatively grotesque, beginning with small lesions developing on the epidermis of the sea star, and progressing to entire limbs falling off (think leprosy for sea stars). Ultimately, SSWD results in the death of a sea star. With the number of species susceptible to SSWD, scientists are eager to learn more about how environmental factors may impact the disease’s mortality rate. Previous studies have pointed towards temperature playing a large role in the disease’s rate, where higher temperatures result in faster mortality.
Methods & Results
Kohl’s team was intrigued by the relationship with temperature and wanted to test if colder temperatures could reduce morbidity or eliminate infection entirely. Previous surveys of the Salish Sea (see Fig. 1) indicated a seasonal component to disease progression in sea stars, where fewer SSWD mortalities were logged in winter months when compared to summer. Following this idea, Kohl’s team collected a small group of Pisaster ochraceus sea stars (seventeen individuals) from two sites in July, 2014. Unfortunately, the disease had spread rapidly through the area in 2013 and no individuals collected were free of SSWD to act as a control.
Sea stars were transported back to the laboratory and set up under two treatment conditions: 9º C water or 12ºC water. Seawater was filtrated and heated/cooled to the appropriate temperature prior to being added to the housing tanks. The experiment became a waiting game as the scientists monitored how the SSWD symptoms progressed.
The main results were partly encouraging. Overall, sea stars housed at lower temperatures survived twice as long as their counterparts in the warmer tanks (Fig. 2A). In terms of disease progression, the lower temperature treatment showed a slower rate of progression where minor symptoms were drawn out before more severe ones emerged (Fig. 2B).
Now, these results are only partly encouraging for the simple fact that every sea star still died of the disease within 44 days of collection. While morbidity (the degree and progression of illness) was impacted, overall mortality was not. We know this disease has been in the intertidal zones for some time, but with concerns over climate change and rising ocean temperatures, the frequency with which we see mass mortality events could increase in the coming decades. In previous studies on P. ochraceus, elevated temperature treatments were set at 14º and 20º C; unfortunately, Kohl’s team found equal mortality rates at 12º C, indicating P. ochraceus populations might be differentially susceptible to temperature stress, depending on local conditions.
Sea stars are keystone species, highly important players in the rocky intertidal zones. Sudden decreases in their populations have the potential to upset the balance in many of these ecosystems. However, work is constantly being done to assess what bacterial or viral vector is responsible for SSWD. And steps taken by scientists, like here in this study, will undoubtedly reveal more avenues by which to slow and eliminate this threat to sea stars.