Kate Gazzo, M.S.
At the rush of incoming waves, algae sways back and forth inside tide pools along the Pacific coast. Sculpins dart in and out of the shadows while purple, orange, and yellow orche starfish (Pisaster ochraceus) cling to rocks. Anyone who was fortunate enough to grow up along a coastline or even to experience tide pooling during a visit knows that one can get lost for hours observing these ecosystems. Unfortunately, tide pools that were teeming with life only a year or so ago, are currently being threatened as is the entire balance of these ecosystems. Intertidal communities are changing as a result of sea star wasting disease. In many areas along the coast you can notice dozens of diseased and dying starfish-a keystone predator in intertidal ecosystems. How long this wasting episode will last is unknown, and scientists have only begun to speculate as to the cause.
An Introduction to Sea Star Wasting Syndrome
Beginning in 2013, researchers in Olympic National Park in Washington observed populations of sea stars dying or, more accurately described, “wasting away”. The disease has been appropriately named sea star wasting syndrome. This disease causes the limbs of sea stars to contort and form large lesions that eventually lead to legs and sometimes even entire bodies of sea stars essentially dissolving. A majority of sea stars that become infected die within a matter of days if not hours (Gardiner 2014).
In 2013, the disease appeared in localized regions along the Pacific coast-first in Olympic National Park, then Vancouver, Monterey, and Puget Sound. During 2014, the disease spread as far north as Alaska and as far south as Mexico and reached previously unaffected areas such as Oregon. While sea stars have undergone disease outbreaks in the past, including wasting events, the geographical extent (4,000 miles ranging from Alaska to Mexico) has never been this widespread. Furthermore, during previous wasting events it was predominately only one species that appeared to be susceptible to the disease; currently, up to 12 species have been affected by this wasting event (Gashler 2014).
Sea Stars A Keystone Predator
The most commonly observed sea stars that are affected are ochre stars (Pisaster ochraceus) and sunflower stars (Pycnopodia helianthoides). Ochre stars are at risk of becoming locally extinct along the Oregon coast where sea star wasting is particularly pronounced. Because of their role as keystone predators, ochre and sunflower stars are vital to the community structure of intertidal ecosystems. During an experiment in 1969, a scientist named Robert Paine from University of Washington discovered that the presence (or absence) of these stars heavily influenced rocky intertidal communities (Gashler 2014). Specifically, Dr. Paine noted that the removal of this species disrupts the balance of other species in marine communities by increasing the number of smaller predators, such as sea urchins and mussels, which sea stars would normally consume. When sunflower and ochre stars were removed from the shoreline, sea urchins began to dominate the intertidal community and kelp, a primary food source of sea urchins, which normally serves as food and habitat for other species, became scarce. Without the top-down predator control provided by starfish, mussel populations also disproportionately increased in relation to other species. The result was the displacement of a number of other species, including barnacles and limpets. Today, similar to the outcome of the 1969 experiment, marine communities are being re-shaped; this time, however, communities are changing not as the result of a small scale experiment but as a result of sea star wasting occurring across thousands of miles.
The Link Between Climate Change and Marine Pathogens
So far, the cause of sea star wasting is thought to be a virus, more specifically a type of parvovirus (Parvoviridae) which affects a range of species including invertebrates and vertebrates. Interestingly, the type of parvovirus-Sea Star-Associated Densovirus, has existed since the 1940’s and also occurs in sea urchins. Why this virus is suddenly causing large-scale sea star die-offs is uncertain (Osgood 2014). A theory that many of researchers are leaning to is that abiotic stressors, such as warmer water temperatures and a lower pH, are impacting the ability of marine organisms to cope with disease (Burge et al. 2013; Bates et al. 2009). Warming ocean waters is one of the most pronounced changes that has occurred within marine ecosystems in recent years. During a small localized wasting event in 2008, researchers at the University of British Columbia first documented an increase in disease susceptibility in Pisatser ochraceus correlated to warmer water temperatures (Bates et al. 2009).
Changes in environment, both on land and in the ocean, influence the intensity of disease outbreaks (Burge et al. 2013). According to Colleen Burge, a marine biologist at Cornell, the number of marine disease outbreaks spike following higher than average water temperatures. Warmer temperatures tend to make marine organisms, such as corals, more susceptible to disease by weakening their immune response and increasing the occurrence of disease. Marine pathogens are also moving toward the poles following the migration of marine organisms (Burge et al. 2013). This may be a clue as to why sea star wasting which once occurred in localized areas and only affected a small percent of populations is now extensively affecting populations thousands of miles apart.
During 2014, sea star wasting spread to Alaska and Mexico. In 2015, water temperatures in the Pacific are predicted to remain elevated as a result of an El Nino weather pattern, further extending the duration of physical stress incurred on sea star populations. Even if sea star populations recover, the intensity and extensive geographic range of this wasting event signifies an alarming trend- the declining health and resiliency of marine ecosystems. It is uncertain whether the loss of sea stars in marine communities will be lasting or temporary. However, one thing is certain, the diversity, composition, and abundance-the entire structure of particular marine communities, depend on the presence of starfish. As ecologists, we tend to look at systems in their entirety, however, starfish remind us that there are defining species within an ecosystem which necessitate a higher level of concern.
(Watch this clip of starfish legs breaking off and moving independently. Caution: disturbing content).
Bates, A. et al. 2009. Effects of temperature, season and locality on wasting disease in the keystone predatory sea star Pisaster ochraceus. Inter-Research Diseases of Aquatic Organisms 86: 245-251.
Burge, C. et al. 2014. Climate change influences on marine infectious diseases: implications for management and society. Annual review of marine science 6: 249-277.
Gashler, K. 2014. Sea star wasting devastates Pacific coast species. Cornell University, Cornell Chronicle.
Gardiner, L. 2014. Sea star deaths along the west coast elicit close study. Scientific American, Blogs.
University of California, Santa Cruz, Ecology and Evolutionary Biology Dept. 2014. Pacific Rocky Intertidal Monitoring, Trends and Synthesis: Sea Star Wasting Syndrome.
About the Author
Kate Gazzo is an ecologist specializing in water quality issues and watershed management. Kate holds a Master’s degree from the University of San Francisco in Environmental Management and a Bachelor’s degree in Environmental Studies with a minor in Biology from Florida Gulf Cost University.