Sea Star Wasting Syndrome (SSWS) describes an immense, ongoing die-off event that has severely impacted multiple species of sea stars, also known as starfish, across the northeastern Pacific Ocean. This phenomenon is characterized by a rapid and often gruesome deterioration of the animals’ physical structure, leading to death. The scale of the outbreak, which began in 2013, has made it one of the largest marine epizootic events ever documented globally. SSWS affects multiple species and represents a significant biological disturbance to coastal marine ecosystems.
Clinical Manifestation of the Syndrome
The onset of the syndrome is marked by subtle changes, such as the sea star exhibiting a deflated appearance or a loss of normal turgor. This is often followed by a refusal to accept food and a general listlessness. Visible symptoms soon emerge, beginning with the appearance of small, white lesions on the animal’s surface, which rapidly spread across the skin.
The affected sea star may begin twisting its arms in unusual ways, possibly attempting to detach the diseased portions of its body. As the disease progresses, the tissue around the lesions decays, leading to the softening of the body wall. The arms may then separate from the central disc (autotomy), before the entire body disintegrates into a white, mushy mass. Progression to death can occur quickly, often within a matter of days or a few weeks.
Geographic Scope and Timeline
The massive outbreak of SSWS was first observed in June 2013 among Ochre Sea Stars (Pisaster ochraceus) along the coast of Washington State. The phenomenon quickly spread with alarming speed, and by the spring of 2014, the wasting syndrome had been documented across an enormous geographic range, extending from Southern Alaska down to Baja California, Mexico.
This die-off affected over 20 different species of sea stars, with mortality rates in some populations reaching over 90 percent during the peak years of 2013 and 2014. Although the intensity has lessened, SSWS has continued to persist at lower levels in many regions since the initial peak.
Understanding the Proposed Causes
The scientific investigation into the cause of SSWS has been complex, suggesting that the syndrome is likely the result of multiple factors rather than a single pathogen. Early research in 2014 identified the Sea Star Associated Densovirus (SSaDV) as a strong candidate, finding it in greater abundance in diseased sea stars than in healthy ones. This single-stranded DNA virus was shown to be transmissible and was initially thought to be the infectious agent responsible for the widespread mortality.
Subsequent studies suggest that the presence of SSaDV alone does not fully explain the massive outbreak. Environmental stressors play a significant part in triggering or exacerbating the disease. Elevated sea surface temperatures, particularly during the 2014–2016 marine heatwave, appear to increase the susceptibility of sea stars.
Another prominent theory focuses on microbial imbalance. The disease is linked to a proliferation of bacteria on the sea star’s surface. Microbes like certain Vibrio species multiply rapidly, consuming oxygen in the thin layer of mucus. This process can lead to hypoxia, causing the animal to suffocate, which triggers the tissue degradation. The current scientific consensus points toward a complex interaction between a pathogen and environmental factors like warmth and organic load.
Ecological Consequences
The loss of sea stars has initiated shifts within the marine food web, a phenomenon known as a trophic cascade. Certain species, most notably the Ochre Sea Star (P. ochraceus) and the Sunflower Sea Star (Pycnopodia helianthoides), function as dominant predators in their respective habitats. Their removal has allowed their primary prey populations to expand unchecked.
In the intertidal zone, the decline of the Ochre Sea Star has resulted in the proliferation of mussels, which are strong competitors for space on the rocks. This unchecked growth can monopolize limited surfaces, significantly reducing the biodiversity of the community. The near-total disappearance of the Sunflower Sea Star has led to a major increase in sea urchin populations. This explosion of sea urchins, which graze heavily on kelp, has contributed to the widespread decline of kelp forests along the Pacific coast.