Sea Star Wasting Disease (SSWD) is a severe health crisis that has profoundly impacted marine ecosystems. Beginning in 2013, the outbreak caused mass mortality among over 20 species of sea stars along the Pacific coast of North America, from Baja California to Alaska. This event is regarded as the largest marine animal disease outbreak ever recorded. Understanding the mechanisms behind this widespread die-off is necessary.
Defining the Disease
The onset of Sea Star Wasting Disease typically presents with subtle behavioral changes, such as lethargy or a refusal to accept food. Affected animals often look deflated due to a loss of turgor pressure before physical marks appear. The visible progression starts with the development of small, white lesions on the sea star’s body surface.
These lesions rapidly spread, leading to the decay of surrounding tissue. The sea star’s water vascular system fails, causing the arms to twist, lose their grip, and eventually detach from the central disc. The final stage involves rapid tissue disintegration, often described as “melting,” which leaves the animal as a white, mushy blob. This process can be incredibly fast, frequently leading to death and dissolution within just a few days.
The Identified Pathogen
The search for a causative agent initially focused on viral pathogens due to the disease’s rapid spread. Early investigations in 2014 identified the Sea Star Associated Densovirus (SSaDV), a single-stranded DNA virus, as the most promising candidate. Researchers found that SSaDV viral load was higher in sick sea stars, and experimental studies could induce SSWD-like symptoms in healthy animals.
Subsequent research complicated the initial viral hypothesis. Later studies found SSaDV or similar densoviruses in healthy sea stars and in museum specimens dating back to the 1940s. This suggested the virus might be a common resident that only causes disease under certain conditions, leading scientists to explore other microbial factors, including bacteria.
More recent findings identified a specific bacterium, Vibrio pectenicida, as a causative agent in sunflower sea stars (Pycnopodia helianthoides). This pathogenic bacterium was found in high levels within the coelomic fluid of sick animals. Laboratory experiments confirmed that exposure to the isolated Vibrio strain could successfully induce the disease signs. The current understanding suggests that SSWD is likely a multifactorial condition, where an infectious agent like Vibrio or a densovirus acts in concert with environmental stressors.
Environmental Amplification
Environmental factors significantly accelerate and amplify Sea Star Wasting Disease. Elevated water temperatures are strongly associated with increased disease rates and mortality. The major outbreak coincided with the 2014–2016 marine heatwave, known as “The Blob,” which brought anomalously warm waters to the Pacific coast.
Warmer water compromises the sea star’s immune system, making them more susceptible to infection. Simultaneously, higher temperatures increase the metabolic and replication rates of microbial agents, such as bacteria. This creates a destructive synergy where a weakened host faces a rapidly multiplying pathogen.
The disease progression is also hypothesized to be linked to localized hypoxia, or a lack of oxygen. The proliferation of microbes on the sea star’s skin consumes oxygen, creating an anoxic layer that starves the animal’s surface tissues. This stressor may be exacerbated by increased organic matter in the water, which fuels microbial growth.
Ecological Impact
The mass mortality event resulted in devastating population declines across the entire geographic range. Some species, such as the sunflower sea star, experienced population declines exceeding 90% in certain areas. The widespread loss of these predators has triggered a trophic cascade.
The sunflower sea star (Pycnopodia helianthoides) is a significant predator of sea urchins. With the disappearance of this sea star, sea urchin populations surged, unrestrained by predation. The unchecked increase in these herbivores has led to intense grazing pressure on kelp forests, resulting in their significant decline in many regions. This shift restructures the entire ecosystem, reducing biodiversity and threatening the health of coastal habitats.