Photos courtesy of Mandy Lindeberg, NOAA and Brenda Konar, University of Alaska Fairbanks.
As a top predator, sea stars can restructure intertidal communities. For example, by feeding on mussels, they open up limited “real estate” in the intertidal zone for other species to move in. Sea stars, like other intertidal species, can tolerate extreme conditions such as the pounding of waves, heat, and desiccation when exposed during low tide. However, the added stress of unusually warm waters during the 2014-2016 marine heatwave may have made them more vulnerable and facilitated the spread of Sea Star Wasting Syndrome and caused sea star populations in the Gulf of Alaska to crash.
The recent sea star die-off event began in the Pacific Ocean on the west coast of North America in 2013 and hit Alaska in 2014. In the warm ocean waters, the disease expanded. This mortality event impacted 20 species of sea stars from Mexico to Alaska and decimated many sea star populations across the northern Gulf of Alaska.
Symptoms of sea star wasting syndrome include abnormally twisted arms, white lesions, deflation of arms and body, arm loss, and body disintegration. They die over the course of days or weeks. The disease was thought to be a virus, but currently, there is evidence that point to bacterial infection resulting from an imbalance in the sea star’s microbiome (microorganisms in a particular environment). A proliferation of bacteria on the surface of sea stars leads to reduced oxygen, which essentially causes the star to suffocate. While this “suffocation” is not transmissible star to star, the bacteria are. Decaying stars exacerbate the situation by providing even more organic material for the bacterial communities to flourish. Impacts vary by species and site conditions, and there is still a lot to learn as more information becomes available.
Much of what we know about the effects of the disease in the Gulf of Alaska stems from our long-term nearshore monitoring activities there. While it appears that the disease has abated, the density and diversity of sea stars remains low at our monitoring sites. The precipitous loss of some species reduced the species diversity that once existed across regions, making these once-unique sea star communities more similar across the northern Gulf of Alaska. Sea star species that are currently present are those that were not impacted by sea star wasting syndrome. In 2020, the sea star species thought to be unaffected by the disease continued to be present (primarily Henricia leviuscula/blood star and Dermasterias imbricata/leather star). The once dominant sea stars that were killed by the disease continue to be rare or absent (primarily Pycnopodia helianthoides/sunflower star, Evasterias troschelii/mottled star, and Pisaster ochraceus/ochre star). Areas in Alaska devoid of sea stars for multiple years have enabled prey species, such as mussels, to flourish.
As sea stars were lost, we began to watch how other species in these intertidal communities responded to changes in the predator population. What we continue to learn from the sea star die-off helps us understand intertidal communities, the species that thrive there, and the environmental variables that influence them.
This article is based on:
Konar, B., T. J. Mitchell, K. Iken, H. Coletti, T. Dean, D. Esler, M. Lindberg, B. Pister, and B. Weitzman. 2019. Wasting disease and static environmental variables drive sea star assemblages in the Northern Gulf of Alaska. Journal of Experimental Marine Biology and Ecology 520: 151209.
More Science Links
Evidence that microorganisms at the animal-water interface drive sea star wasting disease, Frontiers in Microbiology, January 2021
Some bacteria are suffocating sea stars, turning the animals to goo, ScienceNews, 2021
Evidence of increased mussel abundance related to the Pacific marine heatwave and sea star wasting, Prince WIlliam Sound Science Center, 2023
Last updated: July 19, 2023