Wildlife

Still No Causes Found for Sea Star Wasting Disease

Beginning in 2013, a mysterious disease crippled sea star populations up and down the U.S. west coast. Over a matter of months, many sea star species died in record-breaking numbers, though Pisaster ochraceus—a keystone species known as the ochre sea star—was among the hardest hit. Now, researchers at UC Santa Cruz have analyzed just how much the populations of this species have declined, but they have not yet determined what factors might be contributing to the epidemic.

Scientists aren't sure what causes this disease, known as sea star wasting syndrome (SSWS). Some researchers think the culprit could be a viral pathogen, but evidence is inconclusive.

This latest outbreak extended from as far south as Baja California, Mexico, all the way to the southern shores of Alaska.

In a new study published March 20 in PLOS ONE, Miner, Raimondi, and colleagues analyzed marine data collected between 2000 and 2016 from 90 intertidal sites on the North American Pacific coast to determine the impacts of SSWS on ochre sea star populations and try to understand what factors may have contributed to this most recent SSWS outbreak.

The data revealed that, while ochre sea star populations shrank in all coastal areas, sites in southern California and the Channel Islands experienced more severe declines than those farther north. In the south, adult sea stars declined by 99 percent or more at over half of the 39 sites surveyed, whereas only 2 sites in the north experienced similar levels of decline. Miner's group also noted that juvenile sea star mortality in the period after the initial outbreak of SSWS increased by approximately 90 percent compared to pre-outbreak levels.

Ochre sea stars are keystone predators in intertidal ecosystems—the part of the shore area submerged at high tide and uncovered at low tide—preying on organisms like the California mussel. When ochre stars are removed, mussel beds can expand, leaving less space for other organisms to flourish in those habitats. A decline in ochre sea stars could lead to a change in the make-up of intertidal communities on the west coast, although other factors are also at play, Miner said. The epidemic that started in late 2013 is still ongoing, though the major population crashes noted in 2014 and 2015 have since leveled off.