Death of the Sunflower Sea Star Means Ecological Unravelling in the Pacific
Sunflower sea stars seem indestructible. Like other members of their species, these pothole-sized creatures can regrow their limbs — and in the case of the sunflower sea star, all 24 of them. They prowl for food at 40 inches a minute and can swallow an entire sea urchin whole. But something in the ocean is destroying these formidable predators, turning swaths of their population into piles of white goo.
It’s a problem, scientists reveal in a study published Wednesday in Science Advances, that’s causing ocean ecosystems to unravel. Since 2013, sea star wasting disease has killed massive numbers of multiple sea star species all along the Pacific coast of North America, from Mexico to Alaska. The new analysis demonstrates that one of the hardest-hit species is the sunflower sea star: Along the western coastline, in both shallow nearshore waters and deep offshore trawls, there’s been an 80- to 100-percent decline in their numbers.
This rapid, widespread decline is predicted to have serious consequences. Co-lead author and Cornell University professor of ecology and evolutionary biology Drew Harvell, Ph.D., tells Inverse that before the outbreak, the sunflower star was the most common shallow-water coastal star. Now the disease has caused the decline of a once common and ecologically important species, demonstrating the “power of infectious disease to change our ocean biota and the cascading effects on the balance of nature.”
That’s because the decline of the sunflower sea star has allowed one of its preferred food options to run rampant. Urchins have increased dramatically in locations like central California and north of Vancouver, and this burst means more than fresh uni.
“The hordes of urchins now mow down kelp beds and create denuded zones that are not good habitat for fish and many invertebrates that were [previously] sheltering in the kelp beds,” Harvell explains.
Sea star wasting disease did most of its damage in 2013 but continues to kill sea stars. The devastating syndrome literally causes the animals to waste away — first lesions appear in the outmost tissue layer, then decay. Eventually, there is fragmentation of the body and death. In many cases, all that’s left is a pile of white slime.
In this study, the team determined that the timing of the peak declines of sunflower star populations coincided with irregularly warm sea surface temperatures. So while they don’t know the exact mechanism that causes the disease to take root, this evidence suggests that at warmer temperatures the disease can progress faster and kill faster.
The fact that these kinds of large outbreaks are more likely in a warming ocean is a chilling revelation at a time when it’s become increasingly obvious that the ocean is dangerously warm. In January, scientists announced that 2018 was the hottest year ever recorded for the global ocean, and the rate at which the ocean is warming is not only unprecedented, it’s accelerating. Climate models indicate that unless humans significantly reduce the sum of greenhouse gases pumped into the atmosphere, the oceans will continue to warm. If, instead, the world limits the global temperature increase to below 2 degrees Celsius, that same warming could be cut in half.
Sea stars, explains Harvell, are not singularly affected by the changing ocean. Outbreaks are damaging populations of other aquatic creatures — corals, abalone, and salmon. A healthy ocean is a “lifeline for humanity,” and humans have created the outbreak conditions that threaten to unravel it.
Abstract: Multihost infectious disease outbreaks have endangered wildlife, causing extinction of frogs and endemic birds, and widespread declines of bats, corals, and abalone. Since 2013, a sea star wasting disease has affected >20 sea star species from Mexico to Alaska. The common, predatory sunflower star (Pycnopodia helianthoides), shown to be highly susceptible to sea star wasting disease, has been extirpated across most of its range. Diver surveys conducted in shallow nearshore waters (n = 10,956; 2006–2017) from California to Alaska and deep offshore (55 to 1280 m) trawl surveys from California to Washington (n = 8968; 2004–2016) reveal 80 to 100% declines across a ~3000-km range. Furthermore, timing of peak declines in nearshore waters coincided with anomalously warm sea surface temperatures. The rapid, widespread decline of this pivotal subtidal predator threatens its persistence and may have large ecosystem-level consequences.