By: Dana Sackett
A sea star is not usually the first example that comes to mind when picturing a voracious and fearsome top predator. However, many sea stars, the star-shaped echinoderms colloquially called “starfish” and often found in beach-motif decor, are just that: awesome top predators. Moreover, many sea star species are keystone species, or species that have a major impact on the diversity and functioning of their ecosystem. For instance, the sunflower sea star can grow to over 3ft (~1m) and move more than a meter per minute devouring everything in its path, particularly sea urchins and even fish.
The most well-known keystone sea star is the purple or orchre sea star. Scientists discovered that removing the purple sea star from their coastal intertidal habitat allows mussel populations to quickly cover these rocky habitats; the mussels in turn prevent other species from establishing themselves. Similarly, sea stars that prey primarily on sea urchins keep those populations from growing out-of-control and mowing down giant kelp forests that provide food and refuge to numerous aquatic species. Thus, a population of sea stars has important indirect impacts on many species throughout an ecosystem. Also because of their importance, many scientists quickly became alarmed when these top predators began disappearing a few year ago.
Starting in 2013, sea stars started dying off in droves of a gruesome disease called sea star wasting disease (SSWD). The grisly symptoms start with white lesions that lead to a deflated body, slackening heartbeat, limbs falling off, ruptured organs, and death. While this disease has been seen in the past, it had never caused death so quickly, occurred at such a large scale, or occurred for as long as was seen between 2013 and 2015 (now called the Northeast Pacific SSWD event).
During this time more than 20 species of sea stars from Mexico to Alaska were severely impacted. The sunflower sea star was particularly susceptible with 80% to 100% population declines occurring within its ~3000 km range (from California to Alaska in shallow and deep offshore habitats). On the east coast of the United States sea star wasting disease impacted coastal waters from New Jersey to New England also.
The cause of this disease was a mystery until 2014 when researchers were able to link the densovirus to the disease. The pathway and hosts of the disease are still being discovered. One link that researchers have found is that unusually warm waters exacerbate the disease: causing a higher risk of infection and faster progression to death. This was not the first time warmer sea temperatures have been seen to influence the prevalence and severity of marine infectious diseases. However, it was the first time that those unusually warmer temperatures were found to be a key facilitator of such widespread disease-related deaths.
The disease itself and secondary consequences of such a dramatic loss of so many top predators are still ongoing. For instance, with the demise of sunflower sea stars, sea urchin populations have exploded in some areas, substantially reducing kelp forests and the fish and fisheries that rely on them.
Scientists are using this extreme event to learn about evolution. With an average of 81% of the purple sea stars dying across populations within their range, researchers were able to compare the genetic variation of these populations before the disease occurred to the survivors. They found that the survivors had disease resistant traits and that those traits were passed on to their offspring. Thus, natural selection shaped the surviving population and the new generation of purple sea stars.
While it is promising to see that at least one species of sea star may recover with some resistance to this deadly disease there are still too many other factors that could affect this species to truly know how they will fare in the years to come. Another study recently found that while purple sea stars are recovering within their range, more time is needed before these populations are able to resume their role as keystone predators in their ecosystems.
References and other resources:
See our previous article for more information on keystone species here and on how the loss of a species can impact an ecosystem here
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University of California – Davis. “Once-abundant sea stars imperiled by disease along West Coast: Some of ocean’s biggest stars affected by disease and climate change.” ScienceDaily. ScienceDaily, 30 January 2019. <www.sciencedaily.com/releases/2019/01/190130161646.htm>.