- Jan 8, 2023
- 6 min read

HOW ARE SEA STARS DOING?

You may have heard something about the Sea Star Wasting Disease (SSWD) epidemic that came to the Pacific coast of North America in 2013. Though similar die-offs happened in the ‘70s, ‘80s, and ‘90s, the magnitude of each of those was far less and restricted to much smaller regions. The impact of the 2013 event was a real contrast, something like COVID-19 has been to SARS 2003. This mass mortality event was felt from Alaska to Mexico, with these many-rayed echinoderms suffering the most brutal mass die-off ever recorded. The Salish Sea, that stretch of water wrapping around the southern half of Vancouver Island and composed of the Straits of Georgia and Juan de Fuca and Puget Sound, and a global hotspot for temperate sea star diversity, was the worst hit location.

Salish Sea map manipulated from the beautiful original by Dr. Aquila Flower

While there was a lot of commentary and speculation at the time, I haven’t heard much since, now almost ten years later, and have lately been wondering if anyone has assessed the impact and if we’ve sort out an ultimate cause. To being with, SSWD was associated with infection by a sea star-associated densovirus; but later studies showed only clear association between this virus and the illness in one species, the sunflower sea star. Confounded, researchers even attempted to elicit illness in the lab in three other species but ultimately failed to do so. Concern that warming oceans may have resulted in the epidemic, researchers also looked into environmental factors, such as water temperature and precipitation patterns. One study from 2016 demonstrated, to everyone’s surprise, that SSWD was associated not with warmer but with colder water, at least in their study area along the Oregon coast and the Olympic peninsula of Washington. A later investigation, conducted in 2018, looking at six locations from Mexico to Alaska “could not identify a single or combination of parameters of environmental conditions that universally correspond with disease across multiple species or an entire geographic range.” That study concluded the relationship between environment, diverse pathogens, and sea star mortality was far more complex than currently understood.

Interestingly and also still unexplained, there was a tremendous boom of juvenile sea stars of several species in 2014 and 2015. Juvenile sunflower stars in particular were extremely abundant at many locations across the Strait of Georgia and Puget Sound, with populations as much as 300 times their typical numbers. Juvenile mottled stars (Evasterias troschelii) were spotted in large, ultra-dense congregations (almost disgustingly so) in certain locations. But only for a short time. The juveniles disappeared over a period of weeks to months, and no evidence of their abundance has so far been reflected in adult populations. So, just as Descartes suggested, however discernible things may ultimately be, the world does appear to be more complicated and interconnected than our senses or intuition would concede.

Whatever happened to the juvenile population booms and regardless of the disease etiology, what was observed with this epidemic were some devastating impacts. At the time, it was obvious that, although at least 20 species of sea stars were impacted and those populations have always been and remain quite variable by location, sunflower stars (Pycnopodia helianthoides), sun stars (Solaster spp.) and purple sea stars (Pisaster ochraceus) suffered most, with estimates of declines of not less than 90%. And it was a brutal thing, this disease. I mean “wasting disease” is never your friend, obviously; but, it’s also far uglier than it sounds. As for symptoms, first behavioural changes are observed, followed by lesions, and then a noticeable “loss of turgor” (their flesh loses its elasticity and starts sagging.) Then comes the crazy bit: “limb autonomy.” That’s how it sounds, but worse. What happens is the organism’s appendages may become zombie-like, each with a seemingly undead volition of its own, and walk away from one another in a wild episode of self-dismemberment. From there, the disassociated bits then die by way of “rapid degradation” (meaning they melt into a revolting, cloudy mush.) Brutal.

Of course, the above is just the impacts on a stricken individual, however. Obviously, most of what we landlubbers and sea-toppers tend to take in of sea stars is the briefest glimpse of their slow and silent creeping, clinging, and congregating on some dock pilings or among some intertidal crevices. At best, we may poke at them for a moment or two at our favourite tide pool location or within an aquarium’s touch pool. So, most of us don’t get to observe their complex behaviours and interactions these beings have with their neighbours, their prey, and their environment as a whole. And as a result, it’s very easy to have misapprehensions about these beasts. Few of us fully appreciate that sea stars are not anything like the seemingly passive, thick-skinned slugs they come across as but are instead the top carnivore in many of the settings in which they are found. For example, like grey wolves or tiger sharks, the purple sea star is a keystone predator and a hugely impactful actor, quite disproportionate really, within its ecosystem. (As a matter of fact — stick this in your ecology pub quiz — this species, Pisastre ocharaceus, was actually the first ever organism identified as a keystone species.) Purple sea stars thrive on a diet of mussels, chitons, limpets, snails, barnacles, and other small invertebrates; and, though any one sea star may consume fewer than a dozen mussels annually, say, given their numbers, sea stars as a population consume tremendous volumes. In doing so, where they exist they don’t merely control mollusk and crustacean populations but also enable far greater diversity than otherwise. Where few sea stars reside, you can often find tremendous mats of black, teardrop-shaped mussels, for example, and seemingly nothing else. And species such as Mytilus californianus, the California mussel, can reach more than 20cm (around 8in) in length and may be able to live as long as twenty years. But a healthy population of ochre, five-rayed molluscivores will set upon that otherwise entrenched and impenetrable bivalve encampment and pick it apart, enabling other species to take hold. Research suggests that where sea stars are absent the diversity of their prey and similar species is reduced by as much as half. Similarly, sunflower sea stars, that other key predator taken out by SSWD, are the primary consumers of green sea urchins (Strongylocentrotus droebachiensis) who, unchecked, will decimate their own favourite food: kelp. Where this occurs we find formerly vibrant and dense kelp forests, ones teeming with organisms feeding on and sheltering among the kelp, reduced to a desolate landscape known as an “urchin barren.”

Sadly, we’ve observed many diseases running through marine invertebrate species in the past, and with similar cascading ecosystem impacts. Recent examples include: a similar withering syndrome impacting black abalone (Haliotis cracherodii) off the coast of California, resulting in urchin and algae explosions; the epidemic and mass mortality of Heliaster kubiniji, another intertidal sea star and top predator from the Gulf of California, which resulted in an explosion of the population of a local rock snail, Morula ferruginosa; and we’ve similarly observed the local die-off of Diadema antillarum, an urchin, from the Caribbean reef ecosystem, where their absence resulted in their staple food, a micro algae, taking over and swarming local coral with catastrophic results.

So, from what I can tell, as the situation sits today, a decade on, there has been some recovery among sea stars as a whole. Anecdotally, if I go down to the waterfront just about anywhere along the Salish Sea I find purple sea stars. The numbers are not to pre-SSWD levels, and that's obvious, but this is a very good sign. That said, just under the surface things are not looking so good for other species, particularly the sunflower sea star. In late 2021, a collaborative undertaking between some sixty partners (including First Nations, NGOs, universities, and federal agencies across the impacted region), and published in the Proceedings of the Royal Society B., revealed the sorry state of this species. Using information from a broad range of long-term ecological monitoring datasets, the study showed that Pycnopodia now appears functionally extinct (more than a 99% population decline) in the south, from Baja California to Cape Flattery, Washington, on the northwestern tip of the Olympic Peninsula. They also revealed that declines nearly as severe (greater than 87%) occurred between the Salish Sea and the Gulf of Alaska. From that, researchers concluded there was "no evidence of population recovery in the years since the outbreak" and that natural recovery in the southern half of their range is "unlikely over the short term." This means the species is now IUCN-listed as Critically Endangered. Given the certain impacts of this, what we have is a real catastrophe.

On a positive note, however, not only does there appear to be surviving populations in Alaska but also a sanctuary where populations of sunflower sea stars have boomed on the central coast of BC, south of Haida Gwaii and north of Vancouver Island, in the region of the Rivers Inlet and Bella Bella. As a result, as further study into the causes of SSWD continues, a tremendous recovery effort is underway, with active breeding and relocation activities. With any luck, we should be able to bring back the sunflower sea star just as we have so many other species.