Pacific Northwest
Edge of the Salish Sea
In Part 1, I described the 2013 outbreak of the sea star wasting disease or syndrome (SSWD) and showed three of the sea star species that have been most heavily hit by the wasting. Because research groups such as MARINe have been taking baseline measures on our sub-tidal and inter-tidal populations for years, they were in a great position to be able to document the changes following the onset of SSWD epidemic. They have identified over twenty species that have been hit by the epidemic.
In Part 2, I will begin by describing some of the down stream ecological consequences of losing such important players in the balance of nature along our shores. Then I’ll review some of what is known about the causes of these sea star losses and finally look at the prospects for the future of sea stars along the West coast.
Consequences of mass sea star die off:
When a major predator within a balanced ecosystem is suddenly extracted, that ecosystem is going to change and possibly collapse. For example one important ecosystem that is directly affected by sea stars is kelp beds and forests along the western coastal and inland waters. These kelp communities are critical habitats for a number of species’ safety and existence. Such change has already happened. Here is one example among many and more examples are yet to be seen.
A large number of species rely on the kelp environment for their homes and food sources. Kelp forest dwellers include numerous invertebrates, fish, birds, and sea mammals such as seals, sea lions, sea otters. Whales are sometimes found in these kelp forests where they might seek shelter from intense storms. They are critical to the near shore animal population.
Sea urchins (Strongylocentrotus spp) live in and graze on the kelp bottom. If left to their own devices, without predators the urchins can devastate a kelp forest and leave it barren. One of the main urchin predators are sea stars who feed on them and keep their numbers in check, thereby protecting the kelp forest. To demonstrate this, a natural experiment presented itself to a group of ecology researchers who, prior to the epidemic had been measuring the density and growth of kelp forests along central British Columbia coast. In 2015 when the SSWD hit B.C. it devastated the sunflower stars which fed on the urchins in the kelp forests. Having had the previous two years of baseline data detailing the density of the kelp forest before the SSWD hit, they were able to document the change in kelp abundance after the stars disappeared.
Sea otters have recently repopulated this area and their favorite food happens to be large urchins. However, they don’t eat medium or small ones. With the loss of the sunflower sea stars, who do eat medium and small ones, these urchins proliferated by 166% resulting in the kelp forest taking a 30 % decrease in density. Without the sunflower stars, the medium and small urchins were free to feed at will on the kelp. Without the otters, the forest likely would have been lost all together.
Another consequence of sea star loss is change in dominant species within a given habitat as shown by a researcher team from The University of British Columbia. They observed a change in numerical dominance of star species following the onset of the disease. Prior to the SSWD, in the B.C. coastal region that they examined, ochres were the predominant species. After the disease onset, their numbers and size declined while the mottled star numbers which were less severely harmed by the disease rose five-fold. Apparently the mottled stars were less susceptible to the disease than the ochres and without the ochre competition, they proliferated. Long term abundance and other ecological consequences are as yet unknown but there was a clear shift in the dominant species. It may take some time and lots of observation to determine the long term consequences of this SSWD on coastal habitats and ecosystems, but clearly some things have changed.
Other changes that one might see at the beach have to do with the sea stars’ capacity to regenerate lost limbs due to predation or disease. I saw the six armed ochre star shown below on the beach that I monitor. Knowing that they normally have only 5 arms, I inquired with the MARINe researchers and was informed that they too had been seeing an increased number of six and even seven armed ochre stars as well. They hypothesized that perhaps the stars lost an arm to the SSWD and during recovery, the regeneration process didn’t stop with simple replacement of the lost arm but keep going on. Stars also have the capability of sloughing off an arm as a defense mechanism if it will save the rest of the body from harm. And why not if you can just grow another one or two?
I also saw several stars with a short arm when the other four appeared normal. This might well have been do to predation by gulls or other critters but it is also possible that they lost the arm to the disease yet survived and what we see is the regeneration as yet incomplete.
Causal factors: Viruses
As I noted in Part 1, the causal factors leading to this wasting syndrome are not entirely clear, with the possible exception of the sunflower star. A number of hypothesized causes have been and are continuing to be investigated including changed (warmer) water temperatures, pollutants, storms, and microscopic pathogens. One of the first researchers to investigate this mass die-off was Ian Hewson, a marine microbiologist from Cornell University. (This is a link to a video of him describing the epidemic.) Certain clues pointed him to search for some contagious microscopic pathogen. It was noted that sea stars in all aquaria along the west coast had been affected by the SSWD except for those that sterilized the inflowing saltwater with ultraviolet light. Thus, they concluded that the agent was water borne and that direct physical contact with infected organisms was not necessary to transfer the disease.
The first experiments conducted in 2014, used asymptomatic sunflower stars collected from the Salish Sea at sites where the disease had not yet spread. Tissues from diseased stars were homogenized, filtered to screen out all but virus sized particles and injected into healthy stars. Control animals were injected with a similar solution that had been boiled to kill any active organisms. Within 10 to 18 days, all stars injected with the live solution developed symptoms whereas none in the control groups did.
They then took diseased tissue from those stars that had developed symptoms and injected it into other asymptomatic stars. Again, these stars developed disease symptoms while the controls did not.
This was fairly convincing evidence that some pathogen of virus size was responsible for the disease. Through a series of viral metagenomic analyses, they determined that the the virus was something they called “sea star-associated densovirus (SSaDV), later called wasting asteroid-associated densoviruses (WAaDs). And as is often found in viral diseases, increased symptoms were associated with increase in viral load in the animals.
However, this and related viruses are not unique to this outbreak. These same viruses were found on examination of tissues from preserved museum specimens collected on the west coast as far back as 1942. So, the virus has been around for at least 78 years and likely longer. The mere presence of the wasting asteroid-associated densovirus might not be the total cause of this epidemic. Other factors must be involved.
Hewson’s lab conducted another series of studies that were published in 2018. In the first inoculation studies cited above where the symptoms were induced, they only tested the sunflower star. In this second set of studies they repeated these direct challenge procedures with three other species: Pisaster ochraceus, (ochre), Pisaster brevispinus, (pink sea star) and E. troscheli (mottled) . These three species were injected with the virus-sized material from SSWD-affected individuals. In none of these cases did the injections of viruses elicit symptoms of SSWD in any experimental trial. Injections that triggered the disease in the sunflower stars had no observable effect on these other three species tested although these species had been afflicted with the SSWD in their natural environment.
These findings are consistent with other epidemiologic data that have shown no association between WAaDs and SSWD in field samples of mottled or of ochre stars. It appears that the only densovirus association with wasting disease is in the sunflower star. The virus affects some species but not others. Further compounding this association is that simply having the virus present in the body does not ensure that the star will show symptoms as the virus was found in numerous asymptomatic stars as well. And as noted above, these viruses have been present in sea stars for many decades at least and there has never been a massive die-off like this one.
So it is important to explore environmental contributions to the disease.
Environmental contributions
In addition to examining the virus — disease associations, Hewson’ 2018 experiments sought to find associations between disease outbreaks and weather patterns, drought, temperature changes and the like. The results were tantalizing but the patterns were not clear or consistent. Correspondences between these factors and illness were few and when they found something at one site it did not seem to hold at others.
However, other research was underway that more clearly implicated ocean temperature anomalies as being related to the decline in several species of sea stars during this 2013 — 2015 time period.
Although specific circumstances and causal mechanisms are not clear at this point, most researchers agree that the increased water temperatures in the North Pacific during 2013 to 2015 had a hand in furthering this disease and mortality among the sea stars. You might have heard reference to “The Blob” to describe this sea surface temperature anomaly, coined by the Washington State Climatologist, Nick Bond. Another lesser and shorter lived version of the blob showed up this year and was dubbed “Son of Blob.” If such blobs keep popping up, it can not bode well for various sea life that it touches. This possible temperature — SSWD onset relationship has been studied both in the lab as well as studying sea surface temperatures anomalies coinciding with loss of sea star abundance of the west coast.
Subsequent to Hewson’s work, another Cornell research group examined ochre stars in the San Juan Islands and in South Puget Sound of Washington State to investigate the relationship between water temperature anomalies and SSWD progression and death in both field studies and in experimental laboratory studies.
Their field study examined stars, sampled from intertidal rocky shores at 16 sites within Puget Sound and the San Juan Islands. Measurements of star size and presence of disease were recorded as was the water temperature over a 19 Month period.
A total of 6,568 ochre stars were surveyed across the 16 study sites between 31 December 2013 and 17 July 2015. Chances of a star being diseased was related to both its size and with water temperatures. Larger stars were more likely to be diseased and as the water was warmer, chances of finding diseased animals increased. Over the study period, ochre stars decreased by 67% over all and 80% among the larger, adult stars.
To study this temperature effect experimentally, they conducted a series of laboratory studies at the Friday Harbor labs (San Juan Island). They obtained samples of adult and juvenile ochre stars from the waters of San Juan Islands and placed them into tanks at the lab facilities. All stars were symptom free at the time they were obtained from their local habitat although they were from areas where other stars were diseased. Thus they had likely been exposed to any present pathogens such as the densovirus.
Over the 19 day experiment in which groups of stars were variously exposed to different water temperatures, all subjects died of SSWD. However, the water temperature affected the time of survival for the juveniles such that exposure to warmer temperature hastened disease onset relative to onset in the adults.
A similar lab study, also using ochres here in Bellingham at Western Washington University, found essentially the same result. Cooler waters like winter temperatures delayed mortality of ochres relative to those housed in summer-like waters. Although cooler water delayed mortality, they all eventually died.
A more recent field study examined survey data on the abundance of sunflower stars off the west coast from California to British Columbia from 2004 to 2016 covering the periods before, during and after the major epidemic. These surveys measured the biomass of sunflower stars taken from thousands of near shore dives and from deep water ocean trawls.
Their findings were startling on two counts.
First , These stars virtually disappeared from these surveys during and following the epidemic.
In California and Oregon, the average biomass decreased 100% during 2013–2015 ... In Washington, average biomass declined 99.2% during this period. In 2016, no P. helianthoides were collected across the 1264-ha area covered by 692 trawl surveys. The collapse in biomass collection occurred 1 year earlier in California compared with other regions.
Second, the timing of the demise of the stars closely paralleled the increases in sea surface temperature (satellite measured), suggesting that it was associated with and likely facilitated the dramatic die off. However, since the stars continued to die off even in the cooler waters off British Columbia, it was not likely due to the heat per se. This study did not examine for the presence of WAaDs.
So, no single study to date has been able to correlate all of the variables operative with SSWD. However, the densovirus appears clearly implicated in at least the sunflower stars’ demise, and SST might well facilitate the syndrome across various species. They continue to looks for additional pathogens.
Although not conclusively demonstrated at this point, some researchers attribute the occurrences of sea surface temperature anomalies (Blobs) to global warming and points to one more way in which we are harming the oceans and the critters that live there. Warming of seas is only going to continue in the foreseeable future. And as they say, climate change is a threat multiplier. It can activate latent or potential threats into full blown disasters.
Given their various finding as well as those of others described here, Hewson and colleagues concluded:
It is therefore unlikely that SSWD results from a single etiology or is the same disease across its entire geographic range and across all species in which it is observed. Rather, “sea star wasting disease” describes a similar, though not identical, set of disease signs (i.e., “syndrome”) that afflict the same species between different geographic regions, or between species at the same location. We propose that SSWD be re-named “Asteroid Idiopathic Wasting Syndrome” (AIWS) to account for this possibility. The cause of AIWS is therefore likely a complex tango of diverse potential pathogens and environmental conditions.
The future of sea stars on the west coast:
It appears over all that the worst of the epidemic is past although the syndrome lingers in many places and as I noted in Part 1, I found a diseased ochre here just a couple of months ago. The near elimination of the sunflower stars in both off shore and near shore locations continues to be troublesome for their marine ecosystems. The ravages of the epidemic have been variable depending on species, geography, and possibly timing and exposure to the temperature anomalies seen in recent years. The following graph depicts the fall and rebound observations of P. ochraceus at one site, Post Point, here on Bellingham Bay. This site is one of the MARINe study sites and researchers have tracked the stars here from 2009 before the beginning of the die off. This year’s data have not been recorded yet.
It is clear here that ochre stars went from abundant (over 400) in 2009 and 2010 to very few between 20013 and 2018. They then rebounded up to ~ 300 in 2019 and from my own observations they are increasing in 2020. Also important here is that the size of the newer stars that were observed were much smaller than those observed early. This smaller size is probably accounted for by the majority of stars now being younger and smaller since many of the older, larger stars have died off. Recall that size was related to probability of having the disease.
My own observations parallel those of above although I did not conduct systematic surveys as the MARINe team did. As shown in the title photo, the majority of the stars I observed in the past couple of years have been smaller, including many juveniles. The fact that they exist in significant numbers bodes well for the replenishment of certain populations, in this case, the ochres and to some extent the mottled stars. The pace and extent of recovery will likely vary by species.
The greatest concern with return are of the sunflower stars. They have been either totally eliminated or nearly so in most areas studied and seem clearly to be susceptible to the densovirus. Should the virus remain present in the sunflower star’s range, especially if the warm temperature anomalies continue, the future for them is at best uncertain.
The MARINe web site reports concerning the sunflower star:
During 2019, most observations of Pycnopodia (sunflower star) were in the range of 1-3 individuals for a given survey, with the greatest number being approximately 2 adults and 50 juveniles (4-6 cm diameter) at Holmes Harbor in Washington. Prior to the onset of SSWS, some of these same locations had hundreds of sunflower stars. Most current Pycnopodia observations are of juveniles; large healthy adults have become scarce in recent years. No reports have been submitted to us of Pycnopodia south of Washington since January 2018, from Monterey county, in central California.
At this point in time it is difficult to project the outcome as we are only a few years into the epidemic and are just beginning the recovery period. Stars do not reach maturity until five years. Since it was just five years ago that the die off was at its peak and it has continued since, it is too early to know what will happen to them. We’ll have wait and see if the early juveniles do eventually survive and begin reproduction in sufficient numbers to mount a meaningful comeback.
One more bit of both possible hope from one of the authors and researchers whose work is reviewed above. This marine scientist, C. Drew Harvell, wrote an op ed in the New York Times recently describing her team’s work enhancing natural habitats to treat polluted and diseased waters, in this case planting sea grasses to save coral reefs in Indonesia. They are continuing this successful work here in the Salish Sea planting native eel grass to promote healthier waters and hopefully, healthier and more resilient sea life such as sea stars. We’ll want to stay tuned to this work.
To end on a more positive note, below is a favorite photo of mine. This little guy seems to me to be jumping for joy and doing its happy dance on the beach, just waiting to become a full fledged ochre star and hoping to keep all five arms.
