Microbes likely cause of Sea Star Wasting Disease

 

Sea star wasting disease gained notoriety in 2013, when sea stars began dying off in record high numbers all along the U.S. Pacific coastline. U.S. Atlantic coast populations were also affected by die-offs, although not with such prevalence and volume. The dramatic die-offs troubled and puzzled local community members as well as ecologists who study the important role that sea stars play in the rocky intertidal ecosystem. The significant impacts of the mass die-offs jumpstarted years of scientific research to discover the cause(s) and mitigate accordingly. The disease was originally attributed to a virus (densovirus), however recent studies hypothesize that a different mechanism is at play—one that limits the stars’ access to oxygen it needs to breathe.

A healthy Northern Sea Star (Asterias rubens).

When there is a large pulse of organic matter—like phytoplankton, algal blooms, or decaying sea star or mussels tissues, for example—the microbes living in the slime that covers sea stars begin to rapidly multiply and respire. That thin slime layer performs the ultimate life-giving function of gas exchange between the animal and the oxygen-rich water surrounding it. When the layer becomes oversaturated with microbes that are using up oxygen and producing carbon dioxide, the animal becomes hypoxic (deficiency in the amount of oxygen reaching the tissues on the animals’ surface) due to the increasingly anoxic layer that surrounds it. When this happens, the animal becomes starved of oxygen, even though oxygen-rich water circles only millimeters above. This process triggers a response in the sea star to degrade its tissues, ultimately resulting in their dissolution. The response is far worse in sea star species that have a lot of spikes, like Asterias spp., than in smoother species like Porania. The main factors that seem to exacerbate the condition are temperature (bacteria respire more when warmed) and terrestrial sources (which may include anthropogenic nutrient pollution) that may be present in coastal environments. 

Northern Sea Star displaying symptoms of Sea Star Wasting Disease.

In 2017, we reported that sea stars in our tanks were dying off due to sea star wasting disease. While we maintain a water temperature of 55 degrees in our cold water aquaria for native species, there have been instances when temperatures spiked due to a life support system malfunction (usually when a chiller stops working). We now understand that bacterial growth accelerated by increased water temperatures is likely the cause and that the disease is probably not transmissible/infectious toward other sea stars.

So, what are the real-life implications of a mass sea star die-off in the ocean and what can we do about it? The ecological effects can be quite large. Sea stars are often referred to as keystone predators, or predators that are dominant in their ecosystems. Sea stars control a variety of prey species, most notably molluscs, such as dog whelks and blue mussels. This predation effectively stabilizes populations and helps to maintain biodiversity. The loss of sea stars as predators could completely change the population of the rocky intertidal zone landscape. There could also be a “mosaic” response, in which mussel species dominate in some areas and not others. Long term implications depend greatly on the pace and scale of sea star recovery.

Natural systems are complex. Community observations, traditional ecological knowledge and ongoing scientific research help us to better understand the variables at play, effects of various inputs, and to look at change over time. Changes and ebbs and flows in system dynamics are normal and healthy, however when the rate of change is too rapid for a system to keep pace, or is human-induced, it is incumbent on us to do our best to preserve and protect through conservation and mitigation. We know that human health and ocean health are inextricably interconnected—the ocean provides oxygen, circulates heat, influences climate, supports biodiversity and much, much more. A healthy ocean supports a healthy planet.

Helping visitors learn more about the ocean ecosystem is one of the goals of our naturalists and volunteers. Talking about the interconnectedness of sea stars with other organisms in the rocky shore ecosystem, the health and interconnectedness of that ecosystem with the larger ocean biome, and ultimately with humans, encourages scientific thinking and reasoning. Chats alongside the tide pool touch tank help raise awareness and increase understanding of the importance of ocean conservation and biodiversity. We hope these kinds of interactions will encourage people to take actions in their own lives that support a healthy ocean.

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