Pisaster Disaster!

It seems like nearly everyone is sick with the cold or flu these days. But if you think being stuck with a sore throat and runny nose is bad, imagine catching a virus that makes your arms and legs fall off! Whether you believe it or not, there is a virus that does precisely this – not to humans but to sea stars – and has caused sea star populations off the Pacific North American coast to plummet ever since the problem was first observed in 2013.

A sea star inflicted with Wasting Disease, with a white lesion (shown with yellow arrow). Be careful not to confuse the lesion with the whiteish pattern you see all over the rest of the surface, which is a normal characteristic of this purple sea star (Pisaster ochraceus). Image taken from here.

Known as Sea Star Wasting Disease, it’s like something out of science fiction. In the early stages of infection, sea stars exhibit white lesions of dead and dying tissue (shown on the left). The deterioration then spreads throughout the organism, eventually becoming so severe that you sometimes start to see the sea star’s internal organs spilling out through the lesions, or some of its arms falling off entirely. You can’t help but feel sorry for the poor creatures! This whole process happens very quickly, too: within just a few days. One biologist reported leaving her sea stars alone for just 40 minutes and returning to discover that one of them had lost two arms in that short amount of time! Yikes!

Unsurprisingly, the disease is quite deadly, which is bad news for our sea stars here in British Columbia. We set out for the Coal Harbour marina last week to check up on how our local sea stars were faring. Were there still many of them around? Of the ones that were left, how many of them were infected?

Mussels galore! Must have been quite the feast for the purple sea star pictured here.

Mussels galore! It must have been quite the feast for the purple sea star pictured here.

Once at the marina, we didn’t get around to answering all these questions, simply because we didn’t see very many sea stars. Mussels are a sea star’s favourite food, and so you can normally find sea stars hanging around on mussel beds. But despite the thick layer of mussels clinging to the side of the dock, there was only a single Pisaster ochraceus (more commonly known as the “purple sea star” or “ochre sea star”) among them. I was disappointed that we didn’t find any more, but at least the one sea star that we did find did not show any sign of the disease!

A purple sea star shut clamped tightly around a mussel.

A healthy purple sea star clamped tightly around the mussel it is trying to snack on!

So now what? Are sea stars out of trouble yet? Not necessarily. Although the one I found appeared unaffected, purple sea stars as a species have been heavily hit by Wasting Disease. This is a huge problem because purple sea stars are considered a keystone species: that is, they have a much larger impact on their environment than you might expect. (Other British Columbian examples of keystone species are salmon, beavers, and sea otters.) This classification as a keystone species mostly stems from the sea star’s appetite. For example, by eating mussels, purple sea stars normally keep mussel populations in check and prevent them from overcrowding other species.

But once you remove a large proportion of the purple sea stars – say, through Wasting Disease – this balance becomes disrupted. To what effect, no one really knows. That’s what we’re still trying to find out! What exactly is going on with this disease, and how far-reaching might its effects be? It’s a tall order, to be sure, but I am hopeful that with continued research and conservation efforts, Pisaster will soon recover its status as the “star” of the show here along the coast of British Columbia.

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