First report of deadly disease raises concerns for Europe's starfish

One of the first reports of a devastating starfish disease in Europe has raised concern about its potential impact on marine ecosystems.

Sea star wasting disease has decimated starfish populations in the USA, where it kills more than 80% of infected animals on average. Rugose starfish, which have bumpier shells, are thought to be most at risk.

As starfish are keystone predators that help to control populations of other species, such as sea urchins, their loss would lead to a trophic cascade that could cause biodiversity loss amid wider ecosystem changes.

Dr Patrick Collins co-authored a new paper reporting the disease in Crossaster papposus starfish caught in the Irish Sea. 

He says, 'Sea star wasting disease is relatively understudied, and we don't have a firm idea of why it happens.'

'From what we know, it isn't caused by viral or bacterial infection, but instead appears to result from a stress response to very fast environmental changes.'

'That's a worry, because as climate change becomes more pronounced extreme weather events will become more common, and these animals aren't able to cope with that. If sea star wasting disease becomes more common, and it probably will do, this might affect these species more widely.'

The study was published in the journal Biology Letters.

What is sea star wasting disease?

Sea star wasting disease (SSWD) is a condition that causes starfish to fall apart. At first, symptoms of the disease include a deflated appearance, followed by lesions forming on the surface of the starfish. 

As the starfish continues to disintegrate over a matter of days or weeks, internal organs including parts of the digestive and reproductive systems can protrude out of the body, while the arms become contorted. Eventually, the starfish disintegrates to such a degree that it dies.

Not all symptoms appear in every starfish, with some researchers suggesting that SSWD could represent a range of different conditions with broadly similar symptoms. This may also explain why some studies have found links to viruses and bacteria, while others have not.

However it is caused, it has had a significant impact on starfish populations in the USA. Since 2012, outbreaks of the disease on the country's Atlantic and Pacific coasts have affected at least 20 different species and killed millions of starfish. 

Without starfish to consume them, mussel beds have expanded and crowded out other marine species. While starfish may be able to limit the molluscs if their populations recover in the short-term, a longer-term absence may see the mussels become too dominant to be brought back under control.

There is some evidence that starfish may be able to adapt to SSWD over time, but in the meantime, much remains to be understood about the condition as it begins to be detected in Europe.

'In the vast majority of cases, starfish die and can't be treated,' Patrick says. 'It's too late to prevent it, as heatwaves are already happening. SSWD is going to become part of our ecosystems, and potentially cause large changes in how they are structured.' 

How was sea star wasting disease discovered in Europe?

The discovery of SSWD in Europe was accidental, with Patrick and his co-authors having set out to collect starfish for a behavioural study.

While there have been potential outbreaks in the Mediterranean, and reports from aquariums, this is the first time the condition has been reported by scientists in Europe.

'We were looking to conduct behavioural experiments when we collected these specimens from the Irish Sea in March 2022,' Patrick says. 'Unfortunately, they began to die from SSWD and so the experiments failed.'

'That said, we were lucky to find them. As the starfish disintegrate so quickly in the wild, it's hard to study them in time.'

The researchers found that C. papposus starfish which had appeared healthy began to show symptoms of the disease after five days in a holding tank. Of the 12 individuals collected, only four were still alive after a month.

Like other instances of SSWD, the deaths of the starfish were associated with unusually hot temperatures. At the time of collection, the UK was around 2⁰C hotter than normal, and Northern Ireland, where the collection took place, was experiencing 90% more sunshine.

More recent heatwaves, such as the record-breaking highs of July 2022, may have also caused outbreaks of SSWD. However, with many of the affected starfish species living beneath the tidal zone, the disease may be going unnoticed.

Patrick argues that more funding is needed to understand the true extent of the disease, and how it is affecting Europe's marine ecosystems.

'This is not a canary in the coal mine, but a confirmation that things are changing,' he says. 'We were lucky to find SSWD, so now we need to study it more and understand how our ecosystems will be affected.'

'However, there's not been a lot of recent survey work done on the populations of these species, and it's very difficult to gain funding for that kind of fundamental research. It highlights why we need to support this research so we can observe ecosystem changes on a large scale.'

'If we aren't aware of what's going on, then the impacts of this disease, and its potential consequences, could be much more widespread than we know.'