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Where do African fish parasites come from?

The rudd (left) introduced in North African lakes is often infected with several Ligula tapeworms.

When tapeworms of the species Ligula intestinalis infect fish, the consequences are gory. The parasites grow to fill the host's body cavity, leading to behavioral changes and high mortality rates. It's unpleasant for the fish and a problem for the fish farming industry, and this is why scientists keep an eye on where the parasites live and how their geographical distribution evolves.

Recent genetic studies showed that tapeworms in Northern Africa belong to two different families, or lineages: one of them is local; the other is an invasive newcomer. Where do they come from?

Jan Štefka, a biologist based in the Czech Republic, used grid computing to run a comprehensive analysis on genetic data from tapeworms to locate the source of the invasion. The results, published in the journal Biological Invasions, show that the tapeworms arrived in Africa from Europe, hitching a ride with the fish introduced to improve the local lakes.

Shedding light on a cryptic invasion

Ligula tapeworms all look the same to the naked eye and, although they are from the same species, the population is structured in "several lineages differing in their capability to infect various hosts and cause harm," explains Jan, who works at the University of South Bohemia. "Genetics is the only tool to identify such cryptic variation."

"Had we run the analyses on a desktop, we would probably still be running them now."

The tapeworms infecting North African freshwater fish are a good example of cryptic diversity: detailed genetic analysis reveals two hidden lineages within the species, one local and one invasive. The distinction is not just an academic problem; different lineages are likely to infect different types of fish.

So where do the invasive parasites originate?

The story goes back to the 1960s, when the authorities introduced pikeperch (Stizostedium lucioperca) to develop the local aquaculture industry, together with roach (Rutilus rubilio) and rudd (Scardinius erythrophthalmus) to serve as its prey.

Jan and his international team of colleagues suspected that the invasive parasites may have been introduced from Europe at this time, through already infected fish.

To test the theory, the team needed to understand the genetic variability between European and North African parasites, both between and within populations. So they collected a large, diverse dataset of the genetics of the fish parasites found in both Europe and North Africa. In the end, they had hundreds of genetic samples of parasites from both sides of the Mediterranean.

"I am glad to say that Metacentrum representatives provide great support..."

The team then used the grid computing services provided by Metacentrum (the National Grid Infrastructure of the Czech Republic) and centers in the US to analyse the datasets with MIGRATE, a program developed by Peter Beerli, one of Jan's colleagues.

"The computational power offered by grid computing is massive compared to traditional PCs," says Jan, adding that this is important because the models used for his research are computationally very demanding. "Had we run the analyses on a desktop, we would probably still be running them now."

"I am glad to say that Metacentrum representatives provide great support in helping people set up computing accounts and start their work," he adds.

The work shows that the invasive tapeworms were introduced to Africa from Europe. "We have also found that the European parasites remain tied to their co-introduced fish hosts and are not capable of infecting native African fish," says Jan. "At least not yet."

Understanding past invasions to prevent future ones

The world is becoming more connected, with improved transport links and increased mobility of people, animals and goods. This is good news for the local economies, but it also means that accidental introduction of new species into formerly isolated ecosystems is now more likely.

"Vast numbers of species of free living organisms, as well as pests or pathogens, are being transferred between continents due to human activities," says Jan. "Some of these organisms become invasive and harmful to the native ecosystems, some of them not."

"Here we showed that with rich genetic data and advanced computation we were able to trace the direction of an introduction that happened very recently. "

Knowing where the invasive species came from, and how it became established in a new area, is vital for mitigating potential problems. But this is often a difficult question to answer.

"Here we showed that with rich genetic data and advanced computation we were able to trace the direction of an introduction that happened very recently," concludes Jan.

This article was originally published on the EGI website, here.

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