New research uncovers how two marine nematode parasites are interbreeding, in a contact area of their range of distribution

The latest Paper of the Month is, Hybridization and introgression of the mitochondrial genome between the two species Anisakis pegreffii and A. simplex (s.s.) using a wide genotyping approach: evolutionary and ecological implications, and is available open access in Parasitology.

Why look at hybrid parasites?
In the natural world, species don’t always stay neatly separated. Sometimes, when their paths cross, they can interbreed. This process — called hybridization — is surprisingly common and may play a role in evolution.

For parasites, it’s especially interesting. When different species mix, their offspring may pick up unusual mixed traits. They might spread in new ways, become harder (or easier) for hosts to resist, or even change the risk they pose to  humans.

That’s exactly what scientists found happening in the waters off southern Spain. In the Atlantic and the Alboran Sea, two worm-like parasite species — Anisakis pegreffii and Anisakis simplex (s.s.)— often become adult worms in the same marine mammal host, and where their geographical ranges overlap, they don’t just share host space and condition… they sometimes cross-breed.

How do you spot a hybrid parasite?
From the outside, hybrids look just like their parents. So, scientists turn to genetics — like looking at the parasite’s DNA “fingerprint.”

In this study, researchers used a set of powerful genetic tools to compare parasites from the contact zone with those from elsewhere. By checking across many different points in the genome, they could spot which individuals were pure parental species and which were hybrids — even picking out first-generation crosses versus “backcrosses” (hybrids that had bred again with one of the parental species).

Think of it as detective work: every parasite carried a mix of genetic clues revealing who its parents were.

What did the scientists discover?
The results were fascinating:

• Hybrids are real. About 5% of the parasites they found were first-generation hybrids.
• Backcrosses exist too. Some hybrids had bred again with one of the parental species, but these were at larval stage
• Mitochondrial mix-ups. The team even found evidence that A. simplex (s.s.) had passed on its mitochondrial DNA (the “powerhouse” genes normally inherited from the mother) to A. pegreffii — something not seen before, as a result of past or paleo-hybridization events

So while hybridization is rare, it’s definitely happening. And when species overlap, especially if one is much less common, the chances of cross-breeding increase.

Why does this matter?
On one level, this is a fascinating glimpse into evolution in action. Hybridization is possible in these parasites, while reproductive barriers still keep them mostly distinct.

But it also matters for ecology and health. If hybrid parasites behave differently from their parents — perhaps spreading faster, surviving better, or changing how they infect fish and humans — that could have real-world consequences for ecosystems, fisheries, and food safety.

Finally, the study is a reminder of the power of looking at many genes, not just one. If scientists relied on a single genetic marker, they might completely miss the hybrids and misunderstand what was happening in these marine populations.

Parasitology is an important specialist journal covering the latest advances in the subject. It publishes original research and review papers on all aspects of parasitology and host-parasite relationships, including the latest discoveries in parasite biochemistry, molecular biology and genetics, ecology and epidemiology in the context of the biological, medical and veterinary sciences.