Somewhere deep in the rainforest, researchers sometimes come across an undiscovered insect, but it doesn’t happen often that they still find new animal species in our densely populated Europe that has been researched down to the millimeter. And certainly not something as big as a slug.

Yet that is exactly what a group of researchers encountered during an expedition in Montenegro. In July 2019, an international team set out to explore the Tara Canyon. This is the deepest gorge in Europe. In the narrowest part of the ravine, through which they traveled in boats, they found a new species of nudibranch hidden under overhanging rocks. The brownish-gray snails with a relatively sharp back were a whopping 20 centimeters long when stretched all the way. The animals were initially indistinguishable from the keelback snails, officially Limax cinereoniger called.

But a DNA lab at the University of Verona offered a solution. There it was discovered that there is a 10 percent difference in the so-called DNA barcode between the snail in the ravine of Montenegro and the already known keelback snail, which incidentally also occurs in the Netherlands, Menno Schilthuizen tells. Scientias.nl† “For land snails, that’s a pretty big difference,” he explains. “Normally, between closely related species you will find a difference of more than 3 percent but less than 10 percent.” Further dissection also revealed differences in the reproductive organs. That together was enough to speak of a new species. His name? Limax pseudocinereoniger. So he looks a lot like his sister, the Limax cinereoniger.

Limax pseudocinereoniger. Photo: Taxon Expeditions

Longest Penis

The giant keelback snails of the genus Limax are best known for their size – they are big! – and their color: they vary from cream to red and deep black and also have striking patterns of dots or bands. What further appeals to the imagination are their genitals. After barnacles, Limax snails have the longest penises in the animal kingdom, up to seven times their body length. The mating behavior is therefore remarkable. The slugs do long intricate dances, with the hermaphroditic partners entwining their gigantic penises.

The Balkan

Although the large nudibranchs are actually quite well known, it has been clear for some time that there are still many undiscovered species in Europe. Most people don’t expect that. You would think that Europe, a continent full of universities with biologists who love to do research, would have been completely unraveled. And surely something as big as a 20 centimeter slug would have been noticed. It is not exactly a micro-organism or a small insect. How is it possible that it was never discovered before? “That’s because these slugs are very similar externally and internally,” says Schilthuizen. “It is only now that we use DNA barcoding that we are made aware of small differences in shape that we would not have noticed otherwise.”

The discovery is not only very interesting, but also useful. “People often mistakenly think that after 250 years of natural research we will have discovered just about all animal and plant species,” Schilthuizen continues. “But the bitter truth is that we probably only know 20 percent at the most. As species are dying out faster, that lack of knowledge becomes more and more acute: because how do you know what to protect if you don’t even know what you have? That is why we want to use our expeditions to show that there is still so much work to be done and that non-scientists can help too.”

On expedition

Schilthuizen has set up a scientific travel agency, which anyone who would like to participate in biological research can sign up for. “Our expeditions are always aimed at discovering new species, so we go well prepared and focus our attention mainly on groups of animals where we know (based on expertise and the literature) that there are still undiscovered species to expect. What does surprise me is that it is apparently still possible in Europe to discover new species of animals of this size.”

Especially in the Balkans, there is still a lot to be found in the field of new slugs. It was previously established that there are still at least ten slug species on the peninsula that have not been described. Since these are early branches, it is suspected that the entire species originated from that region.

This is not the last expedition in search of new animal species. The scientists and non-scientists plan to return to Montenegro in the summer of 2023 to document the biodiversity. And of course to find new slugs. Who wants can come along!

DNA Barcoding Framework

DNA barcoding is used to identify different species. Short genetic markers in the DNA make it possible to determine whether there is sufficient distinction to speak of a new species. For many organisms, these markers come from mitochondrial DNA (mtDNA). In animals, the CO1 gene is usually used (the cytochrome c oxidase subunit1 gene). In the Netherlands, the Leiden research institute and natural history museum Naturalis determines the DNA barcode of Dutch plants and animals. The aim is to develop as complete a collection as possible of identified species and their DNA barcode.

The participants of the expedition at work. Photo: Taxon Expeditions