Researchers found a new species in the waters off of the U.K. — but they didn't realize it at first

A new species of sea slug has been discovered off the waters of the U.K. Researchers with Centre for the Environment, Food and Aquaculture Science said they originally thought it was a well-known species, Pleurobranchaea meckeli, found near Spain and the Mediterranean Sea – but they weren't sure.

There were no previous records of this type of slug in the U.K., "raising questions around its identity," the center said in a news release. 

The new species of slug, which has been named Pleurobranchaea britannica, is between two and five centimeters long – less than two inches and has gills on its side. It was discovered during a routine survey of sea life in southwest England and the center searched for experts who specialized in this type of slug to help them identify it.

They sent the specimen to University of Cádiz in Spain, where researchers looked at its DNA and physical characteristics, finding it had a different appearance and reproductive system than other known species. So, it was determined it is a new, standalone species.

"The discovery marks the first recorded instance of a sea slug from the Pleurobranchaea genus in U.K. waters," the news release reads. "Scientists believe that this new species could potentially be found from as far south as Spain and Portugal, around the French coast and up to the southwest of the English Channel."

While slugs may be small, they are not just prey – but also predators. 

To protect themselves from predators, some sea slugs will recycle parts of the prey they eat. If they eat prey that has toxins, for example they will secrete the poison into their own skin.

"It is exciting to see that routine fisheries surveys can still lead to such discoveries. It only took a brief inspection of two specimens to be confident that we had stumbled upon a species of Pleurobranchaea," said Ross Bullimore, a marine ecologist who collected the specimens. "This was thrilling because no other species from this genus had been documented in U.K. waters, or even this far north."

Bullimore said they were "blown away to discover that we had identified a third, new species." 

"There is often an assumption that we know everything there is to know about species found in U.K. waters, but this just goes to show that there is still so much to learn in our own backyard," he said.

Hayden Close, a seabed analyst and modeller at the center, said it is "exhilarating" to uncover a new species. "It just goes to highlight the value of these surveys and serves as a reminder of the ocean's boundless unexplored secrets, even in areas we consider well-charted and studied," Close said.