Pleurobranchaea britannica: New species of sea slug discovered off the southwest coast

The new sea slug has been called pleurobranchaea britannica. Credit: Ross Bullimore

A new species of sea slug has been discovered off the West Country coast, experts say.

The creature, whose scientific name is Pleurobranchaea britannica, was identified by scientists from the Centre for Environment, Food and Aquaculture Science (CEFAS) and the University of Cadiz, Spain.

It was collected during routine fishery surveys conducted by CEFAS and the Instituto Español de Oceanografía in 2018 and 2019 off England and in the Gulf of Cadiz.

Marine biologists initially believed the small, shell-less sea mollusc to be a Pleurobranchaea meckeli, due to the distinctive side-gill on the right-hand side of its body.

These are typically found from northern Spain to Senegal and across the Mediterranean.

However, the species had never been found in UK waters before, which has raised questions about its true identity.

This sparked a search for experts specialising in this type of sea slug and specimens were sent off to a team of scientists in Spain for further investigation.

The new sea slug has distinctive characteristics that can barely be seen by the naked eye. Credit: Ross Bullimore.

Scientists at the University of Cádiz concluded this was indeed a new species of sea slug.

A further investigation into its DNA and an identification of different physical appearances and its reproductive systems supported that it was an unclassified species.

The discovery marks the first time a sea slug from the Pleurobranchaea genus has been found in UK waters.

Scientists believe this new species could potentially be living as far south as Spain and Portugal, around the French coast and up to the southwest of the English Channel.

It is thought that climate change may be a factor in this species moving further north.

'We assume we know everything - but this shows there's still much to learn' - CEFAS

Ross Bullimore, Marine Ecologist at CEFAS said: "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.

"This was thrilling because no other species from this genus had been documented in UK waters, or even this far north.

"We 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 UK waters, but this just goes to show that there is still so much to learn in our own backyard”.

A map showing where sea slugs have been found off the Devon and Cornish coast. Credit: CEFAS.

What is a sea slug?

Sea slugs sit at the top of the food chain, functioning as both predators and prey. They protect themselves from predators by recycling parts of the animals they prey on, such as by absorbing toxins from certain prey and then secreting the poison into their own skin.

Their sensitivity to changes in the environment make them valuable indicators of ecosystem health, helping scientists understand the impacts of climate change and human activities on marine habitats.

CEFAS Endeavour. Credit: CEFAS.

"As scientists, we have learnt a lot from this new discovery," said Peter Barry, a marine biologist at CEFAS.

"We are talking about differences between species that are almost impossible to see with the naked eye.

"It shows how easy it is to overlook and miss changes in the distribution and range of species we are familiar with seeing, or in this case, the presence of a whole new species where it isn’t expected," he added.

Dr. Juan Lucas Cervera Currado, Full Professor of the Department of Biology at the University of Cadiz, Spain, said: "This discovery has really helped to advance our knowledge of British and Spanish marine fauna, with the next steps being a more detailed review, including the use of molecular biology techniques, to understand more about P. meckeli and P. britannica."