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Side-gilled sea slug a first for Britain


Pleurobranchaea britannica - note the side-mounted gills (CEFAS)
Pleurobranchaea britannica - note the side-mounted gills (CEFAS)

A new species of side-gilled sea slug has been discovered in Britain – and named Pleurobranchaea britannica.

The nudibranch was collected by scientists on the research vessel CEFAS Endeavour off England’s south-west coast in 2018 and 2019 and has now been described. CEFAS is the Centre for Environment, Fisheries & Aquaculture Science.  

Measuring 2-5cm and cream and clay-coloured with white specks, the slug resembled another common species, Pleurobranchaea meckeli, found from northern Spain down to the Mediterranean and West Africa.

This made identifying the new species challenging, but the team were helped by specialist DNA analysis carried out by its research partners at the University of Cadiz.

Top view of the slug (Ross Bullimore / CEFAS)
Overhead view, showing the widely spaced rhinophores (Ross Bullimore / CEFAS)

No other groups of sea slugs found in UK waters feature side-gilled anatomy, so it was this and aspects of the “head” that alerted the scientists. The leaf-like side-gill resembles a traditional canoe paddle, while the sensory rhinophores at the front are widely spaced.

The discovery marks the first recorded instance of a Pleurobranchaea sea slug in UK waters. With the two other members of the genus, P meckeli and P morosa, known from warmer waters, it is possible that the nudibranchs are migrating as a result of climate change.

“It is exciting to see that routine fisheries surveys can still lead to such discoveries,” said CEFAS marine ecologist Ross Bullimore. “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.”

The study has been published in the Scientific Journal of Zoosystematics and Evolution 

Also read: First recorded UK sighting of rare nudibranch, Decapitated nudibranchs regrow their bodies, Rare nudibranch makes a second appearance

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