Picton said he believed the animal to be an undescribed Lomanotus species, a genus of sea slug.
Being undescribed means it had never been identified, named or documented before, the Marine Biological Association added.
The association has appealed to divers, rockpoolers, sailors and fishers to record anything similar they spot and submit photographs to its data archive centre called DASSH, external.
A spokesperson said the recordings helped scientists to build a clearer picture of how coasts and seas were changing as waters warmed, including sightings of “unusual visitors and shifting species ranges”.
“The discovery is another reminder that the marine life around Britain is ever-changing,” the association said.
In 2025, young rockpoolers who joined the monthly Rock Pool Project Bioblitz, external, found a rare depilatory sea hare on Mount Batten beach in Plymouth.
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