Vicky Barlow has seen "some amazing wildlife" in her time spent searching through United Kingdom rock pools. But over the weekend, she came across something that she says was "extra special" - an "extremely rare" rainbow sea slug.

Barlow wrote in a blog post over the weekend that she had been searching for a spider crab in Falmouth with The Rock Pool Project, a local nonprofit that seeks to connect people to marine wildlife. Then when she picked up a massive seaweed-covered rock, "something bright and unusual" caught her eye. It was the rainbow sea slug.

"But, as far as we can tell, this is the first time this species has been found by a rock pooler in the UK," she said.

She placed it in a pot to get a better view, and the slug "revealed itself in full technicolor," she said, adding that other enthusiasts of sea slugs and similar animals belonging to the nudibranch group "rushed over and all watched the creature navigate the pot in complete awe."

"Like most nudibranchs it had quite the personality, rearing up to get a better look at its surroundings, and using its tentacle-like 'cerata' along its back to make itself appear bigger if one of our hands got too close," she wrote in her blog post.

They eventually returned the sea slug to its place in the rock pool and have uploaded images and information to the Rock Pool Project's database and the National Biodiversity Network.

Seeing a rainbow sea slug, whose scientific name is Babakina anadoni, in the area is part of the "massive changes" sea life has experienced in the Southwest U.K. in recent years, marine biologist Ben Holt, who also works for The Rock Pool Project, told BBC News.

The animals thrive in warm water and are typically found off the coasts of Spain, Portugal and France. Earlier this year, a report from the National Oceanography Centre found that sea surface temperatures around the U.K. have had a "significant warming trend" of around 0.3 degrees Celsius every decade over the last 40 years. Within the next 77 years, the waters are expected to warm by more than 3 degrees Celsius, the report found.

Meanwhile, the weekend finding is one of several discoveries of the rainbow sea slug in recent months. Both Holt and Barlow say there have been a "handful of sightings" since the end of last summer.

"It's an amazing find and I expect we will see more of them," Holt told BBC news. "It is also quite remarkable because rock pools are quite a harsh environment with the tide going in and out."

"It is absolutely amazing what you can find on our rocky shores," Barlow said in her blog post, "and today was a perfect example of the incredible wildlife we have on our doorstep here in Cornwall."

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