Rare ‘Technicolor’ Rainbow Sea Slug Found in England

It’s only the fourth record of the species in the U.K., and experts say it’s a sign of warming waters due to climate change

a pink sea slug with purple, orange and yellow on its spines
This is the first documented rainbow sea slug in a rock pool in the United Kingdom. Vicky Barlow / Alamy Stock Photo

Vicky Barlow was overturning stones in a rock pool in Falmouth, England, looking for spider crabs, when something “extremely bright and unusual” caught her eye, she writes in a blog post. Underneath a large, seaweed-covered rock, she made a very rare find: a rainbow sea slug.

“I knew exactly what it was as soon as I lifted the rock and definitely let out a little squeak of excitement,” Barlow, a volunteer with the environmental education nonprofit the Rock Pool Project, writes in an Instagram post.

She placed the sea slug in a pot to get a better look at its vivid pink, red, purple and orange hues. The critter “unfurled and revealed itself in full technicolor,” she writes, and then it reared up to examine its environment. Other slug enthusiasts rushed over to watch the animal explore the pot and take some photos before Barlow placed it back where it came from. 

Rainbow sea slugs are very uncommon in England—they’re usually found in warmer waters, such as those along the west coasts of Spain, Portugal and France. In fact, there have only been three previously documented sightings of the species in the United Kingdom, writes BBC News’ Jonathan Morris. But those were all spotted by divers and snorkelers, with the first sighted off the Isles of Scilly in 2022. 

“As far as we can tell, this is the first time this species has been found by a rock pooler in the U.K.,” marine biologist Ben Holt, CEO of the Rock Pool Project, tells the Guardian’s Steven Morris. “It’s a warm-water species but it looks as if it’s arrived here.” He adds that the organization’s members have been witnessing “striking changes” because of climate change.

Rainbow sea slug

Waters around the U.K. have seen rapid warming due to the climate emergency. Earlier this year, a report from the National Oceanography Center found that sea surface temperatures around the U.K. have increased by about 0.3 degrees Celsius (0.54 degrees Fahrenheit) per decade over the last 40 years. It also predicted that by the end of the century, U.K. water temperatures will rise by more than 3 degrees Celsius (5.4 degrees Fahrenheit). 

Sea slugs—the common name for a group of marine creatures called nudibranchs—are particularly useful for tracking climate change’s impacts. Most nudibranchs only live up to about one year, which means their populations respond very quickly to environmental changes, reported Marthe de Ferrer for Euronews in 2021. In Australia, citizen scientists have been collecting data on sea slugs for a decade to monitor species and learn about changes in their distribution, including their expansion into historically cooler waters. One of California’s nudibranch species, meanwhile, has extended its range north by about 130 miles since the 1970s. 

Around 3,000 species of nudibranchs exist in waters from the poles to the tropics. These mollusks are soft-bodied and feed on seaweed, anemones and other sea slugs. Unlike other snails, adult nudibranchs don’t have shells to protect them from being eaten by hungry marine animals—they shed them as larvae. Instead, many nudibranchs have evolved vibrant colors to scare off predators such as sea turtles, sea stars and crabs. Some species in the group Aeolidida may even have the unusual ability to steal stinging cells from prey they ingest and shoot them out when threatened. 

“It is absolutely amazing what you can find on our rocky shores,” Barlow writes in the blog. “Today was a perfect example of the incredible wildlife we have on our doorstep here in Cornwall.” 

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