Country diary: Slugs? Dull? This one is pure showbiz | Charlie Elder

I have never seen a species quite so flamboyant in appearance. A couple of centimetres in length, it was curled beneath the submerged rock like a piece of gaudy costume jewellery.

When the cry went up that one had been found, I had to prevent myself from breaking into a run as I navigated the slippery seaweed-covered boulders to catch my first glimpse of this rarity.

A sea slug. And yet a species that does everything possible to dispel any idea that slugs are drab and dull. The extravagant rainbow sea slug comes with a “ta-da!”. Typically found along the coasts of Spain and Portugal, this multicoloured member of the Aeolid nudibranchs family has spread north with warming seas, and was first recorded in the UK off the Isles of Scilly in 2022. It has since secured a toehold in southern bays of Cornwall and Devon.

I had joined a “bioblitz”, run by the Rock Pool Project, on a low spring tide in Falmouth. The community organisation’s monthly challenge engages members of the public in a citizen-science survey of seashore life. The rainbow sea slug was a star find, drawing an admiring huddle of enthusiastic rockpoolers. Breathtaking and bizarre, its outlandish confectionary of candy colours looks almost artificial. The vivid pink body is decorated with a mane of blue finger-like projections known as cerata, which are topped with yellow and orange. A pop-art palette of colours designed to warn predators that it is unpalatable.

This one was found on the underside of a rock beside white circles of recently laid eggs – the next generation of these sensational little sea creatures.

The project volunteer and marine biologist Ellie Maynard told me that she encountered her first rainbow sea slug last Christmas. “Since then I have seen upwards of 20 here,” she said. “It has been really rapid how they have grown in number.”

It was far from the morning’s only rarity. Others included the wonderfully named hair curler slug, which first reached the UK last September and has wavy brown cerata that give it a permed appearance.

The rainbow sea slug we were watching eventually trundled off, like an overdressed performer late for a carnival. Show time was over.

Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount