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.…