An incredibly rare sea slug recently graced British waters as Babakina anadoni, arguably the most technicolor sea slug in the world, made an appearance off the coast of the Isles of Scilly. It’s the first time the species has been seen in the UK, says the Cornwall Wildlife Trust (CWT), and it’s a hella fancy.

The sea slug was spotted by Allen Murray, a volunteer for CWT’s Seasearch Diving project which aims to record marine habitats and species found in UK waters and use the information to identify sites of specific conservation concern. Murray’s sighting is all the more remarkable in the context that, while very conspicuous in its coloration, the rainbow sea slug is but a tiny bean.

“It’s one of the prettiest sea slugs I’ve seen and, given its less than half the size of your little finger, it’s amazing Allen spotted it at all!” said Matt Slater, Marine Conservation Officer at CWT and coordinator of the Seasearch Diving project for Cornwall and the Isles of Scilly, in a release emailed to IFLScience.

“There’s still so much out there that we don’t know about our marine environment. Records like this from our Seasearch divers are vital in helping us understand and better protect our seas.”

As a sea slug, B. anadoni sits within the nudibranchs, which are a famously flamboyant bunch. Among them are species that look like grapes, sheep, bananas, and even one that can decapitate its own head to grow a fresh body from scratch.

As well as being spectacularly colorful, our rainbow sea slug has an unusual sex life. Being hermaphrodites, both parties in a mating pair are equipped with both sets of reproductive organs.

The sea slugs battle it out with their penises to decide who is the “dominant male”, with the victor being whoever first penetrates the body wall of the other sea slug.

The rare sighting of this penis-wielding sea slug around Melledgan, an uninhabited rock island in the Isles of Scilly, is surprising as it’s quite far north for the species. Previously it’s only been recorded along the West coast of Spain and further south in the Atlantic, but never before in the UK.

“We never cease to be amazed at the wildlife turning up in Scillonian waters,” said Lucy McRobert, Communications Manager at Isles of Scilly Wildlife Trust. “From rare and beautiful nudibranchs to violet sea snails to great whales like humpbacks and fins, every time we dive beneath the surface we learn and see something new!”