The Rainbow Sea Slug

The oceans are changing, and one of the outcomes is that new species are frequently arriving on the Cornish Coast. This includes a variety of species of sea slug, including most recently the ‘hair curler’ slug. But by far the most remarkable is the rainbow sea slug.

This almost unreal looking sea slug was first seen off the UK coast around the Isles of Scilly in summer 2022. I was then fortunate enough to find not one, but three of them on a very low spring tide in early 2023, one of, or perhaps the first, sighting off the UK mainland. Since then, the sea slug has been seen increasingly frequently in Cornwall (especially here in Falmouth), and even a few times in Devon further north. In fact, 2026 has been a year when it has been seen in quite large numbers locally.

Finding a rainbow sea slug (well, three) was one of the most exciting encounters I can remember, and even though I have seen them quite frequently since, I still gasp whenever I see one. Their colours are scarcely believable. The slugs ingest small hydroids (relatives of jellyfish) and instead of digesting the stinging cells of their prey, they distribute them into the cerata appendages on their back as use them for their own defence against predators, which they advertise through their gaudy colours.

Rainbow sea slugs have rightly attracted a lot of attention, and have become somewhat of a local celebrity now, though they are also another warning of how our oceans are changing. Nonetheless, they are incredible to photograph, and it’s been really nice that a couple of my images of them have been awarded in photo contests recently.