A gorgeous flash of colour in a murky loch: how nudibranchs won my heart

The Guardian

02-03-2025

It has been a long time since I've thought about nudibranchs, let alone spotted one. But a lifetime ago, as a zoology undergraduate at Glasgow University, I spent hours underwater, swimming through kelp forests, corals and shipwrecks, looking out for the tiny, colourful creatures.
Diving on the west coast of Scotland is spectacular for all the reasons you might expect: the drama of the islands, bays and meandering sea lochs against the mountains, the rugged rocky shores, the awe-inspiring wildness.
I still remember the excitement of diving trips, in a van full of air tanks and weight belts. From the bustle of Great Western Road, past Dumbarton, up the A82 as it hugs the side of Loch Lomond and on, to Gareloch or Loch Long or further afield, left at Arrochar , through the Rest and Be Thankful, to Loch Fyne. Underwater, the lochs are full of beauty, with bright corals, dead-man's fingers, sea pens and queen scallops dancing along the sea floor, spiny lobsters and spider crabs.
In atrocious weather, however, a common occurrence, a choppy loch presents a visibility challenge. Dark skies flatten the colours of the corals and kelp to various shades of brown.
So when, tutored in what to look for by a fellow diver and marine biologist, I spotted my first nudibranch, or sea slug, I was delighted – charmed, even. It was tiny, barely 2cm long, but presented a gorgeous splash of white and orange against a brown seascape, a frondy-gilled thing with almost fluorescent spots, grazing on seaweed. So far removed from its slimy, creepy, land-based cousin and so pretty, like a sea jewel.
The biologist was writing a PhD on nudibranchs and, for a summer, we divers were his willing pupils, happily embracing his project, which felt like searching for brightly coloured, underwater butterflies or flowers. They are shell-less molluscs, their name means 'naked gills', referring to the breathing apparatus outside their bodies.
Mostly carnivores, they feed on seaweed, sponges, jellyfish, anemones and other nudibranchs. Most are minuscule, between 1cm and 6cm long, but many are vibrantly hued, from bright purple and blues to orange, and really stand out. My favourites were almost translucent, with elongated cerata, tentacle-like growths tipped with bright orange. When a nudibranch eats the tentacles of a jellyfish or other stinging animal, the venom-filled stinging cells pass through the cerata, or growths, which it then uses against predators.
That summer, we would fling ourselves backwards off small inflatables into the deep water below to reach inaccessible reefs, or kit up in wetsuits in rain-lashed shores on dives in search of them. We found nine species in all and were rewarded by being named in a study published by the zoology department: A Divers' Guide to the Marine Fauna of the Clyde Region.
I don't know how their populations are faring amid the climate and biodiversity crisis, but for me, they conjure up the joy of the subsea world.
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