My dive guide — a small, sun-weathered woman named Rosa who had been diving the reefs off Tulamben in Bali for twenty years — once told me that she could spend an entire dive on a patch of rubble no larger than a dining table and find something new every time.
I didn’t believe her. Then she showed me.
In forty minutes on that patch of rubble, Rosa found eleven different species of nudibranch. Some were smaller than my thumbnail. One was the colour of a lit match. Another was pure white with what appeared to be electric-blue fringing — a creature so improbably beautiful that I genuinely questioned, for a moment, whether I was looking at something artificial.
I was not. I was looking at a Hypselodoris bullocki, one of approximately 3,000 known species of nudibranch. And I had been swimming past them for years.
What is a Nudibranch?
The name is Latin and Greek combined: nudus (naked) + brankhia (gills). Naked gills. It refers to the gill plumes — called cerata or a branchial plume depending on the subgroup — that sit exposed on the outside of the animal’s body rather than protected inside a shell.
Nudibranchs are sea slugs, but “sea slug” is a functional description rather than a precise taxonomic one. They are shell-less marine gastropod molluscs in the suborder Nudibranchia, and they represent perhaps the most extraordinary expression of colour and form in the invertebrate world.
They range in size from under a millimetre to, in the case of the Spanish shawl (Flabellinopsis iodinea) and some dorid species, around 30 centimetres. They are found at every depth, from the intertidal zone to the abyssal plain, though the greatest diversity occurs in warm, shallow tropical and subtropical reef environments.
And they are, uniformly, astonishing.
Why Are They So Colourful?
There are two primary evolutionary drivers for nudibranch colouration, and they operate simultaneously in many species.
The first is aposematism — warning colouration that signals toxicity or unpalatability to predators. Many nudibranchs are toxic, and their bright colours say so clearly. The mechanism behind the toxicity varies by feeding preference: nudibranchs are specialist predators, each species typically feeding on a narrow range of prey (sponges, hydroids, anemones, corals, other nudibranchs), and many sequester the chemical defences of their prey rather than producing their own.
The most dramatic example of this is the aeolid nudibranchs that feed on hydroids and other cnidarians. These animals consume the stinging nematocysts of their prey without discharging them, transport them through specialised cells called cnidosacs, and store them in the tips of their cerata — the finger-like projections on their back. They then use those stolen stinging cells as their own defence. The technical term is kleptocnidy: theft of stinging cells.
The second driver is mimicry — either of toxic species (Batesian mimicry, where a harmless species resembles a harmful one) or within groups of toxic species (Müllerian mimicry, where multiple toxic species converge on similar warning patterns, reinforcing the predator’s avoidance learning).
In practice, most nudibranch colouration serves some combination of these functions. The result — brilliant oranges, electric blues, hot pinks, acid yellows, and complex patterned combinations of all of them — is one of the most visually extraordinary phenomena in marine biology.
Key Groups for Divers
The taxonomic diversity of nudibranchs is such that a complete treatment would require several volumes, but there are a few groups that divers encounter most frequently and that repay close attention.
Dorid nudibranchs (suborder Doridacea) are the most species-rich group. They have a characteristically oval, flattened body with a branchial plume — a rosette of feathery gills — on the upper posterior surface. Species of Chromodoris, Hypselodoris, and Glossodoris are among the most colourful animals on any tropical reef, typically feeding on sponges and displaying vivid patterns in blue, white, purple, yellow, and red.
Aeolid nudibranchs have elongated bodies covered in cerata — the projections that house their kleptocnidy. Flabellina species are particularly striking, with vivid purple, orange, and pink colouration. Aeolids are commonly found feeding on hydroids, and the two — predator and prey — can often be photographed together.
Phyllidia species are thick-bodied, bumpy, and often boldly patterned in black and yellow or black and white. They feed on cyanobacteria-laden sponges and are highly toxic as a result. Some species of Phyllidia are so chemically protected that they can share territory with predatory flatworms without being touched.
Chromodorids deserve special mention for sheer spectacle. Chromodoris willani, C. elisabethina, C. magnifica, and dozens of others are found throughout the Indo-Pacific in such density and variety that a single dive on a healthy reef can produce ten or fifteen different species on a careful look.
How to Find Nudibranchs
Nudibranchs are there. They are almost always there. The challenge is training your eye to see them.
The technique is to slow down completely. Not “slow diving” — absolute stillness. Hover near a surface — the side of a coral head, a section of reef rubble, the underside of a ledge — and look at the textures rather than scanning for movement. Nudibranchs don’t move quickly and many don’t move at all while you’re watching them.
Look for colour anomalies. A hot pink patch on a grey sponge. A cluster of white with yellow tips on a green hydroid. Anything that seems slightly wrong in texture or colour relative to its background.
Look on the prey species. If you know that a particular species of nudibranch feeds on a particular sponge or hydroid, find the prey and look at it carefully. Often the nudibranch is not just on the prey but perfectly colour-matched to it — a second layer of camouflage within animals that are also capable of being spectacularly visible.
Carry a torch even in daylight. The contrast it creates makes small animals dramatically easier to see, and many nudibranchs prefer the undersides of ledges and overhangs where natural light is limited.
Photography
Nudibranchs have made macro photography one of the great joys of diving. Their small size, their relative stillness, and their extraordinary colouration make them ideal subjects — though the challenges of achieving proper focus and exposure at high magnification, in moving water, while managing buoyancy, take considerable practice.
The standard approach is a compact camera or mirrorless system in a housing, with a macro lens or wet lens attachment, and one or two small strobes or video lights positioned close to the subject. Ring lights work well for uniform illumination; twin strobes allow more directional control.
For absolute beginners: even a smartphone in a waterproof housing will produce usable nudibranch images if you can get close enough and manage your movement. The camera rarely fails. Movement is the enemy.
Why They Matter
Nudibranchs are indicators of reef health. Their presence, diversity, and abundance reflect the health of the invertebrate communities they depend on — the sponges, hydroids, bryozoans, and corals that form the structural base of a functioning reef ecosystem. A reef with high nudibranch diversity is, almost by definition, a reef with a rich and complex invertebrate community.
That’s the thing about looking for nudibranchs. You end up looking more carefully at everything. The rubble patch becomes a landscape. The sponge becomes an apartment block. The reef stops being a backdrop and becomes the thing itself.
Find a Rosa, if you can. Let her show you the dining table reef. You will not see the ocean the same way afterward.
