Ask a diver or snorkeller about their favourite find and there is a good chance they will name a nudibranch. These small, slow-moving sea slugs come in colours that look almost painted on. This guide covers what they are, why they look the way they do, how they protect themselves, and how to find and photograph them without disturbing them.
What is a nudibranch?
Nudibranchs are sea slugs. The Queensland Museum describes them as "naked-gilled" sea slugs, which is where the name comes from. They are molluscs, part of the same big group as snails, cowries and cone shells. The difference is the shell: in nudibranchs it is reduced, internal or missing altogether.
There are around 3,000 valid species, according to the Queensland Museum, and many have colour patterns as elaborate as a butterfly's wings.
Dorids and aeolids
Most nudibranchs fall into two main groups. Dorids carry a feathery cluster of gills on the back, towards the tail end. Aeolids have no gill plume. Instead their backs are covered in finger-like outgrowths called cerata, which help with breathing and, in some species, hold stinging cells taken from their food.
Those "horns" are noses
The pair of horns at the front are called rhinophores. The name comes from Greek words for "nose" and "carrier", and that is their job: they pick up chemicals dissolved in the water, much like a sense of smell. Many sea slugs find their food this way. Most nudibranchs can pull their rhinophores back into a pocket under the skin when disturbed, which protects them from nibbling fish.
Male and female at once
Nudibranchs are hermaphrodites. Dr Bill Rudman of the Australian Museum's Sea Slug Forum explains that they act as both male and female during mating. Because their reproductive opening sits on the right side of the body, mating pairs line up right side to right side.
What do nudibranchs eat?
All nudibranchs are carnivores. Depending on the species, they eat sponges, hydroids, lace corals (bryozoans), anemones, soft corals and sea ferns. Some eat other sea slugs or their eggs, and a few will occasionally eat their own kind.
Why are nudibranchs so colourful?
The Queensland Museum notes that many nudibranch colour patterns come from chemical compounds the animals absorb from their food. Those colours do different jobs for different species:
- Warning: bright patterns tell predators that the slug is toxic or tastes foul.
- Camouflage: some species match the colour and texture of the food they live on, which makes them hard to see while they feed.
- Bluffing: some nudibranchs that are not toxic copy the patterns of toxic species to put predators off. This is called mimicry.
- Surprise: some can flash hidden colours when threatened, which may confuse or startle an attacker.
Research from the University of Queensland shows how this plays out on the reef. In a 2017 study, triggerfish learned to avoid the sea slug Goniobranchus splendidus mainly by its yellow rim, which stays the same, rather than its red spots, which vary between populations. A 2018 follow-up found that red-spotted sea slugs along the east coast look alike to fish but do not all carry the same chemical defences.
How do nudibranchs defend themselves?
Without a shell, a nudibranch cannot retreat the way a snail can. The Australian Museum notes that fish appear to be their main predators, so most of their defences are aimed at fish.
Borrowed chemicals
Many nudibranchs keep distasteful or toxic chemicals in their skin. Some make their own, while others take them from their food. The Australian Museum describes chromodorid nudibranchs getting these chemicals from the sponges they eat, which makes them inedible to most fish. Two Nembrotha species have been shown to take defensive compounds from the sea squirts they feed on and release them in a slimy mucus when disturbed.
Borrowed stings
Some nudibranchs that feed on stinging animals, such as fire corals, keep the stinging cells (nematocysts) from their prey and use them for their own defence. In aeolids, stinging cells taken from prey can end up in the cerata.
Decoys
According to the Sea Slug Forum, most Ceratosoma species have a long, curved "horn" on the back. It acts as a lure, drawing predators to the part of the body that holds most of the distasteful chemicals.
Where to find nudibranchs
Nudibranchs live on coral reefs and rocky reefs. Some sit in the shallows and others live deep: Sea Slug Forum records for one species alone, Ceratosoma trilobatum, include animals found in 2 metres of water and in 66 metres.
You do not need to be a scuba diver to find one. Snorkellers can spot them on shallow fringing reefs. Around Magnetic Island, Queensland Parks names the northern sides of Florence and Arthur bays as good snorkelling spots, with Nelly, Geoffrey and Picnic bays also suitable, especially in calm weather when visibility is best. Read more about the island on our Magnetic Island page.
Tips for spotting them
- Go slowly. Nudibranchs crawl, so it is easy to swim straight past them.
- Think small. Many species are only a few centimetres long. A Chromodoris elisabethina photographed at Lizard Island for the Sea Slug Forum was 24 mm.
- Check their food. Look over sponges, sea squirts and hydroids, since nudibranchs are often found on or near what they eat.
- Look for the rhinophores. On a camouflaged animal, a pair of horns poking up may be the first thing you notice.
- Photograph the details. Several species share similar patterns, and the colour of the gills and rhinophores often helps tell them apart later.
How to photograph nudibranchs respectfully
- Look, don't touch. Leave the animal where it is rather than moving it for a better angle. A disturbed nudibranch pulls in its rhinophores, so you lose the shot anyway.
- Mind the reef. Queensland Parks asks snorkellers to avoid kicking, standing on or touching corals, as they are easily damaged.
- Keep your gear tucked in. Fins, cameras and dangling gauges can bump the reef when you are focused on a tiny subject.
- Be patient. Let the animal crawl into a clear spot instead of nudging it there.
- Know the zones. The waters around Magnetic Island are part of the Great Barrier Reef Coast Marine Park and the Great Barrier Reef Marine Park, and Queensland Parks recommends checking the zoning maps before any activity there.
Take a sea slug home
If you have fallen for nudibranchs, our nudibranch collection has neck gaiters in bright, pastel and outline styles, and our nudibranch tees include a Nembrotha design. Not sure which to take on your next dive? See our guide: diving headband or neck gaiter? Everything is designed in Townsville, made to order by our print partners, and ships free.
Ready to put names to faces? Read our guide to nudibranchs you might see on the Great Barrier Reef.