For the uninitiated, the idea of marine slugs might conjure up images of slimy green blobs plaguing reefs, but this couldn't be further from the truth. Psychedelic, outwardly and all-round marvellous, nudibranchs are some of the ocean's most interesting inhabitants. Nudibranchs are molluscs, but far from being your ordinary snail or slug they come in a fantastic array of quirky shapes and neon-bright colours. The Austin Powers of molluscs, if you will. And with over 3,000 species (and growing) found across the world there are endless opportunities to seek them. Groovy baby (sorry).

Nudibranch on reef

Sense Of Surroundings

Most nudibranchs are benthic, meaning they crawl along the sea floor, however, some are able to swim. One such nudibranch is the Spanish Dancer which, paired with its monumental size (for a nudibranch, anyway) of up to 30cm, is quite a spectacle to behold. To navigate their surroundings, nudibranchs use the two horns located on their head called rhinophores, which are chemical receptors used to find food and a mate. Like a tortoise's head, they can be withdrawn into their bodies for protection.

Flabellina rubrolineata

You Are What You Eat

Nudibranchs will slowly harvest their way through algae, corals and even other nudibranchs - which derives their unique rainbow colouring. The distinctive hues are then used as a camouflage or even as repellent because of the toxins they ingest. Blue dragon nudibranchs, for instance, eat the stinging cells of the Portuguese man o' war and store them as ammunition.



Nudibranchs are rather limited in their surroundings, given their slow and solitary nature - therefore it's important to utilise every opportunity to reproduce. As such, nudibranchs possess both male and female reproductive organs so they can mate with any adult that passes. When mating they perform a dance-like courtship before fertilising each other, then both lay eggs in a ribbon-like spiral which can vary from one egg to well over 25 million.

Purple nudibranch

Where To Find Them

You can find nudibranchs in an extraordinary array of environments across the world, from tropical reefs to sub-arctic waters and even hydrothermal vents of the deep. However, the Lembeh Strait in Northern Sulawesi is one of the best paces to spot them, with 200 species (and growing) so far identified. Some of our other favourite haunts include Milne Bay in Papua New Guinea, Indonesia's Alor, Dumaguete in The Philippines and Mozambique.

"I have always been fascinated by nudibranchs, I like to think of them as the “jewels of the ocean”, coming in all different colours of the rainbow. My favourite dive site for nudibranchs is Nudi Falls in the Lembeh Strait where I once counted 11 different species on one dive, including my personal favourite, the bright purple Hypselodoris bullocki."
Eleanor Seagle, Original Diving Specialist
Macro diving
In Numbers


Estimated number of nudibranch species



Largest nudibranch ever recorded



Number of eggs a nudibranch can lay