Nudibranch: More than just a Slug!


To the untrained diver everything is exciting, crazy colored fish, turtle and pristine reefs. However when I have been training new divers it is very difficult to get them excited about what is possibly (in my opinion) one of the coolest things you will see underwater. Nudibranchs are some of the most colorful creatures on earth, which makes them very popular with underwater photographers and divers who seek out macro life. They come in stunning shapes and most divers (once they know what they are looking for) will spend almost an entire dive looking at a tiny piece of coral trying to find them.

Where can I find Nudibranch?

Nudibranchs can be found in all the oceans worldwide, ranging from tropical coral reefs and to extreme colds such as Antarctica. There is even a species found near the North Pole called the sea angel. They can live in a huge range of depths, they can be found in the intertidal zone, to depths of over 700 meters, although you will find the most species in warm shallow tropical reefs.

How many species of Nudibranch are there?

There are actually over 3000 described species of nudibranch and many have been unnamed. They are often called sea slugs, but many sea slugs actually belong to other taxonomic grounds which are not closely related to Nudibranchs. Nudibranchs are benthic animals, which are usually found crawling over the sand, reef substrate or even the corals themselves. There are certain species that float upside down just under the water’s surface, and some species can swim as well.


Nudibranchs employ various defence mechanisms to help prevent them from being eaten. Over the course of their evolution, they have lost their shells, adopting other defensive techniques. Some species have evolved so their colour mimics the food they prey on (such as sponges or soft corals). Many of them are brightly coloured, so bright they stick out to form their surroundings, which is a natural indicator of either being toxic or highly distasteful. Some Nudibranchs feed on hydroids, and can actually store the stinging cells of the hydroids without harming themselves.

Nudibranchs can also use a variety of chemicals to defend themselves. Some of these toxins are not actually lethal but can incapacitate the attacker while the nudibranch can escape. Other species can release a form of acid from their skin, which is released when the nudibranch is physically touched or irritated by another creature. All known species of Nudibranchs are carnivores. Many feed on sponges, others hydroids. Or many species are known to be cannibals eating other nudibranchs or even their own species!

Physically nudibranchs can look vastly different, apart from their crazy colour variations they vary massively in size too. Although the majority of species are between 4-10 cm some species can grow to 60cm and weigh up to over 1kg! They have tentacles on their heads which they use to smell, taste and touch. They have eyes, but they are around ¼ of a millimetre in diameter that is set into their bodies, which can only deal with light and dark.

If seeking nudibranchs is something that you are interested in, then you should discuss with the dive centre that you are wanting to dive with, as many divers are more interested in seeing an entire dive site than focusing on trying to find some potentially tiny and camouflaged creatures. They can be found all over the world, but some of the best nudibranch diving can be found in Australia, Fiji and Indonesia.

‘Nudibranch: More than just a Slug!’ was written by Mike

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