The Weird and Wonderful World of Nudibranchs:10 things you should know about these tiny sea slugs

Updated: Nov 2, 2020

Nudibranchs are truly some of the most extraordinary creatures you will come across in the ocean. Evolution did not hold back when it was cooking up these critters, using every single colour, shape and pattern under the sun.

1. Nudibranchs get their name from the Latin words 'nudus' and 'brankhia', meaning 'naked gills'. Most species of nudibranch have appendages, usually halfway down their body. These allow them to extract oxygen from the water.

2. The two 'horns' on the nudibranchs head are actually called 'rhinophores'. These are pretty special appendages as they are chemical receptors, essentially acting as a nose for the nudibranch. They allow them to sniff out sponges, corals, anemones and hydroids. If you look super closely, you will see the rhinophores resemble feathers. This increases the surface area of the rhinophore, allowing for superior food detecting abilities.

Left to right: Nembrotha chamberlaini, Nembrotha cristata, Chromodoris annae

3. Not all nudibranchs feed by hunting for food. Some species acquire energy from a symbiotic relationship with algae. The nudibranch will consume certain types of algae and send them to specialised cells in the nudibranchs skin. Their skin is so thin that it allows the penetration of sunlight allowing the algae to photosynthesis, generating energy for the nudibranch!

4. While most nudibranchs spend their time eating sponges or working on their relationships with algae, there is however the occasional individual that will actually betray its own and eat a fellow nudibranch. Some will even consume an individual of the same species! See this brutal cannibalism here.

5. Nudibranchs come in an array of striking colours and impressive patterns, making them some of the most fascinating creatures to see underwater. There is, however, a crucial reason for this spectacular display that is far more important than just looking good to us nudibranch enthusiasts. Ingesting stinging cells and toxins from their food and storing it in the rear of their bodies makes nudibranchs pretty poisonous to potential predators. Therefore, these colours are actually used to let their predators know they shouldn't be messed with. Even the nudibranchs that aren't poisonous tend to be colourful or patterned to ward off predators. It has also been suggested that nudibranchs with colourful and noticeable patterns are more likely to be ignored by predators because of their uniqueness. Essentially, predators are more likely to be wary of something they don't recognise.

Left to right: Chromodoris annae, Andreadoris egretta, Phyllidia varicosa

6. Their impressive ability to turn toxins into a defensive mechanism makes nudibranchs super important to us humans by pretty much keeping us alive (sort of). Recent studies have shown that certain chemicals extracted from nudibranchs show anti-cancer, anti-tumour and anti-microbial properties. Pretty impressive for a tiny little slug.

7. As is often the way, evolution must sacrifice certain qualities to allow for specialisation of others. For the nudibranch, it was their eyesight that had to give way for their highly sensitive rhinophores. They can therefore only see the difference between light and dark. So unfortunately for them, they will never know their own beauty!

8. Nudibranchs are actually hermaphrodites, meaning they have both 'female' and 'male' reproductive organs. The love life of a nudibranch is therefore not a particularly romantic one. They will pretty much reproduce with any other nudibranch of same species that they come across. By taking it in turns to fertilise one another it allows them to both lay eggs in a beautiful and pretty strange looking spiral shape, that often ends up resembling a flower.

Pair of Nembrotha chamberlaini nudibranchs mating and red nudibranch eggs

9. Although most species are benthic (living on the ocean floor), some nudibranchs are pelagic (living in the water column). The blue glaucus nudibranch is one of these pelagic species. They have the ability to store an air bubble in their stomach allowing them to stay afloat in the water column, like a tiny little scuba diver wearing a BCD.

10. Just like their land-dwelling relatives, nudibranchs produce slime as they move around. This slime is filled with chemicals that communicate to other nudibranchs various messages, such as possible dangers or that they are ready to mate!

Top left to bottom right: Tamja morosa, Phyllodesmium briareum, Adreadoris egretta, Chromodoris lochi, Nembrotha kubaryana, Phyllidiella pustulosa

Mimi x


Proksch, P.,1994, 'Defensive roles for secondary metabolites from marine sponges and sponge-feeding nudibranchs', Toxicon

Rudman, W. B., 1991, 'Purpose in Pattern: The Evolution of Colour in Chromodorid Nudibranchs', Journal of Molluscan Studies

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