Exposing Nudibranchs.

Uncovering marine biodiversity on our shorelines.

Nudibranchs are stunning and bizarre creatures, they occur in an almost unimaginable range and combination of colours, including intense blues, vivid yellows and purples, magenta and scarlet. Australia’s tropical waters abound with a great diversity of species,  but it’s our southern shorelines that have offered the greatest assortment of new discoveries.


The conspicuous contrast of Nembrotha kubaryana, a tropical species. Image via flickr


Divers sometimes refer to nudibranchs as ‘butterflies of sea’.  Now I mean no offence to the Lepidopterist community out there (i.e. those that study butterflies) but I’m of the view that nudibranchs put butterflies to shame when it comes to sheer brightness, flamboyance, and a seemingly incongruous combination of shapes, patterns and appendages.

So, what are nudibranchs? What sets them apart from all the other wild diversity of our oceans? Well put simply they are snails and slugs, but they’re about as far removed from your average garden snail or slug as you could possibly imagine. Nudibranch means ‘naked gills’,  they are gastropods, that is snails and slugs, but nudibranchs have rearranged their anatomy and lost their protective shell resulting in their gills being directly exposed to the ocean currents.

They occur throughout the world’s oceans from intertidal rock pools to the deepest trenches. Despite their conspicuous colour and lack of agility nudibranchs are surprisingly poorly known to science with relatively few studies into their ecology and biology.

What we do know is that most nudibranchs are carnivores preying upon marine animals including corals, bryozoans, anemones, jellyfish and occasionally they devour each other. The Blue Dragon, one of the few nudis with a common name, is an ocean going, upside down swimming hunter. This species preys upon jellyfish in the ‘blue bottle’ group of jellyfish, the Physalia. Remarkably it can encapsulate the stinging cells of the jellyfish it preys upon and use these in self-defence. 

The Blue Dragon heading south as our oceans warm. Image via flickr


The ability to harness the toxic power of the stinging cells of one’s dinner is not however confined to the Blue Dragon. This rather filamentous Flabellina rubrolineata positively bristles with defensive cnidocytes. The white points at the tip of each pink fringed filament are where its cnidocytes reside. This elegant marine ballerina inhabits the shores of Port Phillip Bay, clearly it’s not only tropical waters where bright and intricate examples of these creatures can be found.

Flabellina rubrolineata   Image via Wiki Commons

Flabellina can be seen at Blairgowrie pier, as can Mexichromis macropus. This species is relatively common and at times has been observed in large numbers within Port Phillip Bay.

Mexichromis macropus. Image via Wiki Commons

And in case you’re wondering, yes they are posturing to exchange barbs as is the mating behavior of some nudibranchs. For those new to the sex life of marine gastropods allow me to explain. Nudibranchs are hermaphrodites, they copulate by inserting their barbed penis into their mate and exchanging sperm.

Simple, and sounds all rather dispassionate but it can be a little more intimate than that for some. A number of nudibranch have been observed stroking each other’s ‘heads’ and caressing each other before copulation, while the more cannibalistic varieties rather quickly ‘uncouple’ and then attempt to eat each other. A rather odd evolutionary strategy I’d suggest.

Bright colours in nature, are often a warning sign of toxicity but I must admit that many Nudibranchs do look rather edible to me. This next species really does remind me a meringue just out of the oven, with slightly browned crispy, wispy peaks. It is referred to by many divers as the confectionary nudibranch. I think the Meringue Nudibranch would be more apt.

Glossodoris atromarginata, often seen on the east coast. Image via Wiki Commons


And if I were to abandon my studies to pursue a career as a manufacturer of confectionary I would surely model my first product on this little guy, Glossodoris cruenta occasionally recorded along the southern Queensland coast.

Glossodoris cruenta. Would anyone care to suggest a common name? Via Wiki Common

I don’t know about you, but to me, this oceanic eye candy has ‘milk bottles’ written all over it. But, Glossodoris cruenta is an uncommon species in Australia. It’s our southern shorelines which are the center of discovery for nudibranch diversity.


Port Phillip Bay and surrounds is renowned for its range of nudibranchs with over 100 new species described over the past few decades. Surprisingly these creatures are relatively easy to find in rock pools and the intertidal zone. Close inspection of anemone beds and fringing marine sponges will more than likely reveal a nudibranch. One of the more common species is Tambja verconis, it’s a conspicuous nudi with unmistakable coloration.

Tambja verconis

Image via flickr

The vast majority of Victoria’s new discoveries have been made by amateur ecologists Bob Burn, a passionate nudi-nerd and builder by trade.  So, while exploration of the worlds deep ocean trenches often reveal remarkable new creatures it is equally likely that new nudis will be discovered in rock pools by weekend enthusiasts.

New species await discovery, hiding in plain view. They’re in a rock pool near you.



Part one of a four-part series loosely based on the classic elements, water, earth, fire and air.  In each piece I explore an element of discovery, innovation or speculation in ecology reflecting the classic elements.

10 Responses to “Water.”

  1. Rob Dabal says:

    Well….funny you should mention. Indeed there is a citizen science project happening next year that does precisely that!

    The Victorian National Parks Association will be conducting the inaugural Port Phillip Bay Nudibranch Census in April 2018.

    For more information go to https://vnpa.org.au

  2. Tharaka Kaluarachchi says:

    Really excellent article about nudibranchs, truly stunning stuff. I wonder if there are any citizen science projects happening next year in the Port Phillip Bay area where people can get involved in discovering and identifying nudibranchs in our area? Like a census, maybe?

  3. Rob Dabal says:

    Thanks for commenting Nathalie. Yes, dare I say there’s an abyss of knowledge when it comes to understanding biodiversity in our oceans. Depending on your reference there are around 5-6000 species of nudibranchs in the world. Over 350 have been identified on Victorias coastline and we only have 0.15% of the worlds coastline. Certainly there are many species that are pelagic and cosmopolitan but I would expect that there are many thousands of new nudibranch species out there, its just that no one has gotten around to describing them all. Take the Buccaneer Archipelago in the Kimberley WA for example. Over 800 islands, remote, tropical, poorly surveyed ecologically with many large endemic species, Its bound to have hundreds of endemic smaller species.

  4. nbolton says:

    Really interesting read! Love the comparison to lollies.. they do look quite edible! Reading this makes me realise how many oceanic creatures there are out there that I know nothing about.

  5. Rob Dabal says:

    Hi Jennifer, thanks for the feedback and your excellent question, I hope my answer does it justice!

    The ‘why they have evolved to have their gills exposed‘ question is a tough one to answer.

    Our knowledge of basic ecology and evolutionary biology of nudibranchs is extremely limited. We are lucky in Victoria to have had a dedicated enthusiast turned world expert by the name of Robert Burn, who has described over 100 new species with dozens still awaiting formal description but having a basic understanding of the diversity of a group of species is really only the first step in a deep understanding of a groups ecological and evolutionary attributes.

    Regarding the evolution of gill exposure I would suggest that there would be some trade offs involved in the investment in producing a shell versus the exposure of their gills that does not disadvantage them. Many nudis have are highly toxic, they are not often eaten by fish and many have the capacity to assimilate the toxic compounds of their prey items either within their tissue internally or via cnidocyte encapsulation. So exposing their gills seems not to place them in greater danger of predation because they are highly toxic. The greatest predators of nudibranchs are…..other nudibranchs. There are some challenging evolutionary questions here especially for the species that have been observed consuming the larvae of their own species.

    However, it has been suggested that having exposed gills probably makes them more sensitive to pollution and toxicity. Their potential as indicators of environmental health deserves further investigation.

    Hope that helps answer your question.

    Cheers Rob

  6. Jennifer Feinstein says:

    Engaging and interesting read! Love your clever and humorous style of writing (especially the title!). After seeing all the pictures, I think I agree with you that they are more vibrant than butterflies. I’m curious why they have evolved to have their gills exposed.. does this put them in more danger to predators, etc.?

  7. Rob Dabal says:

    Hi Sian. Tambja verconis is around 4-5cm and can be up to 7cm, so fairly conspicuous. Some tropical species can be much larger at around 10+cm or more. Many of the more reclusive species in Port Phillip Bay can be quite small, anything between 5-20mm is fairly typical.

    But the real trick with spotting Tambja is to find its food source. It feeds almost exclusively on a blue bryozoan, Bugula dentata which is fairly clear in the image. It occurs in relatively shallow waters to I think 20+m depth. You’d find Bugula on piers and in deeper rock pools, so Tambja is a species that you’d be able to find snorkelling once you find the food source. Many Nudibranchs have similar colouration to their food sources, in some cases it about camouflage but it also appears that many nudis absorb pigmentation from their food source.

    Cheers Rob

  8. Siân says:

    What stunning colours! Can I ask what sizes are we talking, similar to actual slugs? In particular the Tambja verconis – if it is relatively common I’d like to keep an eye out next time at a beach with rock pools.
    And great concept of theming each blog to reflect the four elements!

  9. Rob Dabal says:

    Thanks Richard. I also worked on dive boats in north Queensland and yes nudis are stunning creatures that get folk excited. And they’re also abundant on the southern coast once you get your eye in. Cheers

  10. Richard Proudlove says:

    Enjoyable read. I was a dive master in Asia in a former life and we loved Nudis since you could always find one or two on every dive to delight clients…whilst other marine treats were often more elusive.