Send Nudis: The Way-Too-Simple Intro to Nudibranchs


If you search ‘Nudibranchs’ online, what you’ll likely find is that it’s a surprisingly overwhelming topic to dive into. See what we did there? Dive? We digress… With over 3,000 known species in our world’s oceans, each more flamboyant and weird than the last, they’ve developed a sort of cult following among divers. Sure, they’re maybe not as exciting as a shark, as fun as a sea lion, or as elegant as a manta ray, but when you delve into a few pictures you might find yourself in the normal internet wormhole marvelling in all their glory. Maybe after getting to know these creatures a little better, you’ll be nudi-hunting in no time.

Before we dig in – let’s give a shoutout to those that submitted their best nudibranch photos you’ll find throughout the blog.

Nudibranch. Heh, Funny Name.

Upon first glance, most people will pronounce it phonetically – nudi (like noody) and branch (like a tree). Thanks to a combination of both Latin and Greek roots, the Latin word for “nude” and the Greek word for “gill” combine to be pronounced “noody-brank.” Think of bronchial tubes and the hard “ch”; same etymology here. Because of the pronunciation, in the plural form you only need to add an “-s” instead of an “-es” like you might think.

“Naked gill” is pretty self-explanatory since nearly all the species have a respiratory system that protrudes from their bodies. There are dorid nudibranchs which have gills on their back end that stick up looking almost like a mini sea anemone. The eolid nudibranchs have cerata (Greek for “horns”) that protrude from their bodies and contain respiratory structure.


How hard do you have to look?

Most of them spend their entire lifespan (about 1 year) on the reefs or seafloor scrounging for food. There are a few varieties that will float or swim as well, and they reside anywhere from the ocean’s surface to hundreds of meters deep. Some species of nudibranchs are incredibly colourful and unique and might stand out against a darker coloured reef, while others have more nuanced colour designed to camouflage themselves. One says “don’t eat me I’m harmful” while the other kind just tries to stay out of the spotlight in general. They range in size from around 4mm to over half a meter. While the smaller, less colourful ones might be harder to spot, they’re not exactly going to impress anyone with their speed; they travel about 10m a day on average.


If I see them, can they see me?

Probably not. Their vision structure is very simple – a lens and less than 10 photoreceptors – which allows them to tell the difference between light and dark. For contrast, the human eye has well over 100 million photoreceptors, which is helpful to us for checking out all these colourful nudis!

I’m still not sure why I should care about nudibranchs…

Look at it this way…if there’s a manta nearby the group, everyone will know. If there’s a whale shark around, you’re going to get a head’s up. The nudibranch search is for those who really want to examine the reef and see something cool. The fact that there are so many types that come in all different shapes and sizes makes them fun to try and spot along the reef. Hmm, maybe you need some cooler facts.


Yeah, do they do anything cool?

You bet your sweet mask they do. Nudibranchs are mostly carnivorous and eat a variety of different things including sponges, barnacles, other nudibranchs, anemones, and even jellyfish. The only reason they can eat toxic prey is due to a special lining in their intestines. Pretty cool in itself, but not nearly as cool as the fact that some species can essentially absorb their prey’s toxins for their own defence, not unlike an underwater Kirby.

Take for instance the Blue Dragon nudibranch that eats Portuguese Man O’ War. Look, but don’t touch, because it’s using the jellyfish toxins for defence.

Beyond utilising their carnivorous traits to their advantage, there are some other nudibranchs that will digest algae from coral and then retain the chloroplasts to produce food. If you don’t remember your primary school science, this means that the nudibranchs are actually using photosynthesis (the same process plants use) in order to produce food for their use. This trait is called kleptoplasty.


Ok, yeah, that’s pretty cool. Anything else?

Well, like other creatures of its kind, the nudibranch is hermaphroditic which means it carries both sets of reproductive organs even though it cannot reproduce by itself. They find partners and reproduce with each other simultaneously. No big deal for a hermaphroditic animal yet, but the truly weird find came from a researcher from Osaka University that found that Chromodoris reticulata (and possibly other species) will detach its own penis after the reproductive act. You might think this would be a one-and-done mating scenario, but it actually has a 3cm penis and only uses 1cm of it at a time, so it can mate up to 3 times before it has to completely regenerate its own sex organ.


Neat party trick. So are these things just everywhere?

Not everywhere, but there are thousands of different species mostly spread across the warmer tropical waters on Earth and there has even been one found near the North Pole! Not only that, but they don’t have a lot of predators – only other nudibranchs, turtles, and some crabs. You would think with their beautiful colours and distinct shapes you’d have seen them in your neighbour’s aquarium by now, but they seem to even have developed a defence mechanism against humans! The micro-ecosystems they live in are so specific that they don’t do well in captivity which means they aren’t being fished out for collectors.


Hey, thanks for the overview! Where can I find out more about nudibranch

Well, there are several books on Nudibranch identification and behaviour – a quick internet search will get you to those and they tend to be broken down by region. There’s also an online catalogue of pictures and classification called that is exhaustive. Alternatively, if you just want to see some of the favourites out there, I might suggest Buzzfeed’s 33 Nudibranchs That Are More Colourful Than Nicki Minaj. Have fun!


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