The Secret World of Sea Slugs
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Nudibranch: It Must Be Slug Love

The Ultimate Nudibranch Guide

What are Nudibranch ?

We have to start our Nudibranch guide  with  “It's just a slug!”  Well, soft-bodied sea slugs actually and they are members of a class called Gastropoda in the phylum Mollusca. Now, complicated names out of the way, most of the colorful ones belong to the suborder Nudibranchia aka Nudibranchs! They can be found just about anywhere in the worlds oceans but are most diverse in tropical waters. Nudibranch have very strange shapes and styles, they can be thick or flat, bumpy or smooth and long or short. Some are so tiny they measure less than a fingernail about 0.6 centimeters to much larger ones. The average lifespan of a nudibranch can vary anywhere from weeks to one year, based on the food available to them. Nudibranch are carnivores and use their radula, a band of curved teeth, to scrape or tear food particles. They feed on species such as coral sponges, eggs, hydroids and sea anemones.

Check Out this Great Video Introduction to Nudibranchs!

What Does Nudibranch Mean?

At some point or another whilst talking to underwater photographers, Nudibranch  come up in conversation. When spoken it is pronounced “ noodi-brank “ its actual Latin word nudus meaning naked and Greek brankhia meaning gills.  This refers to its gills or gill-like appendages which you will find on the backs of Nudibranch. They are very popular with underwater photographers for three reasons. 1) Most are very colourful 2) They are slow moving so easy to click and 3) there are so many types its fun to seek them out and find new ones all the time!

Different types of Nudibranchs

To date, more than 3,000 species of nudibranchs have being discovered and more are getting identified everyday. Of four group types, two main types are used, one is called ‘dorid’ nudibranchs  and the other are called ‘eolid’ nudibranchs.

Dorid nudibranchs breathe through gills that are on their backs. Eolid nudibranchs have cerata - finger-like appendages that cover their back. The cerata can be a variety of shapes and have multiple functions, including breathing, digestion and defense.

When you want to identify Nudibranchs you can start with a Nudibranch Guide or book, but remember to note the size, colour, mantel details where you found them and if a food source was present, which was it? You can use Apps to help identify them like Nudibranch ID, developed by Gary Cobb for the iPhone or website like and

What do Nudibranchs eat?

Nudibranchs spend their time eating sponges, coral, anemones, hydroids, barnacles, fish eggs and other sea slugs and other nudibranchs! ( check out the video Above of Nudi's going cannibal!) Nudibranchs are picky eaters though, individual species or families of nudibranchs may eat only one kind of prey. Nudibranchs get their brilliant colours from the food they eat. This is used for camouflage or to warn predators of the poison that lies within. This Flabellina  nudibranch shown here feeds on a species of hydroid called Eudendrium ramosum, which possesses a pigment called astaxanthin that gives the nudibranch its brilliant purple coloration.

Do Nudibranch have eyes?

They sure do, all nudibranch have small eyes but they have terrible eyesight and cannot see their own brilliant coloration. They can see light and dark with limited vision and their entire sense of the world around them is obtained through their rhinophores and oral tentacles

Rhinophores on a nudibranch look like two hornlike tentacles, feathers, or filaments on the head of a nudibranch. The Nudibranch uses them to help them smell, taste and get around. Since a nudibranchs eyesight is so bad these receptors help to smell  food sources and/or other nudibranchs. Rhinophores often are nibbled on by hungry fish though which is why most nudibranchs have the ability to withdraw the rhinophores and hide them in under their mantle.


What is a Nudibranch Mantle?

The “body’ of a Nudibranch as you look at them is often called the mantle, Usually the top part of the body which is brilliantly covered, is often referred to as the mantle. In some species its thick and extends over the slug, alternatively the mantle may bear tubercles which vary in size, shape and number and are often a character used to identify nudibranchs. In many dorids, acid glands and/or spicules are found in the mantle tissue, these are mainly for defense. In some species, tentacles around the mantle also contain defensive glands that have been shown to produce chemicals distasteful to fish

The Sex life of a Nudibranch

Nudibranchs are hermaphrodites - they have both types of reproductive organs Naturally being slugs, they cant exactly move too fast or far and are solitary in nature, so reproducing, if the situation presents itself is pretty important.
Having both sexual organs means that they can mate with any adult that happens to pass by them, in some situations after ‘sex’ Nudibranch discard their penises and can regrow them inside of 24hrs. Leaving their ‘nudi’ inside another nudibranch allows for better reproduction chances and it is known that a nudibranch can impregnate and be impregnated at the same time!

Nudibranch Guide To Eggs

Nudibranchs lay their eggs in flat ribbons attached to rocks or other objects (dorids) or in tangled masses attached to the sea bottom and other objects (aeolids} They lay masses of spiral-shaped or coiled eggs. The eggs hatch into free-swimming larvae which eventually settle onto the ocean bottom as adults.

It's not just a Slug! Nudibranchs are important to science.

Scientists study the relatively simple nervous system of nudibranchs to learn more about the processes of learning. Nudibranchs may also be the key to developing medicines to help humans in a variety of ways (learn about one nudibranch researcher here, and 'bio-prospecting' here.)

Nudibranch photography Tips

  • Get low, get the rhinophores in focus. If the gills have retracted, be patient and wait until the gills come out.
  • Get close and try to fill the frame.
  • Understand how your aperture controls your depth of field.
  • Think about the kind of background you want - black background, background sharply in focus, or a background nicely blurred. All choices can make great underwater photographs.
  • Learn how to position your strobes for front-lighting and side-lighting. Different sea slugs look better in different kinds of light.
  • To identify a nudibranch, try to get the gills, rhinophores, oral tentacles, etc. in sharp detail
  • If your camera allows you to move your focus points, choose spot-focus, compose your photo, and move the focus point until it lies over the rhinophores of the nudibranch.
  • Read about and use supermacro techniques and modes if  you want to photograph very small nudibranchs.

Please don't harm a sea slug just so you can get a better photograph, or move them from their environment. Sea slugs feed on very specific food sources. 

Best dive sites for Nudibranchs "Nudibranch hot spots"

  Here are some excellent Nudibranch Guide dive locations for sea slugs and nudibranchs.
  • Anilao, Philippines - Nudibranch capital of the world
  • Puerto Galera Philippines also are good for sea slugs
  • Lembeh straits, Sulawesi - you'll find an excellent assortment of sea slugs here
  • Bali, Indonesia - especially Jepun, Biaha, Seraya
  • Loloata Island, Papau New Guinea
  • La Bufadora, Mexico
  • Byron Bay, Australia, near Julian Rocks
  • Nha Trang, Vietnam
  • Pattaya Thailand
  • Samae San Thailand

Any of the muck diving locations in Asia will be excellent for nudibranchs

We hope you enjoyed our guide into the world of Nudibranchs! Thanks for Reading!! Happy nudi hunting. Remember we absolutely adore Nudibranchs, if you have any Nudi picture you would like to share please post them to our Facebook page @ 

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Andrew Jennings
Andrew Jennings
You host and creator of underwater clicks.