Marine Life Focus: Nudibranchs

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Nudibranchs are fascinating! If a couple of years ago anyone would have ever told me that one day I will be obsessed with slugs, I would have not believed a word. And yet, here I am. Who would have thought how much diving can change your perspective on life?

Nudibranchs are much more than “just some slugs”, and if you have not yet come to appreciate them, let’s try to change that!

The first time you ever see a nudi on a dive, you will probably be pretty unimpressed. I sure was. But once you find out how many different species there are and what kind of superpowers they possess, you will probably join the club of nudi fans! At the latest when you start taking pictures on your dives, you will quickly discover that nudibranchs make for a perfect subject to get started with underwater photography.

So let’s learn more about them:

Nudi…what? Nudibranchs.

What is the difference between a sea slug and a nudibranch?

Nudibranchs are often referred to as sea slugs. And they are sea slugs, because they are a group of marine gastropod (=slugs) molluscs. However, there are some sea slugs, that are not nudibranchs (and not even closely related to them). It can be a bit confusing, and some sea slugs are oftern confused with nudibranchs.

Their name comes from the Latin word “nudus” and the Greek word “bránkhia” meaning “gill”. They are basically sea snails that have lost their shell over time (hence “naked-gilled”). Although some have a common name, it is popular to identify them with their Latin name. (This is where 5 years of Latin in High school finally pay off ;))

But enough with the scientific talk!

There are around 3,000 different species of nudibranchs! Generally, you can distinguish between dorid and aeolid nudibranchs. 

Dorid nudibranchs have a branchial plume around their anus. Aeolid nudibranchs have cerata across their back instead.

Dorid nudibranchs (such as chromodoris, or glossodoris) have a branchial gill plume around their anus.

Aeolids (e.g. Flabellinas) have cerata spreading across their back instead of the gill plume. Those cerata prove to be really useful to the nudibranch: they use them both for respiration and for their defense! When feeding on hydroids, aeolid nudis ingest nematocyst stinging cells from them and incorporate them into their own body! The stinging cells are stored in their so-called cnidosac, located at the tip of their cerata. When they are being attacked, they defend themselves by firing those stinging cells from their cnidosac. What is a survival mechanism for them, just looks beautiful to us. 

Aeolid nudibranchs use their cerata for breathing and defending themselves against predators.

Where are nudibranchs found?

Short answer: Almost anywhere 😉 Nudibranchs can be found in seas all over the world: from the arctic to the tropics, at any depth. The most common place to see them are shallow waters in the tropics. Except for three species (the neustonic Glaucus and the pelagic Cephalopyge trematoides, and Phylliroe bucephalum), they are benthic animals. You will most likely find them crawling along a substrate like sponges, coral or the ocean floor, instead of floating in the open ocean.  

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Anatomy

Nudibranchs come in all different colors and shapes which makes up part of their fascination. However, you would be surprised to learn that they can actually not see those colors themselves! 

Although they do have eyes, which are located on top of their head, they can only detect light and shadow with them. (Isn’t it a real shame that they cannot see each other’s colors!) So instead of using their eyes for navigation, they actually use their rhinophores. Rhinophores are receptors located on top of their heads. They are very sensitive chemosensory organs and can feel the ocean current, detect scent and taste. That’s right, nudibranchs can smell!

Photo Tip: When photographing nudibranchs, focus on the rhinophores.

Some species also have an extra set of oral and/or propodial tentacles. Those tentacles are very sensitive to touch, taste and smell. 

Nudibranch behavior: mating and laying eggs 

Nudibranchs are hermaphroditic. This means they have reproductive organs for both sexes, but they cannot fertilize themselves and thus need a partner to mate. Their mating rituals vary depending on the species and have been found to last from a few seconds to several hours. Some have been observed displaying a dance-like courtship ritual!

Because their reproductive organs are always located on the right side of their bodies, nudis will align head-to-tail when mating. After mating, both individuals are fertilized and can lay eggs. Their eggs look like a colorful ribbon-like strand (if you have ever seen those strands on the reef and asked yourself what it was- well, now you know ;)).In order to deter predators, nudibranch eggs contain toxins from sea sponges. If you ever find an egg-ribbon, you should take a really close look! Sometimes you can find other nudibranch species feeding on the eggs, like the favorinus. (Some species are even cannibalistic!) 

After hatching, the juvenile nudibranchs look just like the adult ones, only smaller. Their life-span can reach between several weeks up to one year. 

Photo Tip: If you ever get to witness the mating of nudibranchs, this is a great photo opportunity!

Nudibranchs as underwater photography subjects

Ok, by now you should be convinced that nudibranchs are much more interesting than you first thought.

So what makes them so special as subjects for underwater photography?

  • They are slugs, which means they move sloooooooooowly and you can take your time composition your shots and trying different angles.
  • There are so many variations that it’s like collecting pokemons- but make sure only to “collect” them in your pictures.
  • Talking about Pokemons, there even is a “Pokemon” nudibranch: Pikachu nudibranch (Thecacera picta).
  • You can turn your dive into a real treasure-hunt trying to spot them. Try a battle with your dive guide and see who can find the most 😉
  • Most nudibranchs have really flashy colors: make them visible in your photo!
  • Get creative and try different angles, positions, and compositions
  • Even a “common” nudibranch can make a great subject! Challenge yourself and make it stand out!
  • Before approaching, always make sure it is safe to approach! Check your surroundings and the environment to make sure you won’t cause any damage to the reef.
  • If you find a nudibranch that is not in a good position to photograph, train your patience. Wait until it moves into a better position.

With all those features, nudibranchs are fascinating creatures and the perfect subject for underwater photographers who are just getting started, and professional ones alike!

Did you already encounter them on your dives? Which one is your favorite nudibranch? Let me know (and show me a picture!) in the comments!

Do you know other mind-blowing facts about nudibranchs? Share them with us in the comments! 🙂

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