What are nudibranchs? 5 fun facts

You may already have encountered some of these little colourful creatures during your snorkelling or diving trips. Also commonly called sea slugs (even though this is not technically correct), nudibranchs are present in oceans all over the planet. While they are more common in certain areas, they can be found everywhere in the ocean, deep or shallow, warm or cold.

With over 3,000 known species, there are plenty to observe during your diving or snorkelling adventures. And while they might look like little slugs, there are many fascinating things about them!

Details of rhinopores on a nudibranch

Small but plenty

There are over 3,000 known species of nudibranchs as of today, but it is fair to think that there might be a lot more that have not been discovered yet. They range from very very small (1mm or so) up to over 40cm for the Spanish Dancer, the biggest species of nudibranchs.

With their bright colours, their odd shapes and their usual slow moving pace, they make great subjects for underwater photos!

They are almost blind

They have eyes under the surface of their body that helps them distinguish light from darkness, and that is about it. However, their little “antennas” on their front, called rhinopores, do most of the job: these sensors help nudibranchs find food, mates and more.

Just like snails, nudibranchs also leave a chemical trail when they move slowly underwater. This can warn predators but also serve as a call to other nudibranchs in the area in order to help find a mate!

Chromodoris magnifica nudibranch

They are hermaphrodites

Nudibranchs have both male and female reproductive organs, but they still need a mate to reproduce! When you see two nudibranchs entangled together, it is likely that they are fertilising each other’s eggs. Later on they lay eggs on rocks or corals, and leave them to mature. You might have noticed them before, they look like colourful spirals.

This reproductive system is efficient, as nudibranchs have a short life span. Most of them only live for a few weeks or months, with some of the species making it close to a year.

They are poisonous

With no shell, a slow pace and no weapon per se, nudibranchs are easy prey for a lot of other marine animals. As a consequence, a lot of species developed some defense mechanism. Some of them produce their own poison, some others will become poisonous from ingesting toxins from the food they eat such as sponges or hydras. They store those toxins in dedicated cells in their bodies, that will in turn poison their predators.

Cyerce Kikutarobabai nudibranch, which is about 2cm long

There is more than the eye can see

While they do not look strong or powerful, nudibranchs are carnivorous and have strong jaws to chew on sponges, small invertebrates, and hydras. Some species have a favourite food and will eat only that, while others feed on multiple food sources.

More impressive even, some nudibranch species are solar powered! The nudibranch family known as Phyllodesmium feed on algae that continue to photosynthesize in their body, therefore giving the nudibranch energy.

Clear shot of a chromodoris loci nudibranch

Now that you know all these facts about nudibranchs, you can try to spot them on your next dive or snorkel trip and observe these fascinating little creatures!

They also make great photo subjects as they move very slowly! If you are thinking about starting taking photos during your snorkelling or diving trip, check out our tips to start taking photos with your Iphone.

If you want to learn more about nudibranchs, you can check out this Nudibranch behaviour book.