We are talking about the little fellas of the Ocean – the nudibranch. These tiny ocean dwellers are sometimes difficult to spot because of their size. Often they are just the size of a fingernail. For many experienced divers, finding one during a dive and being able to photograph it is something special. Many divers have no idea what they are and pass them by.

These little colorful friends are creatures with truly special features. Let’s start by defining what they are.

Nudibranchs are a group of soft-bodied marine gastropod molluscs, belonging to the order of Nudibranchia. They shed their shells after their larval stage. They are famous for their extraordinary colours and striking forms. Nudibranchs have colourful nicknames to match, such as “clown”, “marigold”, “splendid”, “dancer”, “dragon”, and “sea rabbit”. There are about 3,000 species of nudibranchs.

The word nudibranch comes from the Latin nudus ‘naked’ and the Ancient Greek branchia ‘gills’. Nudibranchs are often casually called sea slugs. Many sea slugs actually belong to several taxonomic groups that are not even closely related to nudibranchs. There are more than 2,000 known species of nudibranch, and new ones are being identified almost daily. They are found throughout the world’s oceans, but are most abundant in shallow, tropical waters. Their scientific name, Nudibranchia, means naked gills, and describes the feathery gills and horns that most wear on their backs. Generally oval in shape, nudibranchs can be thick or flattened, long or short, ornately colored or drab to match their surroundings. They can grow as small as 1/2cm or as large as 30cm long.

Although they look pretty harmless most nudibranchs are carnivores feeding on anemones, corals, barnacles, sponges and even other nudibranchs. To identify prey, they have two highly sensitive tentacles, called rhinophores, located on top of their heads. Nudibranchs get their coloring from the food they eat, which helps in camouflage. Some even retain the foul-tasting poisons of their prey and secrete them as a defence against predators.

Top 10 Nudibranch Facts

FACT 1: A Nudibranch’s colour is a warning
Nudibranchs come in every colour under the sun. Like for most brightly coloured, slow-moving species, these colours are a warning to predators to stay away. They rely on toxins or unpleasant tasting chemicals for protection, which they advertise with their extremely vivid colours.

FACT 2: Nudibranchs use tentacles to smell
Nudibranchs have two rhinophores on top of their heads which they use to smell. These worm-like tentacles can be a target for peckish fish. Most nudibranchs have the ability to retract their rhinophores into a pocket in their skin to avoid getting them nipped off.

FACT 3: Nudibranchs are hermaphrodites
Slow-moving nudibranchs are called sea slugs for a reason. Being extremely slow makes finding a mate tricky. To maximise their chances for reproducing, nudibranchs have a curious adaption. Depending on if the other nudibranch is male or female, the other will change its’ sex so that they are always able to mate.

FACT 4: Some nudibranchs produce solar power!
Some nudibranch, like the sap sucking slug produce solar power by eating corals. Corals contain an algae, called zooxanthellae, which nudibranchs absorb allowing them to photosynthesize. This energy source allows nudibranchs to survive for several months without eating.

FACT 5: Nudibranchs have varied diets
You are what you eat, and for nudis the prey they consume often gives them their bright colours. There are also cannibals amongst them, while others are fussy eaters and will only eat one type of prey. Like we have already learning a true nudibranch is a carnivore and uses a powerful jaw, known as a radula. Some nudibranch feed on toxic animals and then store the toxins in specialised cells to aid in their own defence.

FACT 6: Both nudibranch parents lay eggs
As nudibranchs are hermaphrodites, after a sexual encounter both become pregnant and will therefore both give birth to hatchlings. Double the nudisbranchs for just one encounter!

FACT 7: Nudibranchs can cut off body parts to survive
One of the most unusual nudibranch facts is that they can survive getting rid of body parts. Maybe you know that a gecko can ditch its tail to escape and grow it back. Nudibranchs can get rid of their rhinophores, branchial plumage or a fragment of their mantle in order to save vital parts of their body, which it will then regenerate later.

FACT 8: Some nudibranchs do not eat
They have the ability to store photosynthetic algae inside their bodies. They only need to eat for the first few weeks of their life and once they have enough, the algae provides enough energy from photosynthesis for the both of the algae and the nudi.

FACT 9: Nudibranchs communicate with slime
Similar to their land counterparts – slugs – nudibranchs leave a trail of slime behind them. This trail is a unique marker filled with chemicals. They use it to communicate with other nudibranchs. While we don’t speak nudibranch, this might be to find a potential partner to mate with or even warn others of danger.

FACT 10: Nudibranchs live short lives
While most nudibranch species typically live up to one year, some may only live a few weeks! The oldest nudibranch recorded was four years old.