photo by Wakatobi guest Richard Smith

Nudibranchs: Beautiful but Dangerous Marine Creatures

Published May 8, 2013 in Invertebrates, MARINE LIFE

With the creation of Nudibranchs, Mother Nature expressed her wildest indulgence of colors and forms. This month in Wakatobi, as the water temperature drops to 27 degrees, there seems to be a simultaneous increase in the size, variety and amount of Nudibranchs. The dive team decided to focus on these amazingly beautiful “butterflies of the sea”.

Nudibranchs belong to the Phylum Molusca, along with other sea shells or molluscs. They are part of the class of the gastropods, or univalves and belong to the sub-class Opisthobranchia, along with sea hares and slugs. These colourful and bizarre creatures have astonishingly remarkable and diverse lifestyles, features and shapes.

There are more than 3000 described species of nudibranch, with new ones being identified almost everyday. Their size ranges from just a few millimeters to around 300 mm. They can be found in all the oceans of the world and within most habitats, but are more abundant in shallow, tropical waters.

The scientific name “nudibranch” comes from the Latin nudus, naked, and the Greek brankhia, gills – “naked gills”, which is quite appropriate as many nudibranchs have dorsal gills at the rear, shaped like bushy plumes, from which they breathe – ‘naked gills’.

Nudibranch -TOM REYNOLDS

photo by Wakatobi guest Tom Reynolds


Almost all of the Opisthobranchs are very interesting critters, fully functional hermaphrodites, born with male and female sex organs, thus have a set of reproductive organs for both sexes.

Most mature nudibranchs have reproductive genital openings on the right-hand sides of their upper bodies close to the “neck”. Once members of the same species have recognised each other by smelling each other and tactile body contact, there is often some courting. They manoeuvre around in opposite circles to get their reproductive pores aligned. One might follow the other around touching on the edge of its tail for some time before the one in front gets the message. The well-documented behaviour of ‘tailing’ can be a prelude to mating, as well as a lead to food behaviour.

Mating itself happens when a nudibranch pair reach a ‘head to tail’ position, with their ‘necks’ touching, lining up their genital papillae. On contact, each penis everts from the neck and seeks out the female genital duct to pump in sperm. This can last from minutes to days!

Some species of Opisthobranch may form communal mating, or spawning groups, while others just pair up after some ‘courting’, copulate, then move on to lay their respective egg ribbons. The egg ribbons can come in an amazing array of size, shape, colour and design, depending on species and are usually laid on, or near food sources.

The majority of parents then abandon the eggs and move on. Most egg masses are toxic, to deflect predators. The next stage of development is again dependent on species – direct development where the young emerge, crawling out of their egg, or planktonic larval dispersal, drift around in the currents until they settle and metamorphose into juveniles.


Opisthobranchs are as diverse in their food preferences and habits as they are in shape, pattern and colour. Some are voracious cannibals, feeding on other Opisthobranchs, while others slowly ply their way forwards, grazing on, but not limited to algae, sponges, anemones, corals, barnacles, fish eggs and crustaceans.

They have two highly sensitive tentacles, called rhinophores, located on top of their heads, which allow them detect odours in the water currents. They often use touch and tactile senses to detect the presence of prey.

Chromodoris Nudibranch

photo by Wakatobi guest Richard Smith

Colouring and Defense

Opisthobranchs include some of the most colorful creatures on earth. They use a variety of weapons to aid in protection, from chemical and biological to swimming escape responses, warning (asposomatic) colouration and camouflage.

Sea slugs, similarly to land snails, can leave a trail of slime behind, which provides a great deal of information to other nudibranchs. An attacked sea slug can release chemicals into its slime trail to warn other slugs, it can follow a slime scent to find a mate.

Some Opisthobranchs evolved with textures and colors that mimick surrounding plants to avoid predators. Others, as seen especially well on the nudibranchs chromodorids, have an intensely bright and contrasting color pattern that makes them especially conspicuous in their surroundings. This is believed to be an example of aposematic coloration; the shockingly bright coloration which warns potential predators that their prey-to-be is distasteful or poisonous.

Some even retain the foul-tasting poisons of their prey and secrete them as a defense against predators. After the first unpleasant encounter with a noxious nudibranch, the predator learns its lesson and subsequently avoid that badly tasting animal.

Nudibranchs that feed on hydroids can actually store the hydroids’ stinging nematocyst cells in their body without harming themselves. The Flabelinas digest the hydroids and can then concentrate the specialized projectile-emitting cells for future discharge at the end of their tips, a fantastic defense against potential predators.

This well known Chromodoris magnifica has a shell only during its larval life. It depends on classic aposematic coloration to warn predators that it is toxic or extremely distasteful.

The sponge-eating phyllidia varicosa is very common on Wakatobi’s reefs and is part of a group of nudibranchs with a repellent chemical defense, that fishes and crustaceans then associate with its blazing colors and learn to avoid.

One taste of the lovely Hypselodoris bullocki is usually sufficient to teach a predator that it is a bitter mouthful. Scientists believe that the natural toxins produced by reef invertebrates such as this may have future commercial or medicinal applications.

One of the highlights of the last dives was a free swimming Bedford’s flatworm. Some of our lucky guests managed to not only observe, but actually take pictures while this colorful flatworm swam along. Flatworms are similar to Nudibranchs in many ways, although unlike most Nudis Flatworms have no external gills. Some have marginal tentacles at one end which may contain simple eyes, and other species may have dorsal tentacles issuing from the back near the “head”.

On Wakatobi’s reefs, there is a very wide variety of Nudibranchs and Flatworms right now, although the majority of them are present year round. They thrive on healthy reefs and this is yet another sign of just how pristine Wakatobi’s protected reefs are.