There is no denying it, nudibranchs are beautiful. They come in what seems to be an endless array of shapes, sizes and not least colours. Scuba divers, especially underwater photographers, seem to go crazy for these little guys becoming more and more obsessed with finding the smallest, the most colourful and the rarest ones to photograph and show off to their dive buddies. But what exactly are these fascinating littles creatures that seem to generate such passion and obsession? We take a closer look and dive into the wonderful world of nudibranchs.

A goniobranchus hintuanensis nudibranch displaying its rhinophores — A *goniobranchus hintuanensis* nudibranch displaying its rhinophores

Nudibranchs: What are they?

Chromodoris lochi- an example of a dorid nudibranch — *Chromodoris lochi*- an example of a dorid nudibranch

Put simply, nudibranchs (pronounced new-dih-bronk) are a type of sea slug that tend to dwell on the sea floor and can be found all over the world from the warm, shallow reefs of southeast Asia to the cold waters of Antarctica. Nudibranchs are shell-less gastropod molluscs which, as mentioned before, are part of the sea slug (opisthobranchs) family. However, while all nudibranchs are sea slugs, not all sea slugs are nudibranchs. Currently there are over 3000 distinct species of nudibranchs known to science with new ones being discovered all the time! The name nudibranch is derived from latin meaning “naked gills”, nudi being the latin for naked, and branchia being latin for gills.

Flabellina exoptata - an example of a eolid nudibranch — *Flabellina exoptata* - an example of a eolid nudibranch

There are two main classifications of nudibranchs, dorid nudibranchs which have their gills on their posterior and eolid nudibranchs which have a number of finger like appendages called cerata on their back. These gills are what allow the nudibranchs to breath filtering out the oxygen from the water. At the front of the nudibranch you’ll often see two horn like tentacles sticking out which are called rhinophores - these rhinophores are what help the nudibranchs to locate their prey by smell, in fact although nudibranchs are some of natures most brightly coloured creations they have very poor vision and are only able to see in shades of light and dark. The rhinopohores can be retracted back into the main body of the nudibranch senses danger.

Although they may seem small and innocuous, nudibranchs are in fact carnivores that feed upon other animals such as barnacles, sponges, anemones, other sea slugs & eggs and from time to time have even known to perform acts of cannibalism and eat other nudibranchs. They eat using a radula, a tooth like structure used to scrape prey off of rocks. Like most of us they are quite fussy eaters, some species will only eat one type of food and it is their food that gives them their bright colours. These colours are often used to warn prey off as in the animal kingdom bright colours often mean that a subject is poisonous. Often some dorid nudibranchs can produce their own toxins or absorb them from their prey and release them at a later time to fend off predators. A number of species will create their own food by eating corals and absorbing the algae chloroplasts which then in turn provide nutrients using the sun, other will store and farm zooxanthellae, housing them in their digestive gland which gives a steady supply of nutrients.

Nudi’s are also hermaphrodites meaning that they have both male and female sexual organs which is useful for a creature that is mainly solitary as whenever they meet another member of their species they can mate. Although they are shell-less they do have shells during their larval stage which are later shedded. In terms of life span they can live from a few weeks up to a year.

Nudibranchs move on what is know as a “foot” and often leave a slimy trail which can be used to let other nudibranchs know where they are, a useful trick to attract a mate or to warn of danger in the form of other predators. In terms of size there is a large variation across all species with some being absolutely tiny at around 4mm all the way up to a whopping 60cm (600mm).

a pair of chromodoris colemani — a pair of *chromodoris colemani*

Why are divers so fascinated by them?

Scuba divers and especially underwater photographers often go crazy and become obsessed by nudibranchs, but why? Well, although they can be found the world over they can be difficult to find and there is a sense of satisfaction and pride at being able to spot them. Then there is the process of trying to identify them, often after a dive you can find groups of scuba divers crowded around a small book arguing over whether they spotted a chromodoris or a nembrotha and arguing well into the evening about it - all in good nature though! Photographers love them because although they can be small, they are slow moving, sometimes appearing to be almost stationary or motionless which makes them ideal subjects for photographs along with the bright, contrasting colours. It has to be said there is something rather satisfying about a well framed picture of a nudibranch with the rhinophores in sharp focus. Nudi’s often have great sounding nicknames such as blue-dragon, spanish dancer, shaun the sheep or pikachu which adds to their appeal and mystique as well.

So, there you have it a brief look at nudibranchs, what they are and why we love them. Here at southeast Asia diving we a huge fans of them and hope you are too. Next time you’re scuba diving take a bit of time to really look around you and see if you can spot a few of them, it takes time and practise but once you get you’re eye in you’ll be able to find them on nearly every dive, especially in and around southeast Asia.

Nudibranchs - What are they and why do we care?