The Naked Truth about Sea Slugs - Thrills, Gills and All!

Author: Max Hand, July 2007

Slug-tastic

Glossodoris atromarginata at King Cruiser Wreck

Most of us who think of slugs would feverishly reach for the slug pellets to protect our vegetable patches and flowers from being eaten. Yet, if I was to tell you that one of the most beautiful things you can come across whilst diving in the sea is a slug, many would think me seriously deranged! The only thing these underwater beauties would have you reaching for is your camera. Please let me try and explain my geakish tendencies.

Sexier than a Manta?

Sure when you see a manta ray you never grow tired of their majestic awe. However, for those of you out there who love underwater macro, and I for one am one of those, these slugs will simply stop you in your tracks. Also known as Nudibranchs they derive their name from the Latin "Nudi (nudus)" meaning "naked", and "Branch (brankhia)" meaning "gills". Just saying "naked gills slug" immediately conjures up a whole new sexy image...or is that just me?!?

Chromodoris annulata at Koh Bon island

Nudibranchs breathe through these gills which on some are set on their backs, shaped like a forest of feathery fingers. All nudibranchs also have two tentacles on their heads known as rhinopores which are sensitive to smell, taste and touch.

There are some 3000 plus different varieties of these little critters to choose from to get those hairs on the back of your neck excitedly standing up. They range in size from 4 mm to 1000 mm (that's 1/8th of an inch to 4 inches in old money). So as you can imagine they are not so easy to see. Being slow-moving and small you would think they would be an easy meal for so many predators. Not so.

Naturally Obnoxious!

Unknown nudibranch

Some have evolved to be expertly camouflaged to match their surroundings. They normally acquire their invisible defense from what they eat, such as the red nudibranch which happily munches away on the red sponge. Others are decorated with an explosion of bright colours, safe in the knowledge that they are noxious or sometimes even toxic to their would-be predators. They again extract their toxins from what they eat, and coat themselves with a defensive noxious/toxic mucous. Mmmm, lovely. Apart from tasting awful who would really want to eat them? Come on !

These nomadic naked soft-bodied creatures gracefully crawl along feeding and giving us a chance to see them in their splendor. All are carnivorous, feeding on sponges and bryozoans (tiny animals that build up their community on corals, also known as moss animals). Some are also known to indulge in cannibalism, ferociously devouring other sea slugs or even their own kind !

Let's Talk About Sex!

Nudibranch egg spiral

All nudibranches are hermaphrodites with a set of both male & female organs positioned close together on the right behind the head. These organs are wonderfully known as the "gonad". Despite the close proximity of the gonad they cannot self-fertilize. If you see two nudibranchs head to toe on the right side of each other then there's a pretty good chance that they're having some fun.

As to who plays what role, who knows? Maybe they play a game of "paper, scissors, stones" - best of three! Once the deed is done they can store fertilized eggs until the environmental conditions are good. The eggs are laid in a gelatinous spiral covered in a healthy mucous from their proud parents to be.

Varicose wart slug

Thanks, Mum!

Parents can be cruel when naming their children. Please spare a thought for some of the nudibranch names like the Varicose Wart Slug (latin Phyllidia Varicosa) which probably explains why these self aware creatures are so shy!

We Want Nudis!

Okay, by now you should be totally hooked and asking "But Max, where can we find these magical Naked Gilled Sea Slugs?". Well funny you should ask, but I'm glad you did, so let me tell you. The answer is "anywhere". They are found in salt and fresh waters. The most spectacular are in our oceans. Even once you get past the profusion of colour adorning the reefs, they can be so very hard to find. So you have to remember to switch hunting mode and set your eyes to macro vision.

They can be found on specific corals, rocks, reefs, wrecks, walls, caves, sand, even free swimming (normally the backstroke!). I have seen them between depths of two metres and forty metres so they are out there. When you are out diving next keep your eyes peeled at all times to catch a glimpse of their splendour, and please watch your buoyancy and leave those slug pellets behind!

About The Author

Max Hand - loves all things small!

Max hand works as a divemaster and instructor for Sharkey Scuba in Phuket. As you can probably tell his main passion is for these small unique creatures and Max never gets tired of looking for them. All of the photos of nudibranches in this article were taken by Max around Phuket or in the Similans islands.

Max's favourite local dive site is Koh Doc Mai, probably the best place around Phuket and Phi Phi for the "small" stuff. If you are interested in utilising Max's expertise to go in search of these critters then the good news is he is available for hire! Just send us an e-mail to request his services but book him early as he is not called "Mr Popular" for nothing!




PADI student


Whale shark

Manta ray

Equipment Rental Discount

Equipment rental discount

Multi-day rental,

get up to

20% discount