WIDE EYES STARED BACK AT ME through a mask. If his jaw had dropped any lower his regulator would surely have fallen out. In the mesmerised state he was in, I wasnt entirely sure that he would even have noticed.
     He dropped his eyes back to the reef, his mask pressed almost to the reef wall.
     I prodded him to look at my slate. Its a Nudibranch, I scribbled.
     He nodded excitedly and grabbed my pencil. Bloody Brilliant! he scrawled across the slate, with a bold exclamation mark. And to be honest, although I might have chosen more poetic words myself, he was absolutely right. Brilliant in its remarkable red and pink hues, brilliant against the pink coralline algae it was sitting on, and brilliant in its bizarre lifestyle.
     I could tell that David was quite taken with the critters, because on the rest of the dive he managed to miss two sea snakes, one devil ray and a monster barramundi cod. His eyes scouring the reef for more of the rainbow-coloured little creatures, he missed anything bigger than 7cm long and more than 5cm from his mask.
     It was one of Davids first dives on a coral reef. It was his first encounter.
     Funny, earlier that week, two photographers with us had been discussing sea slugs and David said he couldnt see what all the fuss was about.
     No, its the big stuff I want to see, he assured me. So it was with some amusement that I watched him fall hopelessly in love with the enigmatic sea slug. He was well and truly smitten.

Nudibranchs, tiny but potent. Describing them as sea slugs does them no justice. It conjures an image of a brown or grey formless lump trailing around the vibrancy of the coral, when in fact they are genuine rare jewels going about their daily fascinating lives.
     A brilliantly coloured group of organisms that enchant those who come across them, nudibranchs occur throughout the world and number around 2000 or more species, with more new ones being found every year.
     Varying in size from something the size of a fingernail to 30cm heavyweights, most nudibranchs measure in at a respectable 6cm or so.
     They display bold poster colourations in often-surreal designs. The word dull isnt in their vocabulary. Great for us to look at and photograph, but there is a serious warning in those brilliant colours. They yell right into your mask: Hey, HERE I AM, hey, DOWN HERE.
     They also yell to the more regular residents of the reef: DONT EVEN THINK ABOUT IT, PAL!
     If that particular resident has had a go at chomping the critter in the past, the certain trauma involved will come rushing back to it, in glorious Technicolor of course, and it will sensibly leave the terrifying bully of a 2cm nudibranch to its own devices.

I have a clear recollection of sympathy watching a large wrasse happily nip down to the reef in the mistaken belief that he had found a tasty morsel for lunch. The expression on his face was, I imagine, similar to mine at realising that the chopped tomatoes I had loaded onto my dinner plate were in fact the hottest chillis in existence.
     Out came the little nudibranch, unflustered, and off went the wrasse in some amount of shock and, if my chilli experience is anything to go by, a definite degree of pain.
     Nudibranchs have perfected the you are what you eat slogan. Some would say they have taken it to extremes. They are masters of defence. Many secrete an acidic substance from their skin but others have evolved a cunning use of local resources.
     They swallow intact the stinging cells from food such as jellyfish, anemones and hydroids and pass the cells, unfired, to a number of little sacs on their backs called cerata. And there they lie until some unsuspecting fish decides to have a go at turning the nudibranch into lunch.
     This is chemical warfare at its most advanced. Its hard to believe that the tiny pink Flabellina carries around such a highly sophisticated arsenal.

Not only do nudibranchs make themselves disgusting to predators, but some manage to camouflage themselves so well that you would need a hi-tech sea-slug detector to find them. Check out a yellow sponge and you will find a yellow nudibranch with an impossibly matching hue. For a predator, a blue nudibranch sat happily chomping on a matching blue sponge is invisible.
     It is odd to describe these tiny creatures in the same vein as a lion or even a shark but they too are carnivores. Divers need not worry because it is the more sedentary, smaller creatures that interest nudibranchs - sponges, sea mats, sea squirts and jellyfish.
     The sex lives of nudibranchs show up we humans for lack of imagination in the bedroom department. They have the dubious honour of being one of those, some would say lucky, creatures we call hermaphrodite - both male and female at the same time. They all carry eggs and sperm and can cross-fertilise each other depending, perhaps, on whether they are in touch with their feminine or more butch side on a particular day.
     But because all nudibranchs are both guy and girl, their genitalia is, perhaps depressingly for the nudibranch, the same tube-like structure. Nature, of course, thinks of everything and it seems that a nudibranch getting its own sperm or eggs is impossible.
     A fortunate design, as that could lead to all sorts of legal complications, but at least the kid will know who mum and dad are. But love for nudibranchs, unlike their cousins the sea hares, who are more prone to group displays of affection, occurs in romantic pairs.

Back on the reef, David pointed out a pink and yellow couple tip-to-toe in an embrace. He held his fists up to indicate that they were fighting.
     I countered with an equally graphic hand gesture to indicate that, no, in fact they were doing quite the opposite.
     Mating among nudibranchs can last anything from a few snatched moments to a marathon several hours. We left the amorous pair in peace. No one likes to be stared at in such a compromising position.
     After mating, both nudibranchs will lay eggs. It may take a few days until fertilisation is complete but when it is, you will see the beautiful, delicate ribbon-like egg sheaths that the nudibranchs will lay on their favourite foods, such as sponges or algae.
     These sheaths contain thousands, perhaps millions of eggs. Such massive spawning numbers increase the chance of as many as possible of the nudibranchs reaching adulthood.

Explaining all this to David back at the resort, the real wonder of nudibranchs came back to me. This, after all, is a critter that many miss on a dive, but one so bizarrely adapted to its environment that its hard to believe the engineering embodied in your run-of-the-mill Chromodoris magnifica sitting quietly on that piece of yellow sponge.
     I leave the last words to Dr Hans Bertsch: Nudibranchs are a happy bunch. They look like theyre all dressed up going to a party. They look like they are where they want to be. And that is the basic success of evolutionary adaptation.
     Which really, I suppose, was what David was telling me in his own Ive met my first nudibranch sort of way.
     Brilliant was about all I could get off him the entire trip back to the resort. I couldnt have put it better myself.

Some 2000 nudibranchs have already been identified around the world and most are known by their Latin names, like this Halgerda malesso laying its eggs.
The tiny but well-armed Flabellina exopata goes about its business
Chromodoris willani. The nudibranchs in these pictures were spotted around Malaysia
Glossodoris cruentus.
Love is a beautiful thing - a pair of Hypelodoris bullocki get intimate