IT'S NOT OFTEN THAT YOU FIND anyone raving about slugs and snails, other than with reference to the damage they do to their gardens. Yet divers you might otherwise consider quite sensible get excited by the sort of slugs that live in the sea. Why?
One reason is their sheer variety. There are reckoned to be around 6000 identified species of opisthobranch or sea slug, of which almost half are nudibranchs, the shell-free group or "clade" we see most often.
Another explanation is that sea slugs can look extremely pretty, like little gems. It's not just the bright colours and extravagant patterns but the textures. Examine these surrealistic doodles closely and you'll see every finish from gunmetal to sock.
Nudibranchs are the saving grace of more than a few otherwise dull dive sites, splashes of colour among drab seascapes. This is especially so in Europe, though Continental sea slugs tend to be less flamboyant than their tropical counterparts.
But it's the phenomenal growth of digital underwater photography that provides the key to the current popularity of nudibranchs.
These small creatures can be hard to spot on a crowded reef wall (though easier in a muck field), but once located, they're easy meat for the photographer.
They have the self-confidence to sit out in the open. They'll never cause motion blur. They stay put as you manoeuvre round them to get the best angle, and you don't feel you're upsetting them as you gather enough shots to guarantee a result.
They're the perfect amateur photographer's model, and the alien life-form you later see blown up in Photoshop can be quite a surprise.
On dive trips, especially in the Indian Ocean and North Pacific, I started to accumulate nudibranch images. I wanted to identify these critters, not just in a train-spotting
way but because magazines require accurate captions.
So I sought help at www.nudipixel.net which, with 24,000-plus contributed photos of some 1800 species at the last count, is a haven for international nudibranoraks.
"I fell in love with nudibranchs even before I had a camera," Bali-based co-founder Erwin Kodiat told me.
"I agree that there is increasing interest in nudibranchs - one of the indicators is that lots of new nudibranch books have been published recently, and it could be related to the growth in digital underwater photography."
Nudi Pixel is "user-friendly, as it is designed with non-scientist users in mind". You can study multiple images of the same suspect, classified by species, genus, geographic location and so on.
From one species you can link through to similar varieties, too, rather like Amazon's "If you liked this DVD, you might like..."
"Lots of nudibranch specimens have not been identified, and sometimes there are differences of opinion among experts worldwide," says Erwin.
So definitive ID isn't always an easy process, with apparently similar slugs turning up in different families, considerable overlap, and some turning out not to be the nudibranchs you thought they were, but polyclad flatworms (that old rookie error - they're not even molluscs!), velvet snails, polychaete worms, even sea cucumbers and other slow-moving sea creatures.
Sometimes these are non-toxic species mimicking the toxic for protection.
Nudi Pixel also carries many pages of unidentified sea slugs, an open challenge to the expertise of its board of volunteer verifiers. These submissions are either awaiting attention, difficult to identify because of the picture quality or angle, or simply previously unknown.
IT SEEMS I'M NOT THE ONLY non-marine biologist to be daunted by ID'ing sea slugs. "Taxonomy is a pitfall at the best of times, especially now, with the digital age and no bodies to examine!" confirms Dr Nathalie Yonow, the only UK-based expert on Nudi Pixel's panel.
Nathalie is always waiting for the next coffee-break and a chance to tackle the email attachments that keep pouring in.
"Everyone expects us to be able to ID photos. If it were so easy, we'd all be out of a job!" she says, pointing out that any one nudibranch species could be found from the Indian Ocean to the Pacific, but might display colour and pattern variations along the way. "Others are found only in one place - and if divers mix up their localities..."
The lack of user-friendly names doesn't help the newbie. "Some species, like the Spanish dancer or pyjama nudibranch, are familiar to most sluggers, but the Latin name belongs to only one species, whatever the location and whatever its language," points out Nathalie reasonably.
Nathalie started snorkelling as a youngster in Saudi Arabia, and diving while an undergraduate at Swansea University: "Not that I ever saw any sea slugs when diving in the UK, because the visibility was too poor. I'd find them by turning over rocks on the beach.
"I couldn't identify fish or crabs to save my life, but sea slugs for some reason I could. Girls seem to be the best at actually spotting these small things - guys tend to be too busy looking out for sharks." Yes, that was me at one time - my feminine side must be kicking in.
Nathalie teaches biology at Swansea, and specialises in the study of tropical marine opisthobranchs. Her book Sea Slugs of the Red Sea came out in 2008 - "I reckon I can identify 99% of Red Sea species" - and she has authored or assisted in naming 20+ species. Another, Nembrotha yonowae, bears her name.
"There are loads of websites, mostly specialising in particular parts of the world, but I got involved with Nudi Pixel because it's the best, and probably the most up-to-date taxonomy-wise," she says. "It seems to be taking over the world - it certainly takes up a lot of time, but it is addictive.
"Some of us work together and we're constantly cross-checking. With some species I can say I'm 100% sure, but others are hit and miss from the photos provided."
How did sea slugs develop in so many extraordinary ways? "All gastropods started out with shells, like snails, so they effectively had their bums twisted behind their heads," says Nathalie.
"Over millions of years, some managed to get untwisted, and gradually lost their shells, evolving so that what remained was like a piece of elastic, giving them great flexibility." Nudibranch is of course Latin for "naked gills".
"Without houses on their backs, they could go anywhere, in between rocks or into the sand, and exploit every possible habitat, from seagrass beds to rock and coral reefs. But without shells they also needed to find new methods of defence.
"These turned out to be chemical or mechanical, from manufactured toxins to sponge spicules lined up in their backs to form a tough surface. Some also developed projections that fish can bite without doing any real harm to the necessary organs, such as gills and rhinophores [horns at the front end].
"Their rhinophores help them to find food, find each other or sense predators, and their bright colours and patterns are generally a warning sign to predators."
Nudibranchs' diets can vary, with sponges, hydroids, gorgonians and anemones included on their menu, and they often match their colouring to their preferred food source.
They move like any gastropod, on the sole of their single foot, though some Red Sea species have learnt to swim; one, Notarchus indicus, has even developed a system of jet propulsion. And all opisthobranchs are hermaphrodite, with both male and female sexual organs.
The greatest diversity of nudibranchs (and fishes) can be found in the Coral Triangle in south-east Asia, bordered by the Philippines, Indonesia and PNG.
Based solely on Nudi Pixel contributions (and bear in mind that the website is based there), the motherlode is in Indonesia, with 508 species, Bali being the hotspot with 318, followed by Lembeh and Manado. Helmut Debelius in Nudibranchs and Sea Snails calls Bali the capital of sea snails.
Then comes Australia, the Philippines (Anilao stands out), Malaysia and Kenya (Malindi seems to be the western Indian Ocean's nudi stronghold).
The most globe-trotting nudis (by number of countries) are Pteraeolidia ianthina, Phyllidia varicosa, Phyllidiella pustulosa and Phyllidia ocellata. Nudi Pixel's sea-slug photographs were taken at 435 dive sites in 85 countries.
HOW BEST TO FIND OUT exactly which specimens you clocked on your last trip? "Use a good picture book, so that you can at least identify the genus - say, Chromodoris," says Nathalie. "Look at the colour and pattern, gills and rhinophores. Then go to the Chromodoris section on Nudi Pixel. If there is still a problem, try a specialist regional site, or use Sea Slug Forum in Australia."
What I found most intriguing in talking to Nathalie Yonow was her assertion that if you or I spot and photograph any 50 different species of opisthobranch, there's every chance that more than one of these will turn out to be a slug that has yet to be classified.
"In the Indian Ocean this is 20-30% likely, while in the Coral Triangle as many as 50% could be new!" she says.
And there was you thinking you'd never come across anything new under water. Don't give up just yet!
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