 Text and pictures by Mala & Adi Gehri of ABC DIVERS, Koh Samui Another Flabellina sp., common in the Indian Ocean side of Thailand Amazing colour, beautiful form, funny appetites, unique defenses and one that practices feminism and masculinity, the nudibranch has it all.While the sights of larger reef inhabitants amaze other divers, my hubby and I dive with magnifying lens strapped on to our BCD. Many a time, I have got amused laughs, curious stares and a flow of questions regarding this strange piece of dive equipment. Why? Playing Sherlock Holmes underwater has many rewards especially when hunting down nudibranchs.
Nudibranchs have fascinated divers and continue to hold their audience captive with dazzling colours and shapes thus earning the distinction of being trendsetters. Oddly enough, many divers have passed one by without recognition, as these trendsetters are masters of forms. Some have two horned tentacles (rhinophores) on their head, others feathery gills on their back. Many are brightly coloured though some blend into their environment. While most are small (1-8 cm) there is the Spanish dancer that can grow to 50 cm. With at least 3,000 species in the world’s oceans, there are no strict rules as to where they may be found. However, the vast majority inhabit the warm waters of the Indo-Pacific reefs.  Spanish Dancer (Hexabranchus sanguineus) is found among soft coral at Anemone Reef, off Phuket.
What Are They? Nudibranchs, more commonly known as the true sea slug, are closely related to Gastropods (shell bearing organisms). The species lacks a shell and are soft-bodied creatures that use their foot-like appendage to crawl over the surface of the reef in search of food. If you have ever seen one move, they do not seem to be in much hurry to find the next meal at all! Nudibranch literally translates as ‘naked gills’, a perfect description of its breathing apparatus. In contrast to their relatives the gastropods, who have their gills well hidden from prying eyes, most nudibranchs take great pride in displaying their gills in the form of feathery structures on their back or sides. Despite their variety, all nudibranchs can be grouped into 4 main types which is indeed a blessing with 3,000 of them on the reefs!
 Chromodoris quadricolor also known as
the ‘ pyjama ‘ nudibranch Harlequin Nudibranch (Doridaceans)
The largest group, these nudibranchs are characterized by the combination of a pair of tentacles on the top of their head and a tuft of feathery gills on the rear part of the back. But beware, just when you thought that's simple, here come the phyliids who also belong to this group but lack conspicuous gills and have lumpy ridges on theirs backs!! Did I say it was easy?  Glossodoris symmetricus seen around Koh Phi Phi is a colourful nudibranch inhabiting the intertidal rocky reefs. Tubercular Nudibranch (Aeolidaceans)
The second largest group are the easiest to recognize with their long tapering bodies covered with numerous finger-like appendages (cerata) that are non-retractable.  Members of Risbecia tyroni are generally found crawling in pairs, one attached to the back of the other.
Side-gilled Nudibranch (Dendrotaceans)
These have several feathery gills along each side of the back as well as a cup-like sheath around each rhinophore on the head that can be retracted.
 The flamboyant Nudibranch Nembrotha kurbayana is my favourite nudibranch. Veiled Nudibranch (Arminaceans)
With a fleshy extended lobe on the head, some members possess longitudinal ridges on their back while others have well-developed cerata.
Life As A Nudibranch
Nudibranchs lead a nomadic life appearing when and where they please, from 100m abyssal walls to shallow, sandy flats. It is because of this characteristic that they often elude biologists. How can you study them  Comparison of size: the magnifying glass was 75mm in diameter and the nudibranch Nembrotha kurbayana was about 2.5cm  Glossodoris atromarginata is quite common in shallow reefs around Koh Samui and on wrecks in shallow waters. When they cannot keep an appointment at a certain place and time, What do they eat? All nudibranchs have a brittle tongue called the radula to feed and use their tentacles to locate food and potential mates. Surprisingly enough, most are carnivores with a craving for hydroids (organisms with stinging cells called nematocyst), corals, sponges, fish eggs, crustaceans and tunicates (sea squirts). Others feed on algae and bryozoans (moss-like animals). But the most industrious have taken up farming by growing algae (zooxanthellae) on their cerata which produce carbohydrates for the nudibranch through photosynthesis. They carry breakfast, lunch and dinner with them as they travel. Such talent deserves applause!  Reproduction, as with all living organisms also takes place in the life of the nudibranch. The nudibranch is hermaphroditic meaning that both sexes are present in a single nudibranch but self-fertilization does not occur. Eggs are deposited on the reef as colourful ribbon strands, often on favourite food such as sponges. Once hatched, the larva is free swimming and may travel with oceanic current until a suitable surface is found. How long do they live for? Nudibranchs that feed on slow growing organisms such as sponges and sea pens can live up to a year and others that feed on transitory organisms such as bryozoans have short life spans of a few weeks.
Survival for the nudibranch is dependent on chemical warfare. Their bright colors serve as a warning to would be predators of their inedible qualities. Their flesh is frequently toxic or distasteful due to various chemical secretions. The aeolids feed on sea anemones and hydroids with an ulterior motive! They accumulate the nematocysts (stinging cells) from these animals, concentrate them in their finger-like appendages and use them as weapons. Eating for a reason comes takes a new meaning with these critters! Designers, farmers, warriors and survivors, remember to look out for these colorful reef inhabitants with their amazing abilities on your next dive.
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The Phyllidiidae family of nudibranchs are the most commonly seen nudibranch in the Gulf of Thailand. Picture shows Phyllidia varicosa |
