THE NUDIBRANCH started its evolutionary journey as a mollusc with a hard outer shell. Then it turned into a carnivore, sloughed its armour and acquired a new set of biological and chemical defences.
Today this shell-less snail is a highly sophisticated organism. There are at least 3000 known species of nudibranch worldwide, with a cluster of 900 in the South Pacific and approximately 130 in the Mediterranean and Atlantic.
Identification and classification is not easy. Nudibranchs are often mistaken for other shell-less creatures like Sacoglossa, Cephalaspidea, Notaspidea or Aplysidae, and they may even be confused with flatworms.
The word nudibranch means exposed gill - these creatures have fringe-like feathers (cerata) on the rectum that act like gills. They can also take in oxygen through the skin, which is often enlarged by protuberances and warts.
Nudibranchs are shaped like elongated ovals - flat or thick, long or thin - and are usually seen crawling very slowly along the seabed in search of a good meal. Some, like the 20cm-long giant nudibranch (Dendronotus iris) found in the Northern Pacific, are able to swim by thrashing their bodies from side to side.
Nudibranchs are virtually blind but have highly sensitive tentacles on their heads (rhinophores) that allow them to touch, smell and taste. They use a tongue, or radula, containing thousands of tiny teeth, to feed on algae, sponges and polyps. Marine biologists can identify the four nudibranch groups - the Doridacea, Dendronotacea, Aeolidacea and Arminacea - by the shape and contours of the radula. They can also be identified by external features such as the gills or the shape of the fringe-like cerata on the back.

Nudibranchs have few friends in the marine world. They secrete an evil-tasting and potent substance as part of a highly sophisticated defence mechanism, so if a fish chooses to snap at one, it will soon spew it out.
Dorids, the largest group of nudibranchs, all have this chemical potential, but some also have mechanical defences in the form of small calcareous stilettos called spicules.
Nudibranchs are carnivores and will eat anything from tunicates, sponges and anemones to hydroids and small crustaceans. Some sponges play an important role in helping nudibranchs defend themselves. For example, Cadlina luteomarginata, found in the Pacific, likes to eat Spongia celia. This sponge contains a chemical which the nudibranch digests, then secretes to drive away predators.
The suborder Dendronotacea has large tree-like cerata on its back and mucus glands for defence. The lions nudibranch (Melibe leonina) is slightly unusual in shape - it has no radula, but has a large expandable oral hood in which it traps crustaceans. Melibe can be found in kelp beds mingling in large groups, and when touched they secrete a mucus that smells like lemon.
Aeolidacea are often brightly coloured, and can be identified by characteristic cerata on their backs which they use for protection. These nudibranchs eat stinging lifeforms, such as hydrozoans and anemones, and they pass the stinging cells out of their digestive tract into pockets at the tips of the cerata. When touched, the sting is released.
The fourth and smallest suborder of nudibranchs is the Arminacea. They can be recognized by their long stripes but can sometimes be mistaken for Aeolids. Tropical varieties feed on corals and other coelenterates but others like the Dirona albolineata in the North Pacific - better known as the Fallen Archangel - prefer small snails and mussels.
Marine nudibranchs sometimes have elaborate and outlandishly patterned body contours - more than once these exquisite animals have been described as marine butterflies in literature. In a densely populated world, gaudy colour compositions serve as an optical warning or as a way of confusing the enemy.

It is sometimes easy to overlook a nudibranch in its natural habitat. They get their tissue colour from their food, which allows them to change colour and camouflage themselves when necessary, and some barely grow to the length of a fingernail. Growth up to 25cm is unusual, specific to particular species like the orange peel nudibranch, although some Pacific varieties can weigh as much as 1.5kg.
Exotic nudibranchs are the most highly developed. They are elegant and beautiful, with only a small number of predators. However, their lifespan can be as short as a month. Ironically, the more highly developed the nudibranch, the shorter the lifespan tends to be, as their food supply tends to be specialised, and limited to only a few weeks at a time. Spawning tends to take place only when food is plentiful. Nudibranchs are hermaphrodites - with both male and female sexual equipment - and they meet in groups, attracted by pheromones. Two nudibranchs meet, exchange sperm and fertilise one another simultaneously. This can take hours and is sometimes accompanied by gentle caresses, or occasionally by dart wounds and other aggressive behaviour.
Up to a million eggs are produced, contained in long, rather pretty strings of jelly. These develop into big-eyed wandering larvae with small shells which are abandoned as soon as the creature locates a feeding ground. Then, and only then, can it be called a baby nudibranch.