They may be small, but behind that colourful, vivid exterior is a mini army load of defensive tricks and schemes.
Nudibranchs are exceptional little Gastropods, have an enormously wide distribution covering all seas and oceans including Polar Regions and hydrothermal vents 
With over 3000 species all with their own distinguishing features, distribution and behaviours, it is not plausible to detail all the varieties, colours, and behaviours in a short article, indeed scientists have set up entire web sites dedicated to the identification of these species and openly ask for diver contributions and assistance in logging them.[1,2,3]
The taxonomy is still a burning scientific issue, it is now generally agreed that they fall into the subclass opisthobranch and from there divide into two groups the Dorids, and the Eolids .studies have used DNA sequencing which has so far leads researchers to accept this . This area is still under a great deal of investigation, so the full story of nudibranch classification is by no means a closed subject.
The vast majority of Nudibranchs are part of benthic communities, and in most cases feed on sessile animals like bryozoans, sponges and sea anemones. It is for this reason they can often be found grazing in among coral reefs and wreck communities, where there are plenty of opportunities for their slow predation. These mini carnivorous predators eat their pray using their radula to rasp off food as they manoeuvre around the prey.
Some members of the Eolid grouping are quite notorious for feeding on Cnidarians, eating the nematocysts (stinging cells) these animals produce, store them and then redistribute them to be used in their own appendages known as cerata.
Whilst some members of the mollusc phylum are famous for how sophisticated there eyesight is. The nudibranchs are far less endowed. They do have eyes, but have very poor eyesight, only good enough really to detect between light and darkness. This hindrance is made up for with the use of sensory organs around the mouth area 
There soft shelless bodies, poor eyesight and small size should leave them extremely vulnerable to a vast array of possible predators. Feeding habits would also leave them as easy targets yet there remarkable adaptations and behaviours allow them to remain a very successful group of animals.
The colour variations and patterns whist beautiful to look at are designed to outwit and confuse would be attackers, allowing the animal to disappear among the rocks or reef. The variety of methods used by nudibranchs is long and exhaustive, many use Aposematic colouration which is where colouration is used as a warning marker, in the hope that the predator will avoid taking the chance of being injured or killed. Some species use mimicry of other harmful species again in the hope that any potential predator will not take the risk. [1, 3, 5]
Some species however are more proactive in their defence strategies, using acids and toxins in their skin often harvested by preying upon specific animals which produce these chemicals naturally. [1, 3, 6]
An example of this behaviour is the Nudibranch Glaucus atlanticus which preys on the Portuguese man of war. It uses air bubbles to float on the surface until it comes across its prey, it attacks and eats the trailing tendrils of nematocysts though it does not digest them, it re uses them as its own defensive stinging cells within its own skin.
So the next time you venture into the sea and your thoughts are of the vast array of animals that live there just spare a thought for the underestimated and often overlooked myriad of defensive wonder….the Nudibranchs.
 Polyphyly across Oceans: a molecular phylogeny of the chromdoridae (Mollusca, Nudibranchia). L.M Turner, N.G Wilson (2008)
 Text book: Invertebrate Zoology. Ruppert, Fox, Barnes.
 First chemical Study of Patagonian nudibranchs. A new seco-11, 12-spongiane, tyriannal, from the defensive organs of Tyrinna nobilis. A Fontana, C Muniain, G Cimino (1998)
 The Natural History Museum, www.nhm.ac.uk
All pictures sourced from Wiki Commons.