A delight for shore explorers, these naked slugs or nudibranchs may be encountered on many of our shores. On coral rubble and rocky shores with sponges and other encrusting animals, as well reefs and even seagrasses.What is so fascinating about these colourful slugs?
What are nudibranchs?
Nudibranchs are relatives of snails and clams but lack shells as adults. Indeed, 'nudibranch' (pronounced 'noo-dee-brank' to rhyme with 'bank') means 'naked gills'.
Some nudibranchs breathe with a flower-like feathery external gill on their backs. These elegant black-edged nudibranchs (Glossodoris atromarginata) have black-edged feathery gills which are constantly rotating in the water.
This is behaviour is believed to help improve respiration.Other nudibranchs lack this feathery gill and their gills are hidden between the body mantle and the foot. The gills of this burrowing nudibranch (Armina semperi) is hidden in the folds on the body sides.
This arrangement probably better suits such a burrowing nudibranch, than a feathery gill on the back.If they are naked, how do they protect themselves?
To protect themselves, some nudibranchs produce distasteful substances, toxins and even acids. They advertise this with bright warning colours.
These tiny nudibranchs with bright colours are probably not very tasty for predators to eat.
See the colourful feathery gills on their backs!When the Black phyllid nudibranch (Phyllidiella nigra) feels threatened, it secretes a milky substance (see closeup in photo at right).
The Pustolose phyllid nudibranch (Phyllidiella pustolosa) may have blue or pink nodules.
They too may release a milky substance when they feel threatened.Other phyllid nudibranchs are sometimes seen too. Like the gaudy Varicose phyllid nudibranch (Phyllidiella varicosa) at bottom left.
And the amazing Eyed phyllid nudibranch (Phyllidia ocellata) on the right.The Ceratosoma nudibranch (Ceratosoma sp.) has lobes on its hard body that are armed with glands secreting distasteful substances to discourage predators.
These nudibranchs absorb the toxic chemicals in their sponge food and incorporate these chemicals into their mantle glands.Hidden in plain sight
Other nudibranchs are camouflaged to match their surroundings.
This large mottled brown nudibranch (Discodoris boholensis) is actually quite commonly encountered, but requires an experienced eye to spot as they blend well among the coral rubble.
This very large, rather fugly nudibranch (Ategema spongiosa) is easily overlooked as just another lump of coral rubble or scummy bit of rock. 
Those that eat colourful creatures such as sponges or corals, may themselves be colourful to match their prey.
This bright red or rose nudibranch (Dendrodoris fumata) is actually quite hard to spot among the blobs of colourful ascidians and sponges growing on a rock.
Many are quite small and flat, so they can also easily hide in narrow places, like these really tiny nudibranchs.
Stored stolen stingers: Aeolid nudibranchs have long, slender bodies with clusters or rows of elongated finger-like portions called cerata. 
Some nudibranchs, like Cerberilla nudibranch (Cerberilla sp.) protect themselves with the stingers of the sea anemones or corals that they eat.
These stingers are passed, undischarged, to the cerata. The cerata of these nudibranchs have special sacs at their tips that contain the stinging cells of their prey. Here, the stingers remain 'live', ready to fire off and protect the nudibranch.What do they eat?
Most nudibranchs are carnivores, each species usually specialises in a particular victim. Being small and slow, they feed on immobile creatures like barnacles, sponges, ascidians, hard corals, soft corals, sea anemones, zoanthids, peacock anemones, sea pens and eggs of other creatures including other nudibranch eggs.
The commonly seen Jorunna nudibranch (Jorunna funebris) is believed to eat blue sponges.
Indeed, these nudibranchs have been seen with their egg masses next to chewed up blue sponges.
This large bobbled nudibranch with bright blue spots (Dendrodoris denisoni) also eats sponges, possibly sponges that live in murky, mucky sites.
It lacks jaws and a rasping tongue so it can't chew its food sponge. Instead, it secretes digestive juices onto the sponge and then sucks up the softened sponge!These nudibranchs in pajamas (Armina sp.) are suspected to eat sea pens.
They are burrowing nudibranchs and often seen near half chewed up sea pencils (the white stick like object in the photo).The colourful Cuthona nudibranch (Cuthona sibogae) eats hydroids, a kind of immobile colonial animal that looks like an orange bush.
While these rather cartoon-like Gymnodoris nudibranchs (Gymnodoris sp.) eat OTHER nudibranchs!
There's photos and stories of a Gymnodoris eating another nudibranch on the colourful clouds blog taken during a recent dive at Pulau Hantu!One of the MOST AMAZING nudibranch predators must surely be the marvellous Melibe!
This large nudibranch has an expandable hood at the front with which it captures tiny crustaceans! WOW! More about this beast on the wildfilms blog.Making more nudibranchs
When we see many nudibranchs gathered together, this may be because they are congregating on some food,
or some hanky-panky is about to take place. This pretty nudibranch (Chromodoris lineolata) is commonly seen, but seldom in such large groups as in the photo on the right.Nudibranchs are simultaneous hermaphrodites, that is, each animal has both male and female reproductive organs at the same time. They practice internal fertilisation. So each nudibranch has a complex system of tubes to avoid self fertilisation, to introduce sperm while at the same time receiving sperm from a partner, and for laying eggs.
Nudibranchs mate in pairs, lining up side-by-side, facing opposite directions in order to exchange sperm.
Then they go their separate ways and each lays its egg mass, usually on the prey that they eat or on a hard surface nearby.The egg mass often looks like a frilly spiral of ruffles.
In most, the eggs hatch into free-swimming larvae which have shells.Often, the larvae only undergoes metamorphosis and settles down when it is near its particular prey. The juveniles lose their shells and eventually turn into adult nudibranchs.
Nudis and us: Nudibranchs don't do well in captivity due to their specialised diets and are thus not extensively collected for the aquarium trade. Moreover, some nudibranchs such as the phyllids produce toxins that may kill their tank mates. However, they are part of the attraction for divers and other visitors to natural habitats.
Status and threats: None of our nudibranchs are listed among the endangered animals of Singapore. However, like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Trampling by careless visitors and over-collection can also have an impact on local populations.
Links to more
photos of nudibranchs seen on Singapore shores on wildsingapore flickr
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