Defence:

Amongst the marine snails, bivalves and other related molluscs, the shell is the major means of defence. The animal can retract into the shell to protect it against predation. The loss of the shell by nudibranchs has exposed them to a higher risk and many different defence strategies have been developed including counter shading, mimicry, cryptic, deimatic display or aposematic colouration, autonomy of body parts or the release of stinging cells.

Colour is believed to be an important defence in many species and can come from external sources e.g.. the "farmed" (zooxanthellae) algae, the pigments from consumed sponges or be produced by the nudibranchs themselves. Among the nudibranchs are species that are masters of camouflage, beautifully blending the texture, shape and colour of their body to their host.

The Spanish Dancer, Hexabranchus sanguineus, startles predators with a deimatic display of colour when it is disturbed and displays its brilliantly coloured mantle.

Many of the chromodorids have spectacular and obviously coloured mantles and use this aposematic colouration as a warning. Many of the species have foul tasting toxins from sponges to produce foul tasting acid secretions, which are usually stored in glands in the mantle of the animal. Animals with similar colouration benefit via mimicry from their toxic cousins as fish, etc., learn not to eat them.

Glaucilla marginata and Glaucus atlanticus use counter shading as a form of protection. They are blue on the top to disguise them from birds etc., and silver underneath to resemble the water surface from below.

Others ingest and recycle the stinging cells (nematocysts) from Cnidarians or coelenterates (sea anemones, corals, hydroids, jellyfish etc.). The cnidarians are eaten and nematocysts retained for reuse in their own defence. It is believed to be possible to pass the nematocysts through the animals body and into the cerata because they are physically but not yet physiologically mature.

Nudibranchs tend to be slow moving and some species e.g.., Discodoris fragilis autotomise (cast off) body parts as a decoy. Many of the aeolids throw off cerata. The wiggling cast off distracts the prey allowing the animal time to escape.

Species bury themselves in the sand, hide during the day (nocturnal) coming out at night, hide in caves or overhangs or under rocks or coral.

Dendrodoris denisoni with its blue spots be mistaken for the venomous blue ring octopus. Both has similar blue spots.
© Wayne Ellis 1999

Some nudibranchs can swim to escape by “flapping” their mantle. Hexabranchus sanguineus has the common name Spanish Dancer due to the resemblance when “swimming”.
© Wayne Ellis 1999

A group of red spotted chromodorids on Australia's East Coast, use mimicry (see australasian nudibranch NEWS #2). Some of the species have toxic glands in their mantle and species that resemble these have a higher rate of survival, as predators have learned not to eat the toxic species.


Chromodoris splendida © Steve Grail 1999	C.splendida from the Sunshine Coast © Steve Grail 1999

The colouration in this group changes from Sth Qld and Nth NSW, to those further south. A noticeable difference occurs from Coffs Harbour, North or South. Spots range from large single to multiple small ones. The Sunshine Coast seems to have an orange variation, where specimens with a large orange spot or spots has been sighted by different observers.

Species are known to mimic the food source they live on. Several of the Phyllodesmium (Family: Glaucidae) resemble the polyps of the soft corals (octocorals) on which they feed. Only close observation, egg masses or their movement will give them away.

Mimicry may occur between flatworms (Platyhelminthes) and nudibranchs. Phyllidiella pustulosa (phyllidiid nudibranch) and Pseudoceros imitatus (Pseudoceritid flatworm).

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