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Possible the most colourful sea creatures. Underwater photographs by David and Janet Morgan.
Nudibranchs
Nudibranchs are marine
snails (gastropod molluscs) that
have no shell at maturity.
Nudibranch means “naked gills” because their
gills are on the rear of the back (posterior
dorsal) of some species or along the side
covered by a protective flap as these species
tend to bury into sand or mud. The gills
can be used for identification. Some species “wave” their gills as they move or feed.
The largest species grow to 40cm, the
smallest, microscopic and are easily able
to move through grains of sand. Most are
smaller than 10cm.
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.
Anatomy of a Nudibranch
- Mantle - In all nudibranchs the shell is only present in the larval stage. As adults the mantle replaces the shell and the operculum. This is easily seen in the dorids. Their thick mantle extends over the foot and may be covered in small lumps (tubercles) varying in size, shape and number. Many nudibranchs have a colourful mantle making them easy to see, while others have cryptic mantles allowing them to blend in. Around the mantle margin are sometimes found small poison glands, used as a deterrent to inquisitive fish and other predators. The arrangement of these glands can assist in identification.
- Rhinophores - Chemical sensors on their head are used to“smell/taste” chemicals in the water which leads them presumably to food or other members of their species. The rhinophores vary in shape. When observed closely these sensors look like antennae used to receive signals electromagnetic waves (radio, television, etc.).
- Gills - Nudibranchs use external gills or the cerata to breathe. The dorids have a branchial
plume posteriorly on the mantle. The gills are a feathery structure surrounding the anus sometimes in the form of single leaflets (pinnate), or with smaller side branches (bipinnate or tripinnate).
- Foot - All species have a foot that extends the length of the body. Strong muscular action
sends waves along the foot in the opposite direction to which they are travelling. The
foot can be used for swimming when they need to escape or find food. Again strong
muscular sideways (lateral) or up-down (dorso ventral) convulsions are employed to
help the animal “swim”.
Types of Nudibranchs
There are four main groups of Nudibranchs, each with its own families:
- Aeolidoidea
- Arminoidea
- Dendronotoidea
- Doridoidea
 Aeolidoidea - The body is long and slender, with two cephalic tentacles or feelers on their head, and rhinophores on top and at the front. These animals have finger like cerata of their digestive system extending down the back of the animal. Aeolids lack gills and it is believed they absorb oxygen through diffusion. They feed on sea anemones, corals, soft corals and gorgonians, jellyfish, and mollusc eggs.
These animals can pass the stinging cells or nematocysts through their digestive system without triggering them and use them in their own defence. Some species extract algae from their prey and “farm” them. Size ranges up to 15cm.
This group includs: Aeolidiidae, Embletoniidae, Eubranchidae, Fionidae, Flabellinidae, Glaucidae and Tergipedidae.
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Phanerobranch Dorids retract their gills into a tight group on the back. The gills do not retract into a branchial pocket. Size ranges from 10 cm down to 5 mm. This sub group includes: Corambidae, Goniodoridae, Onchidorididae, Polyceridae, Gymnodorididae, Aegiretidae and Vayssiereidae. |
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Cryptobranch Dorids constitute the most diverse group of dorid nudibranchs. They can fully retract their gills into a branchial pocket. All are sponge feeders and slow moving. |
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Gnathodoridoidea is the most basal group of Dorid nudibranchs. Only a few species, belonging to this group, are known, all of them live in the cold waters in the Arctic, Antarctica and the deep-sea. All gnathodoris are omnivorous |
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Armina sp |
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Chromodoris |
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Chromodoris |
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Chromodoris
elisabethina
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Chromodoris kuniei |
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Hypselodoris |
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Nembrotha |
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Pseudobiceros |
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hypselodoris |
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hypselodoris bullock |
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Unknown |
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Unknown |
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Arminoidea - This is the smallest and most diversified group of nudibranch. The body is flattened and elongate with a mantle which extends into an oral veil. The anus is towards their front. The rhinophores have vertical plates. The gills are often along the side under under the mantle. They feed on Coelenterates and bryozoans and their size ranges from about 1-7cm.
Dendronotoidea - These nudibranchs look similar to Aeolids. They have tubular sheaths that surround the rhinophores, a mid lateral anus on the right side, with tufts of gills along the sides of the broad mantle. The head usually has an oral veil. The mantle skirt may be absent or reduced to a small ridge with paired cerata which range from simple to elaborate, containing branches of the digestive glands and gills along each side of the body. 
