What are Nudibranchs?

Trapania rudmani

Trapania rudmani Photo: Steve Journee. Spotted recently under Wellington wharves

Nudibranchs belong to the very large invertebrate Phylum Mollusca, which comprises an enormous range of terrestrial and marine animals, which vary widely in shape and size. Structure varies so much that it is hard to define molluscs in a way that provides some single or few characteristics which are common to all. Most people will be familiar with sea shells, derived from limpets, paua, sea snails and whelks as the most common characteristic, but molluscs also occur without shells,

Molluscs also include the cephalopods, such as squid and octopus, but nudibranchs are found amongst the Gastropods, where there is also wide variance. Gastropods include sea snails, where the shell is coiled on itself, but also limpets which have a coiled shell only in larval stages, while paua, for example, are gastropods with an uncoiled shell. Sea slugs are Gastropods which have a vestigial shell or the shell is shed at an early stage in growth.

Working on through the structure of this large phylum we come to Nudibranchia, or nudibranchs, which are sea slugs with a shell lost in the larval stages, but mostly known for their exotic shapes and often brilliant colours. There are other sea slugs in different groups with bright colours, which may be confused with nudibranchs. There are some 3000 species of nudibranchs distributed across marine habitats from Antarctica to the tropics and virtually all depths of salt water. Their greatest numbers and sizes are seen in warm, shallow waters up to and including the intertidal zone.

The word nudibranch comes from the Latin word for naked and the Greek word for gills, referring to the feathery growths near the rear end of the animal, which function as breathing apparatus. They are mostly ‘benthic’ animals, which means they are usually found crawling on the seabed substrate, though there are exceptions which float upside down just under the sea’s surface, or pelagic, meaning swimming in the water column. All known nudibranchs are apparently carnivorous, feeding on sponges, hydroids, bryozoans, other sea slugs, including other nudibranchs or even their own species. Barnacles and anemones are also known prey.

Tambja verconis Ian Anderson

Tambja verconis Photo: Ian A Anderson

More than 80 species are known from New Zealand seas. Some are brilliant, such as the spectacular blue and brown gem nudibranch (Dendrodoris denisoni), and Verco’s nudibranch, the gold and blue (Tambja verconis). In contrast, New Zealand’s largest species, the Wellington nudibranch (Archidoris wellingtonensis), is an less beautiful creature. Capable of reaching 20 centimetres in length, its orange-brown body is covered in wart-like growths.

Archidoris wellingtonensis Steve

Archidoris wellingtonensis Photo: Steve Journee

Some say that nudibranchs don’t need shells because they have developed other defence mechanisms. Their colours often blend in with surrounding reef flora as camouflage. Others appear to deliberately flaunt highly different colours as a way of warning that they are poisonous, so ‘don’t bother eating me’! Jason’s nudibranch, for example, feeds on hydroid polyps, and, without being harmed themselves, incorporates the hydroid’s stinging cells into growths on their backs. A bit like using captured weaponry in your own defence!

Jason mirabilis W Farelly

Jason mirabilis Photo: W Farelly DOC

Other nudibranchs adopt poisons from sponges to defend themselves and some can generate their own toxins, e.g. to release mucus to deter a would-be predator. In 2009 dogs died after eating a rather drab-coloured nudibranch species washed up on Auckland beaches. Testing found that the nudibranchs contained the toxin tetrodotoxin, which if ingested would also be fatal in humans. Signs were put up warning people not to touch the nudibranchs and to watch their dogs and children.

(Sources for note: Wikipedia; Beu and Marshall in ‘The Taputeranga Marine Reserve’ edited by Gardener and Bell, VUW; and Maggy Wassilieff in Te Ara, Encyclopedia of New Zealand)