Sea Slugs (Nudibranchs)
A student web page designed by Matt Kirby and Chelsea Wooff

Flamboyantly decorated in brilliant colour, sea slugs are thought by many to be one of the ocean's most beautiful animals. Sea slugs are also known as nudibranchs, a word which means 'naked gill', as their gills are exposed on the outside of their bodies. Sea slugs belong to the Order Nudibranchia, which lies in the Class Gastropoda, in the Phylum Mollusca. Unlike their snail relatives, nudibranchs do not have a hard shell to house and protect their soft bodies, so they have come up with other means for protection.

One defensive strategy that nudibranchs use is called 'cryptic colouration', otherwise known as camouflage. Many types of nudibranchs are coloured to match their substrate (which is usually also their food source) so they blend in to their surroundings and are difficult for predators to spot.

On the other hand, some nudibranchs are so brightly coloured that you can't help but see them. These more striking nudibranchs, secrete toxic chemicals (like the orange peel nudibranch below) or fire stinging cells called nematocysts (like the giant dendronotid below) to defend themselves. Nudibranchs that have chemical defences are brightly coloured on purpose to warn predators that they taste bad.

Combined with crypsis and chemical defence strategies, some nudibranchs like the Giant Dendronotid (described below) have the ability to swim for a short time to escape the immediate danger of predators.

Nudibranchs feed on a variety of organisms that are hardly ever eaten by other marine animals, including sponges, bryozoans, hydroids and even barnacles. Most nudibranchs are specialized for a very specific diet that may be limited to a single species of plant or animal. For eating, nudibranchs have a ribbon of teeth called a radula, that are adapted to match the favourite food source of the individual nudibranch. For example, nudibranchs that feed on sponge have broad radulae with numerous teeth that are good for scraping. Nudibranchs that feed on hydroids and bryozoans have narrow radulae and stronger jaws for crunching. Some nudibranchs will change colour depending on what they eat.

The Northern Pacific Ocean is home to more then 200 species of sea slugs. Two examples are shown below:

Giant Dendronotid (Dendronotis iris)

The Giant Dendronotid is one of the largest and most varied sea slugs in the Pacific Ocean. Giant Dendronotids can come in many different colours including white, grey, orange and red and can grow up to 200 mm long. When under attack, the Giant Dendronotid has the ability to swim away or let the predator taste its chemical toxins. They live in sand and mud at a depth of about 200 m between Alaska and Mexico. Their main source of food is tube dwelling anemones.

Orange Peel Nudibranch (Tochuina tetraquetra)

The Orange Peel nudibranch is deep yellow in colour, with white tipped tubercles. It is the largest nudibranch in the world with individuals reaching 300 mm in length. The Orange Peel nudibranch is typically found in the shallow subtidal from Alaska down to Los Angeles, California. The diet of this nudibranch consists of hydroids, orange sea pens and soft coral. The Orange Peel nudibranch's is the only nudibranch known to be eaten by humans; this sea slug is eaten raw or cooked in the Kuril Indians (part of the old Soviet Union) where it is known as 'Tochni'.

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