Nudibranchs

Nudibranchs are marine snails that have no shell (or in which the shell is significantly reduced). The largest species of nudibranchs grow to 40 cm, the smallest are easily able to move through grains of sand. Most are smaller than 10 cm.

Nudibranch / Chromodoris magnifica / Family Chromodorididae

Nudibranch / Flabellina rubrolineata / Family Flabellinidae

'Nudibranch' means 'naked gills' because their gills are on the rear of the back of some species or along the side. The gills are covered by a protective flap, on species that tend to bury into the sand or mud.

Nudibranch physiology

In all nudibranchs the shell is only present in the larval stage. As adults, the mantle replaces the shell and the operculum. Many nudibranchs have colourful mantles making them easy to see. Around the mantle margin are sometimes found small poison glands, used as a deterrent to inquisitive fish and other predators.

Some nudibranchs have spicules in the mantle also to deter predators. In others the mantle is reduced to a ridge down the side of the body with tentacles protruding.

Chemical sensors on their head are used to 'smell/taste' chemicals in the water that leads them to food or other members of their species. When observed closely, these sensors look like antennae used by us to receive electromagnetic wave signals (radio, television, etc.)

Most nudibranchs have a file-like organ (radula) which is only found in molluscs and is used to scrape off their food. This organ is covered on top with rows of teeth.

Reproduction

Each nudibranch is both male and female producing sperm and eggs. To mate, two nudibranchs come together side by side and pass sperm sacs through a tube in their 'neck' to each other. Both then go their own way and lay egg masses that may contain millions of eggs.

Feeding

All nudibranchs are flesh eaters (carnivores). They feed on other invertebrates such as sponges, soft corals, anemones, sea pens, Portuguese man-of-wars and hydroids. Certain species are specific about their diet and feed only on a single or small selection of hosts, others are generalised browsers.

Certain species eat the eggs of other nudibranchs. These tend to be pale in colour so that they can go unnoticed on the egg masses.

Defence

Nudibranchs have an amazing array of defences. Colour is believed to be an important defence in many species. Others ingest and utilise the stinging cells from soft corals.

Colour can come from external sources (zooxanthellae or pigments from consumed sponges) or can be produced by the nudibranchs themselves.

Some species use toxins from sponges to produce foul-tasting acid secretions, which are usually stored in glands in the mantle of the animal.

Particular species bury themselves in the sand and others hide during the day coming out at night.