Greenspeak

by Ashleigh Wyett, Environmental Educator

Nudibranchs are an extremely beautiful and diverse group of shell-less marine molluscs with over 3000 different species identified. Australia is home to over 300 of these. The word ‘nudibranch’ comes from the Latin word nudus meaning ‘naked’ and the Greek word brankhia meaning ‘gills’. This is because nudibranchs breath through external gills arranged along either the sides of their body or in a cluster on their back called a gill plume. It is this feature that makes them different from other sea slugs.

Nudibranchs live at almost any depth of salt water and range from 2 to 60cm long but reach their greatest size and variation in warm, shallow waters. So far the long longest life-span recorded was a year.

Their head offen has two pairs of tentacles. They have a mouth opening at the end of a short snout and a tentacle on each side. These tentacles help the nudibranch with their sense of touch, using them to feel it’s way around. The second pair of tentacles are called rhinophores and are regarded as a sort of nose, with sensory receptors helping them find food. Rhinophores are in constant danger of being nibbled by predatory fish! To protect them, most nudibranchs can withdraw their tentacles into a special pocket in their body. Often there is a raised flap around each pocket which can be clamped shut when it is disturbed for even greater protection.

The eyes are very small and usually invisble, but can sometimes be seen as little black dots hidden deep beneath the skin behind the rhinophores. Nudibranchs can’t see clear images like humans do, but are able to distinguish between light and dark. They can therefore detect shadows, and can tell the difference between day and night.

Nudibranchs are hermaphroditic, which means they have both a female and male set of reproductive organs, however they can rarely fertilise themselves. Mating pairs inject sperm into each other by a penis and mare ost nudibranchs then deposit their eggs in a jelly-like spiral. Most juveniles are free-swimming and have a tiny shell, which they lose as an adult. They swim around between shallow and deeper water until they are ready to settle on the bottom.

Because adult nudibranchs have bad eyesight and no shell, they need other ways to protect themselves. Bright and complex colour patterns act as a warning sign that they are toxic and distasteful to predators such as fish, which have well-developed vision. Another reason for their colour is camouflage. This means they can hide by matching the colour of the food they eat. A few nudibranchs appear plain but actually have bright colour patterns which they can display quickly to startle and frighten off a predator.

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