Seaslugs - Part III. Nudibranchs

Contributed by Ákos Lumnitzer

I wish I had a pound for every time I heard a diver say: 'Did you see the beautiful nudibranch down there?' - not knowing, that the creature was only another opistobranch and not a true nudie?

Order Nudibranchia - True Seaslugs

Some of the most imaginative colour patterns, shapes and textures await the observing diver when seeking out members of this highly diversified order. If Rembrandt or Picasso could have seen these flamboyant slugs, they surely would have drawn some inspiration from the spectacular arrangements of colours and textures.

They have been referred to as the butterflies of the underwater world. Some nudies can even swim. The Spanish dancer, Hexabranchus sanguineus, swims by undulating its mantle much in the same manner a flamenco dancer would wave her skirt.

Chemical defence systems are common amongst nudies, especially in the family Chromodorididae. Many slugs in this family extract toxin from their diets primarily based on sponges and store these toxins in their mantles, giving fish such a distasteful experience that they will not attempt to eat the slugs again. Bright, noticeable colouring in Chromodorids, called aposematic colouration, is also an obvious sign for potential predators to back off. In fact there are many examples within the animal kingdom to show that vivid colour displays can be a form of defence, showing predators that the quarry is not edible.

Sometimes groups of sea slugs from certain geographical areas have evolved similar colour patterns to warn off their predators. One example of this mimicry is in south-eastern Australia, where a group of about ten species developed red spots thus making identification of species at times difficult, especially for fishes, which would rather leave these alone due to the slugs' similarities.

A very unique form of defence had evolved in the sub-order Aeolidacea. These slugs eat cnidarians, such as hydroids and anemones. The nematocysts from the cnidarians are then stored in the cerata on the dorsum of the aeolids. These appendages, when attacked by a fish, release the stinging nematocysts so the feisty little fishy gets a mouthful of stinging cells instead of an easy meal. Aeolids may also retain zooxanthellae, single celled plants, from their cnidarian prey, thus they have the ability to photosynthesize. Solar-powered nudibranchs? Yes, there is such a thing right down there under the sea. Just when we thought that it was already too unbelievable.

Recommended dive sites for Sydney's nudibranchs: anywhere there is saltwater! All you need is your camera and the ability to shoot from about 1:5 down to 1:1 magnification and you will be very satisfied indeed!

Stay safe and happy diving!

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