ISSUE 19 ARCHIVE - THE SEA DOC INVESTIGATES: NUDIBRANCHS

Dr John Carlin

My wife is passionate about nudibranchs and gets excited every time she spots one when we go diving together. This passion for this weird but beautiful marine creature, and knowledge of where they might be found, was developed when we were both lucky enough to work out on a remote marine research station in Indonesia.

She fondly recalls her time on the ‘Nudibranch Research Project' so this article is written in her honour. I hope you enjoy learning 10 things about an animal that has been called the ‘marine butterfly' of the oceans or, the less poetic, ‘colourful sea snot' by some of my wreck diving friends!

1. Nudibranchs are types of mollusc: Nudibranchs belong to an animal class called Gastropoda which includes snails, slugs and limpets. Nudibranchs have a shell in their larval stage which they shed before they reach adulthood. They are known for their striking colours and forms which is possibly where the ‘marine butterfly' reference comes from. To solve a regular nudibranch argument - all nudibranchs are sea slugs but not all sea slugs are nudibranchs.

2. What's in a name? The name nudibranch (pronounced nooda-brank) is derived from the Latin word ‘nudus' meaning naked and the Greek word ‘brankhia' meaning gills, in reference to the gills or gill-like structures seen on the backs of many nudibranch species.

   Confusingly, however, not all nudibranchs have visible gills on their back. The family of nudibranchs known as the Phyllidiidae lack conspicuous gills and instead have lumpy ridges on their back.

3. There are over 3000 different types: Of nudibranchs across the world with scientists predicting more are yet to be discovered and named. They range in size from just a few millimetres up to species such as the Spanish Dancer which can grow up to 40cm+. They can be found on seafloors all over the world usually in the intertidal zone but have been spotted at depths of over 700 metres.

4. Walking or swimming: Just like their relatives, the snails, nudibranchs are slow moving and as a result with their spectacular colours and forms, they are excellent subjects for photography. Again similar to snails, they move on a flat broad muscle, known as a foot which leaves behind a slimy trail. Whilst most nudibranchs use this method to move, some species can swim short distances by flexing their muscles in an undulating motion to propel them through the water column.

5. Bad eye sight: Whilst we marvel at the colours of different nudibranch species, nudibranchs themselves only have ‘simple' eyes which mean they cannot appreciate their own colours as they can only differentiate between light and dark. Scientists believe that nudibranchs use specialised tentacles on their head called ‘rhinophores' to touch, taste and smell their immediate environment.

6. You are what you eat: Nudibranchs get their colour from the prey that they eat. They are carnivorous animals and depending on the species of nudibranch, their prey can be sponges, corals, anemones, barnacles, fish eggs and sea slugs. Nudibranchs can be fussy about their food with some species only eating one kind of prey. Some nudibranchs are cannibals and will eat other species of nudibranchs.

7. Don't mess with a nudibranch: Some species of nudibranch have developed a system of protection by feeding on anemones and swallowing the anemone's own defence system of stinging ‘nematocyst cells' whole. They store these swallowed stinging cells in sets of protruding organs on their back called ‘cerata' and use the ‘stolen stinging defence' system to ward off predators.

   Some nudibranch species use their bright colours to warn predators since they are toxic if swallowed whilst others are able to emit unpleasant chemical odours to deter potential predators. Some nudibranchs attempt to hide from their predators by using their colours as camouflage to blend in with their backgrounds. Species which employ this form of defence will often take on the colour of their preferred prey such as the brightly coloured sponges they feed on.

8. Powered by the sun: I have been thinking about how I can go green and reduce my carbon footprint. However, some species of nudibranch are several steps ahead of me and have effectively harnessed the power of the sun to help feed themselves!

   The nudibranch species Berghia major feed on an anemone to absorb algal cells called zooxanthellae into their cerata. The zooxanthellae continue to perform photosynthesis and the nudibranch benefits from the sugars produced.

9. How do you tell if a nudibranch is male or female? The simple answer is you can't. Nudibranchs are hermaphrodites which mean they possess both male and female productive organs. As all nudibranchs move relatively slowly and are solitary in nature, it is important to be able to reproduce whenever they meet another individual of the same species.

   Species attract mates by secreting pheromones into the water column and once they meet they exchange sperm and fertilize each other. In some species there is a certain amount of courting when they come together. Before the deed is done they have been seen slowly dancing together, manoeuvring around in circles to get their reproductive organs pores into the right position. Tailgating in nudibranchs has a completely different meaning to ‘tailgating on a motorway!'

   In some cases, upon meeting one nudibranch will ‘tailgate' the other by following it around touching the edge of it's tail for some time before the nudibranch in front gets the message!

   Upon a successful mating, over a million eggs can be produced and these are laid in masses of spiral shaped or coiled egg ribbons which hatch into free–swimming larvae before settling onto the ocean bottom as adults.

10. Nudibranchs only have a short life span: Just like butterflies, nudibranchs live from just a few weeks up to a year depending on the species. Researchers believe the more developed the nudibranch, the shorter the life span as their prey can be specialised and limited to only a few weeks at a time.

Nudibranchs can be found all over the world and even here in our own temperate waters. So look out for these wonderful ‘rainbows of the ocean' next time you are out diving in your local coastal waters or wherever you might be diving next.