Underwater Colour Extravaganza

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Extravagantly colourful delights of the underwater world await you beneath the waves of coastal northern New South Wales, Australia.

Just below the surface of our coastal seas, are found an astounding variety of enchanting marine creatures; the Flatworms and Nudibranchs. Not well appreciated until Man took to the oceans on a voyage of underwater discovery with the newly-invented aqualung, our understanding of these magnificent but often elusive creatures has grown over the last 40 years. Not fish, not plants, but distant relatives of common garden slugs and snails, they occupy their own ecological niche and have been around for eons of time.

In nature these slithery survivors, whilst potentially food for their own cannibalistic relatives, appear to have few other natural predators. Still, they have ample strategies for their own protection including venoms and toxins; all advertised by vivid colours where it certainly “pays to be seen”. Although the Sea Hare has been used in Chinese medicine, and one other type of Nudibranch is eaten by native Aleutian Islanders, these delightful creatures are fortunately not immediate candidates for mass extinction due to the culinary predilections of Mankind. And, collectors beware, a Nudibranch in your fish tank will release toxins poisonous to your prized tropical fish.

Along the north coast of New South Wales, Australia, are two fantastic diving hot spots: Julian Rocks, at Byron Bay; and, the North Solitary Islands, east of Wooli. Our coastal waters are nutrient rich and bathed in the southward flowing Eastern Australian Current (the “EAC” of Nemo fame), which brings tropical marine life. There are overlapping populations of both cold water and warm water marine life, showing considerable diversity (over 500 types of marine life at Julian Rocks alone), so tiny “Nudis” live side by side with large Grey Nurse Sharks.

Both areas are protected by marine parks;  Julian Rocks – part of the Cape Byron Marine Park:

and the North Solitary Islands – part of Solitary Islands Marine Park:

The classification of these slitherers at first seems overwhelming, but approached systematically is quite logical. There are two basic but separate groups (which are call Phylum) and in these are placed the flatworms, and the molluscs.

The marine flatworms belong to the Phylum Platyhelminthes and include Callioplanidae, Euryleptidae, and Pseudocerotidae. They are related to intestinal tapeworms, but are much more interesting and do look better in the outside world. The flatworms show less external structure development than the molluscs. Flatworms fold their mantle to form a rudimentary sensory organ at their rostral end. They present a variety of colours and patterns: for example, the camouflaged Polyclad flatworm; bright colours, Pseudoceros rubronanus; and, disruptive markings, Bedford’s Pseudobiceros also known as the Persian Carpet Flatworm.

Nudibranchs belong in the Phylum Mollusca, class Gastropoda. Of this class, the subclass Opisthobranch includes Nudibranchs and related orders and suborders: Just for a moment leave aside the Nudibranchs, and consider the following:

Order Cepalaspidea, headshield slugs.
Order Anaspidea, sea hares.
Order Sacoglossa, sap sucking slugs
Order Notaspidea, sidegill slugs

Order Cepalaspidea, bubble shells and headshield slugs. An external shell with a fine-lined pattern is observed with the translucent Wavy-lined Bubble shell Micomelus undata seen here browsing on green weed. The Psychedelic Sagaminopteron shows a rostrally placed trunk, and external gills to one side on its back.

Anaspidea, the sea hares, are rare in these waters, although you can see them if you walk in off the beach at Lord Howe Island at similar latitude.

Sacroglossans have sharp teeth for piercing algal cells to suck out the cellular contents, hence the term “sap sucking slugs”. Instead of frondy rhinophores, theirs are rolled and form part of the body shape. Examples are the Blue Spotted Elysia, and the Ornate Elysia.

The side gill slugs, Order Notaspidea, are considered an evolutionary precursor to the Nudibranchs. I have only seen one of these at the Solitary Islands; the Umbrella Shell with a rudimentary top shell and pale green-blue knobbled mantle, a hat much too small for the body below.

Now we come to everybody’s favourite, the Nudibranchs. with their own Order; Order Nudibranchia. Under this heading are four Suborders: Doridacea, Dendronotacea, Arminacea, and Aeolidacea.

Nudibranchs have a number of distinguishing features including external gills, and sensory organs called rhinopores. How do they breath underwater? It is believed that the gills act by diffusional gas exchange. Gills can take several shapes, from graceful fronds to spiky cerata. Some, like the Elegant Tritoniopsis wear them as filigree fronds; in others, the gills are hidden or placed at the side, even protected with an overarching plate as in the Purple-Edged Cerastoma to discourage predators seeking a tasty nibble.

How do they see and feel? If you look carefully at the Snakey Bornella , you will notice tiny rudimentary eyes. Most Nudibranchs actually do not have eyes; but rely on their rhinopores, specialized multipurpose sensory organs probably affording taste, rudimentary vision, and tactile sensations all in one. In some Nudibranchs, such as the Black-margined Glossodoris, rhinopores can be withdrawn completely. The components of rhinopores are usually disc-like, presumably to increase surface area, like the plates of the outer segments of the human photoreceptor. The Serpent Pteraloidia and the Red-lined Flabellina have tentacles also. Nudibranch behaviour is fascinating; they can be seen posturing like this Bennet’s Hypselodoris with its head up sensing which direction to go for the next meal (or mate).

Nudibranchs are usually brightly coloured, especially the Sweet Cerastoma, the Yellow Ridged Cerastoma, and the Collingwood’s Chromodoris, all advertising a warning to predators. Their colouration appears artistic and pleasing to our eye and makes them a favourite with underwater photographers; one of my favourites is the Red Gilled Nembrotha, photographed at the North Solitary Islands. Interestingly, the Splendid Chromodoris follows the same basic colour pattern, but no two individuals have the same shaped red splotch on their backs. Disruptive camouflage is widely practised as is seen with the Jackson’s Hypselodoris, and the purple lines of Emma’s Hypselodoris with its yellow on orange background; whereas, other are bold and go for gold, like the Fleshy Doriopsilla and its relative the Scribbled Doriopsilla. White can also be fashionable: pure white, like the Tritaniopsis, or shades, the Bicolour Flabellina, and the Hidden Chromodoris. Sometimes, the colours help them blend in with their background, and food source, like the Jackson’s Hypselodoris with its gaudy orange markings. This Imperial Risbecia has chosen to blend into a coarse sandy background and is easy to swim past.

How do they move? Flatworms move on a carpet of mucus, but can also go for a swim, especially to escape from pesky underwater photographers. Opisthobranchs move by ciliary action along the base of the foot, and sometimes the foot moves in waves. Occasionally they will be free swimming like the Spanish Dancer that undulates through the water with colourful motion reminiscent of a Spanish Flamenco dancer.

How and what do they eat? The Nudibranch’s mouth contains a serrated tongue called a radula with which it grinds off bite size portions. Nudibranchs are found munching away on algae, bryzooans, sponges, hard and soft coral, ascidians, hydroids, other Nudibranchs, and even eggs of other Nudibranchs. Yet with this veritable smorgasbord of briney delicacies, they are fairly specific as to their diet. They may show their dinner in their body colour: The Ornate Elysia ingests chloroplasts and has a giveaway green tinge which also enhances its camouflage. At Julian rocks in May, 2012, Atkinson’s Okenia ” was out in abundance feeding on the Pink Bryozoan “Pleurotoichus clathratus”. Pteraeolidia feed on zooxanthellae-rich hydroids, retain the zooxanthellae within their bodies, which then provide nutrition to the host via photosynthesis, analogous to corals.

How do they do “it”? Paired, and even multiple groupings are often seen, so they would do “it” rather creatively. No one doubts that Nudibranchs may themselves have “touchy-feely” qualities, like this pair of White’s Hypselodoris, and Tyron’s Risbecia–these also often follow each other along, a phenomenon known as “tailing”. Some have sex organs that are humongous compared to their body size, witness Bennet’s hypselodoris see here in a mating posture. The reproductive openings are generally found on the right side of their bodies, and most are hermaphrodites. Each of the pair may lay many eggs, with characteristic ribbons being found near their food source. Eggs can be released to the currents (which is known as planktotrophic larval dispersal) or baby Nudibranchs may grow from sticky egg masses. The Spanish Dancer leaves a characteristic pink spiral egg mass.

Defence is a forte of the Nudibranchs: they can be highly camouflaged and difficult to see in the first place; cover their backs with sharp spicules like portable barbed wire; or, can take flight by swimming; or, stay and fight with offensive, even toxic secretions. Pteraeolidia, the serpent, feeds on stinging hydroids and stores undischarged nematocysts in its cerrata as defence.

The presence and distribution of all Flatworms and Opisthobranchs is thought to be dependent on season, and phases of the moon, at least; but no one truly knows for sure. Most of the time they may be “hibernating” undiscovered in rock crevices, waiting to emerge at the call of Mother Nature for feeding, mating, and laying of the eggs. You may dive an area and find lots of Nudibranchs, but come back a week later and there are none to be found.

The rich marine habitats of Julian Rocks and North Solitary Islands are home to both flatworms and many Opisthobranchs. At Julian Rocks over 120 different nudibranchs have been identified, although you will never see them all at once. The North Solitary Islands are less well catalogued; my personal tally is 15. Both are served by established dive operators, Julian Rocks from nearby Byron Bay, and the North Solitary Islands from Wooli, an iconic coastal village about two and a half hours south by road from Byron. So, leave the wide angle lens at home, and come diving to enjoy seeing these fascinating and colourful creatures — eye to rhinopore! It is all here on the northern coast of New South Wales, Australia.


The author, an Eye Surgeon by profession, is a keen voyageur, and a published under water photographer and writer. He is currently working on the design of a sailing marine research vessel which will be named “Here Nui”.

Dive with Sundive, Byron Bay: (02) 6685 7755,  www.sundive.com.au,  or Wooli Dive Centre, Wooli. (02) 6649 7100  www.woolidive.com.au.

Useful references:

Coleman, Neville: “Neville Coleman’s Marine Life eGuide”.

Coleman, Neville: “Nudibranchs Encyclopedia, Catalogue of Asia/Indo-Pacific Sea Slugs”, Neville Colemans Underwater Geographic Pty Ltd ACN 002 043 076, January, 2008.

Debelius, Helmut: “Nudibranchs and Sea Snails, Indo Pacific Field Guide”, 4th edition, IKAN-Unterwasserarchiv D-65933 Frankfurt, Germany, 2004.


www.surg.org.au: website for Solitary Island Underwater Research Group (SURG).

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