The term ‘sea slug’ may conjure up drab, lumpish-type creatures, but thankfully the names of its 11 sub groups, such as nudibranchs, sea butterflies, sea hares and sea goddesses, more evocatively describe these strangely-shaped, flamboyantly-coloured species. Beyond their beauty, there is more than meets the eye – sea slugs have evolved over millions of years to pick up some very strange behaviours such as shucking their shell, a taste for the distasteful, and a tongue like no other. In the first of a series, we use NIWA’s marine identification e-guides to discover some of these amazing species found within the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa Moana, a biological hotspot.
In New Zealand there are approximately 130 species of sea slugs, many of which are endemic, and others which we share with eastern Australia and the south-west Pacific. The majority of these species are from the sub-group called nudibranchs (pronounced nudi-brank), which translates to “naked gills”, which we will concentrate on. In the Hauraki Gulf you can find numerous species including our endemic pink and white Jason mirabilis, to the planktonic, fish-shaped Phylliroe lichtensteinii, which floats on the ocean’s surface and luminesces brightly at night.
True to their name, sea slugs are a slippery species to classify. Until recently, these 11 sub-groups were thought to be related to each other and were all classified together as the subclass Opisthobranchia. But, in fact, sea slugs belong to the higher molluscan class Gastropoda, most of which have a thick, external shell into which the animal that made it can withdraw for protection. Over time, each of these 11 sub-groups has undergone parallel evolution to either reduce the external shell to an internal plate, or to simply shed their shell during the larval stage, such as the nudibranch.
But with this effective means of protection gone, how does a sea slug protect itself from the predators of the ocean? The answer lies in their habitat and food. Many sea slugs camouflage themselves within the bright colours of sponges, corals and bryozoans, while others yet consume them and display their pigments in their skin, or show their food through translucent skin. Others simply synthesize their own colours. Some species go one step further, eating toxic organisms (such as certain sponges), incorporating the poisons into their own tissues to ward off predators.
Which leads us to how they eat. If ever you needed inspiration for a fearsome monster’s mouth (think Dune’s sandworm or Star Wars’ Sarlacc), look no further than the sea slugs’ radula. The radula is a tongue-like ribbon on which are positioned rows of teeth. Depending on the sub-group and what they have evolved to eat, the numbers of rows of teeth, teeth per row, and tooth shape varies. Those that eat sponges for example (which are deceptively hard, despite their name), have many rows of scythe-like teeth that rasp and scrape at the sponge’s surface. The teeth of an aeolid, a type of nudibranch that eats cnidarians (such as jellyfish and sea anemones), has only a few rows of teeth with 1-3 teeth per row. They instead use a well-developed jaw to grip onto the tentacles of its prey, plucking out the flesh with its meat-hook-shaped teeth.
So, what species can you find while rockpool hopping, snorkelling or diving in the Hauraki Gulf?
To the north and south of Long Bay beach are low rocky reefs where you can explore the rock pools at low tide, or snorkel around the ledges and overhanging rocky shelves where colourful nudibranchs feed off the sponges, bryozoans, ascidians, small corals found here. While your chances of seeing a nudibranch are better underwater, species such as the beautiful gem doris (Dendrodoris krusensternii), often claimed to be New Zealand’s “loveliest nudibranch”, can be found in the rockpools. Growing as large as 60-70mm, it has a light fawn to grey mantle, ornamented with high, rounded, gelatinous pustules that feature white spots and streaks, while the smooth areas between are coloured chocolate brown with vivid peacock blue spots. Interestingly, it eats siliceous sponges by expelling saliva with digestive enzymes to liquefy the food, which it then sucks in.
Cape Rodney-Okakari Point Marine Reserve
Summer in Auckland isn’t complete without a snorkel out at Cape Rodney-Okakari Point Marine Reserve (Goat Island), near Leigh. As well as the gem doris, you can find the smaller (20-40mm) clown nudibranch (Ceratosoma amoenum), called so for its bright orange spots. It has purpleantler-like structures called rhinophores that are used to smell, and a clump of purple feathery gills near its rear. They are often found living on sponges, on which they feed and sequester toxins, concentrating the poisons into their glands to make them distasteful to predators.
For keen divers, wherever you can find the tree-like hydroid Solanderia ericopsis in the Gulf, you are likely to find the stunning endemic nudibranch that feeds on it, Jason mirabilis. Common places to see both are the tip of the Coromandel Peninsula as well as the aforementioned Cape Rodney-Okakari Point Marine Reserve. Jason mirabilis is an aeolid (type of nudibranch), distinguished by many long dreadlock-like projections found on their backs called cerata, which are blood-filled tubes that function as both a gill and a digestive tract. Jason mirabilis are one of the biggest aeolids in the world (60mm) and one of the most beautiful, with a delicate translucent pink or lavender body and milky-white cerata.
In the waters around the Mokohinau Islands, another gorgeous nudibranch called Tambja verconis feeds exclusively on the erect, branching bryozoan Viridentula dentata. Belonging to the dorid subgroup, you can’t miss Tambja verconis, which is a large (70-90mm) brightly coloured nudibranch, with a background of bright yellow and large sky-blue spots and streaks. Its body surface is noticeably wrinkled, and its striking gills are blue with black pinnae (the feathery extensions of the gills).
Another nudibranch that shares a love for the bryozoan Viridentula dentata, is actually named for this group of islands where it was first discovered – the Janolus mokohinau. Equally as beautiful, it’s body (18-20mm) is almost entirely covered in cerata which are a translucent, pale green, with bright orange tips, and golden line that runs down the midline of the back. From the numerous cerata, you would assume this is an aeolid species, but it in fact belongs to the nudibranch subgroup, proctonotid.
A species that you are unlikely to see, but should definitely know about, is the nudibranch Phylliroe lichtensteinii. This species has a body so modified that it is shaped more like a fish than a nudibranch, being greatly compressed sideways with a forked tail fin; the only physical giveaway being its long rhinophores. It is 35mm long at adulthood, a translucent brown or pink colour, with hundreds of tiny light organs appearing as tiny purplish speckles, which luminesce brightly at night. It is found floating in the surface waters of the Hauraki Gulf and Kermadec Islands, where it swims with graceful undulations of the whole body. Adults feed on a range of planktonic cnidarians such as jellyfish, while juveniles feed almost exclusively on the hydromedusan Zanclea sp. (a small type of jellyfish), attaching parasitically to the interior of the bell (main body), where they remain until adulthood.
We hope you get to spot a nudibranch on your explorations of the Hauraki Gulf this summer. But this comes with a warning: nudibranch-spotting is very addictive, heightened by the fact that new species are found year on year. For inspiration, check out this ‘nudi-blog’, written by diver Alison Perkins, who recently discovered a new species of Trapania nudibranch with fellow diver Cameron Russo.
Love it, Restore it, Protect it.