Nudibranchs: sea slugs of the Hauraki Gulf

Source: NZ Department of Conservation

Here in Aotearoa, we punch well above our weight when it comes to sea slugs. We’ve got around 130 species cruising our coastlines — some are found nowhere else on Earth, others we share with our mates across the ditch in eastern Australia and the wider south-west Pacific.

Most of our locals come from a sub-group called nudibranchs (said nudi-brank), which literally means “naked gills”. Charming name, right? But don’t worry — it’s not as weird as it sounds. These are the colourful, frilly, show-off sea slugs you often see in ocean photos, and these are the ones we’re going to talk about.

In the Hauraki Gulf alone you can spot a whole cast of characters here’s just some of them and where you can find them this summer.

Head north or south of Long Bay and you’ll hit rocky reefs perfect for a low-tide rock-pool mission or a snorkel when the tide comes in. These ledges and overhangs are hotspots for colourful nudibranchs snacking on sponges, bryozoans, and tiny corals.

While nudibranchs are easier to spot underwater, one local favourite — the gem doris (Dendrodoris krusensternii) — sometimes shows up in the rock pools. Often called New Zealand’s “loveliest nudibranch”, it grows up to 70mm and sports a fawn-grey body with jelly-like bumps and bold chocolate-brown and peacock-blue patterns.

Its eating style is just as dramatic: it dissolves its sponge food with enzyme-packed saliva, then slurps the whole thing up. Classy? Not really. Fascinating? Absolutely.

Cape Rodney-Okakari Point Marine Reserve

No Auckland summer is complete without a snorkel at Cape Rodney-Okakari Point Marine Reserve (Goat Island) near Leigh. Alongside the gem doris, you might spot the cheeky little clown nudibranch (Ceratosoma amoenum), only 20–40mm but impossible to miss with its bright orange spots.

It sports purple, antler-like rhinophores for sniffing out food, and a feathery purple gill tuft at the back. These little guys love hanging out on sponges, which they not only munch on but also turn into a defence weapon — storing the sponge’s toxins in their glands to keep predators at bay. Tiny, colourful, and armed: not your average sea slug.

Marine reserves are fully protected areas, from the sea surface right down to the seafloor. Everything’s off-limits — no taking fish, shells, rocks, or even driftwood.

That doesn’t mean you can’t enjoy them, though. Swimming, snorkelling, boating, and diving are all fair game, as long as you leave the marine life exactly where you found it.

New protections for the Hauraki Gulf include extensions to two marine reserves.

Cape Rodney-Okakari Point (Goat Island) Marine Reserve: Nearly quadrupling its size, this expansion would better protect species like blue maomao and snapper.

Te Whanganui-o-Hei/Cathedral Cove Marine Reserve: More than doubling in size, the reserve would safeguard its hard rock reefs, plants, crustaceans, molluscs, and fish.

Make sure you familiarise yourself with the new rules and extended marine reserve boundaries before heading out.

Diving

For keen divers, if you spot the tree-like hydroid Solanderia ericopsis in the Hauraki Gulf, chances are you’ll also see the stunning endemic nudibranch Jason mirabilis. These spots include the tip of the Coromandel Peninsula and, of course, Cape Rodney-Okakari Point Marine Reserve.

Jason mirabilis is an aeolid — a type of nudibranch — decked out with long, dreadlock-like cerata on its back. These blood-filled tubes double as gills and digestive tracts. Growing up to 60mm, it’s one of the biggest aeolids in the world and easily one of the prettiest, with a delicate translucent pink or lavender body and milky-white cerata. Truly the show-off of the Gulf.

Another nudibranch with a taste for the bryozoan Viridentula dentata is named after the islands where it was first spotted: the Janolus mokohinau. Just as eye-catching as its cousins, this little 18–20mm critter is almost entirely covered in cerata — translucent pale green with bright orange tips and a golden stripe running down its back.

You might assume it’s an aeolid from all those cerata, but nope — it actually belongs to the proctonotid subgroup. Small, colourful, and a little bit deceptive: classic nudibranch behaviour.

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.

It’s pretty awesome that the Hauraki Gulf is home to colourful, flamboyant creatures like nudibranchs. The new protections mean these little beauties, and their habitats, will have a safer home. That’s why it’s important we all know the rules, follow them, and help others do the same. Look after the Gulf, and it’ll keep looking after these amazing species for generations to come.

MIL OSI