How does a sea slug smell? — Norfolk Island's Reef

Two *Platydoris cinereobranchiata* mating. Rhinophores on the right

Etymology of ‘rhinophore’ (Oxford English Dictionary)

Rhino-: Derived from the Ancient Greek word rhis (genitive rhinos), meaning ‘nose’.
-phore: Derived from the Ancient Greek -phoros (‘bearing’ or ‘carrying’), a derivative of pherō (‘to bear or carry’).
Literal meaning: A mixed Neo-Latin and Greek formation translating to ‘carrying noses’ or ‘nose-bearer’.
History: The term entered scientific usage in the 1860s (recorded around 1868) to describe the tentacle-like sensory structures on the heads of sea slugs.

And in other trivia: rhinophore occurs in modern written English 0.02 times for every one million words.

Which probably goes a long way to explain why it’s a word you may not recognise.

Two *Elysia* showing their scroll-like rhinophores

In this blog post, I am going to take a look at some of the different rhinophores sported by sea slugs in Norfolk Island’s Emily and Slaughter Bays. It was a little green-coloured Elysia with fetching blue dots that sent me looking into all this. I’ve photographed this species plenty of times before – mollusc specialist Dr Richard Willan thinks it may even be an as-yet undescribed species – but while processing a close-up recently I suddenly noticed its rhinophores (left image, above). They looked like two tiny scrolls of tissue, rolled inwards along their length.

There were actually two Elysia in the photograph, mating, but for once that wasn’t what caught my attention. It was those scrolls. Inevitably, I started going back through my other sea-slug photographs.

I had been photographing rhinophores for years without paying terribly much attention to just how different they are. Once I did, there were scrolls, ridges, stacks of little plates, tubes and what look remarkably like rabbit ears. The variation is wonderful.

The ‘nose’ comparison is simplistic, of course. We smell volatile chemicals carried through the air. Sea slugs are detecting chemical information dissolved in seawater. Rhinophores are packed with sensory tissue and are particularly important for detecting cues at a distance. Depending on the animal, those cues can include food, predators and other members of the same species. Comparative work on sea-slug sensory organs has found considerable differences in their external shape, while the basic cellular organisation of the sensory system underneath is much more consistent.

There’s some great experiments showing the nudibranch Tritonia diomedea, with intact rhinophores, following the odour of prey upstream and turning away to evade predator odours. When researchers removed the rhinophores, those responses disappeared.

White-speckled sea hare - *Aplysia argus*

Sea hares have been particularly useful animals for investigating this. In Aplysia spp., the rhinophores appear to be important for distance chemoreception and also rheoreception – detecting water flow – whereas the oral tentacles at the front of the head seem to have a greater role in contact chemoreception and touch. Researchers have also identified that sea hares can detect water-borne pheromones – chemical signals associated with mating.

In the image of the white-speckled sea hare, you can clearly see a pair of rhinophores on top of its head and a second pair of structures, the oral tentacles, nearer the mouth. The rhinophores are often rolled or enrolled, and their resemblance to rabbit ears is where the common name ‘sea hare’ comes from. Once you know which bits you are looking at, it becomes much easier to see them in my blunt-end sea hare, Dolabrifera brazieri, dwarf sea hares - Aplysia concava, Stylocheilus striatus and white-speckled sea hare photographs.

Blunt-end seahare - *Dolabella auricularia*

The Elysia version is different again. Rolled or grooved rhinophores are characteristic of the genus, although the details vary between species. In some taxonomic descriptions the groove runs almost the entire length; in others the tip itself is rolled.

Then there are the nudibranchs. In Goniobranchus, Halgerda, Dendrodoris, Roboastra and the pustulose wart slug, the rhinophores are much more obviously ridged or lamellate. Some look like a stack of very fine plates around a central stalk; others have broader, more widely spaced folds. The number, shape and arrangement of those lamellae can be sufficiently consistent to help taxonomists distinguish different species.

Those folds aren’t just for show. More folds mean more surface area exposed to the surrounding water – and therefore more area for sampling dissolved chemicals.

My pleurobranchs bring us back towards the scroll idea. The Forskål’s and Peron’s pleurobranchs have curled rhinophores, as do pleurobranchs more generally, although again the exact form differs. The umbrella sidegill has rolled tubular rhinophores tucked away at the front of what is otherwise a fairly improbable-looking animal.

Slender Roboastra - *Roboastra gracilis*

Pustulose wart slug - *Phylidiella pustulosa*

Which made me wonder: why all these different shapes?

We know a good deal about what rhinophores detect, something about their sensory cells and nerves, and quite a lot about their anatomy. We also know that rhinophore form and structure varies strongly among different groups and is useful in identifying the animal. What I couldn’t find was good evidence that lets us look at the very fine lamellae on one species and say confidently: that shape evolved because it eats this food, lives in that habitat, or needs a more sensitive ‘nose’. Some of the variation may well have functional consequences. Some of it reflects evolutionary history. Almost certainly both are involved. But unfortunately it is not possible to assign a neat ecological explanation to every scroll, fold or frill.

PhD anyone? In the meantime, I’ll keep looking at rhinophores. I have a feeling I’ve only just started noticing them!

*Pseudobiceros sp.* with the lamellae on its rhinophores clearly visible

References

Cummins, S. F., Erpenbeck, D., Zou, Z., Claudianos, C., Moroz, L. L., Nagle, G. T., & Degnan, B. M. (2009). Candidate chemoreceptor subfamilies differentially expressed in the chemosensory organs of the mollusc Aplysia. BMC Biology, 7, 28. https://doi.org/10.1186/1741-7007-7-28 PubMed

Göbbeler, K., & Klussmann-Kolb, A. (2007). A comparative ultrastructural investigation of the cephalic sensory organs in Opisthobranchia (Mollusca, Gastropoda). Tissue and Cell, 39(6), 399–414. https://doi.org/10.1016/j.tice.2007.07.002 PubMed

Klussmann-Kolb, A., Croll, R. P., & Staubach, S. (2013). Use of axonal projection patterns for the homologisation of cerebral nerves in Opisthobranchia, Mollusca and Gastropoda. Frontiers in Zoology, 10, 20. https://doi.org/10.1186/1742-9994-10-20 PubMed

Lisova, E. D., & Vortsepneva, E. V. (2022). New data on nudibranchs rhinophore morphology and their spicule complex in Onchidoris muricata (Doridina, Gastropoda). Zoologischer Anzeiger, 296, 58–70. https://doi.org/10.1016/j.jcz.2021.11.003 ScienceDirect

Wertz, A., Rössler, W., Obermayer, M., & Bickmeyer, U. (2006). Functional neuroanatomy of the rhinophore of Aplysia punctata. Frontiers in Zoology, 3, 6. https://doi.org/10.1186/1742-9994-3-6 PubMed Central (PMC)

Wyeth, R. C., & Willows, A. O. D. (2006). Odours detected by rhinophores mediate orientation to flow in the nudibranch mollusc, Tritonia diomedea. Journal of Experimental Biology, 209(8), 1441–1453. https://doi.org/10.1242/jeb.02164

Side gill sea slugs

Forsskål's pleurobranch - *Pleurobranchus forskalii*

Peron's pleurobranch - *Pleurobranchus peronii*

September 27, 2026

How does a sea slug smell?

It started with a close-up of a little Elysia and two rhinophores that looked like tiny rolled-up scrolls. Once I noticed them, I started going back through years of sea-slug photos and realised just how wildly different these sensory organs can be – ridged, rolled, curled, tubular and, in sea hares, distinctly rabbit-eared. So this week’s rabbit hole is all about how sea slugs ‘smell’ underwater, and why their noses come in so many different shapes.

September 19, 2026

The Elite Fleet sails in

Nearly six years ago, my very first blog post was about the strange blue and violet creatures that live at the ocean’s surface – a group I christened Norfolk Island’s ‘Elite Fleet’. Since then I’ve seen plenty of bluebottles and the occasional violet sea snail, but never a by-the-wind sailor. Until this morning. Velella velella is not a true jellyfish, but a colonial hydrozoan with a transparent sail that lets the wind help carry it across the ocean. So what exactly is this extraordinary little creature, how does it ‘sail’, and what connects it to the rest of the fleet?

September 18, 2026

In a nutshell – where I stand on the proposed Norfolk Island Marine Park

I support the proposed expansion of Norfolk Marine Park and its renaming as Norfolk Island Marine Park. There is much to welcome – strong offshore protection, no mining, and recognition of the importance of sustainable local fishing. But I also have some reservations. Why does Cascade need to be removed from the Marine Park at all? How will more than 430,000 km² actually be monitored and enforced? How do we make sure Norfolk people have a genuine role in management? And, above all, how do we ensure that the enormous new park does not draw attention away from the small, stressed reef immediately beside us?

September 13, 2026

No seagrass, but plenty of turtles

Norfolk Island does not have seagrass, although one of our most conspicuous green algae is often mistaken for it. Feather and grape Caulerpa can both form striking patches across the lagoon. I take a closer look at what Caulerpa actually is, why it is so easily confused with seagrass, and where our green turtles fit into the story.

September 6, 2026

Watching El Niño from Norfolk Island

El Niño is strengthening across the Pacific, but that does not tell us exactly what the coming months will bring to Norfolk Island. After a long wet spell, I am watching our rainfall and water supplies more closely again – and, underwater, keeping an eye on sea temperature, algae, coral recruits and the fish that turn up on the reef.

September 2, 2026

What the world’s reefs are telling Norfolk Island

Two major new coral reef reports carry a clear message for Norfolk Island: reefs can recover when pressures ease, but the time available for recovery is shrinking. Norfolk Islanders were raising concerns about water quality, runoff and the health of Emily and Slaughter Bays nearly forty years ago. Today we have much better science, a changing climate and a far clearer understanding of how closely the catchment and reef are connected. This article looks at what that means for water quality, long-term monitoring, dredging and how we manage Kingston from land to sea.

August 25, 2026

When the map changes

A fish once thought to belong only to Norfolk turns up elsewhere; another appears here hundreds of kilometres beyond its known range. Cardinalfishes and a Sabre squirrelfish show how our maps of species distributions are revised.

August 24, 2026

Endemic, native, undescribed – what do these words actually mean?

A Norfolk chromis that turned out not to be Chromis fumea, a blenny found nowhere else, and a tiny goby that may not yet have a name. A look at what we mean by native, endemic, undescribed and newly described – and why scientific names sometimes change.

August 16, 2026

What’s grazing in the green stuff?

A proliferation of Bryopsis on the reef has brought some beautiful little underwater grazers with it. I follow the trail from sap-sucking sea slugs and egg masses to a bigger question – why is there so much Bryopsis around at the moment?

August 8, 2026

The clam beneath Emily Bay

I’ve seen the beautiful shells of Laciolina chloroleuca in Emily Bay for decades, but I’d never really stopped to wonder where the living animals were. It turns out they may be buried half a metre beneath the sand. A closer look at an easily overlooked clam, its extraordinary shell markings, and the hidden life beneath our lagoon.

Out on A Swim

References

Side gill sea slugs

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