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*

November 3, 2026

Familiar faces – getting to know the reef’s residents

Six years of swimming the same little reef have introduced me to some unforgettable characters that I’ve followed over time. From familiar fish and anemones to turtles and a snail’s eggs, this final article in my photographic series explores what I’ve learned about the lives of individual animals simply by returning, watching and taking photographs.

November 1, 2026

When algae move in – the changing reef neighbourhood

Algae are a natural part of Norfolk Island's reef, changing with the underwater seasons. But sometimes their beautiful green growth conceals a rather sad story. In the fifth article of my six-part photographic series, I revisit familiar places to see how algae have changed over the years – and what might be hiding underneath.

October 30, 2026

The slow disappearance – disease on the reef

What happens after coral disease has run its course? In the fourth article of my six-part photographic series, I revisit colonies I've watched lose living tissue, sometimes alarmingly quickly. From a potentially century-old brain coral that lost most of its living tissue in less than 100 days to the algae-covered skeletons left years later, these photographs reveal the longer-term consequences of coral disease on Norfolk Island's reef.

October 28, 2026

When storms rearrange the reef

Sometimes the sea rearranges a reef in a matter of hours! In the third article of my six-part photographic series, I revisit corals broken, overturned or swept away by powerful swells and storm surges. Some disappeared altogether, while others found surprising ways to survive. Five photographic sequences reveal just how dramatically Norfolk Island's reef can change.

October 26, 2026

When the reef fights back – coral growth and recovery

Not every coral story ends badly! Among six years of underwater photographs are some wonderful examples of growth, survival and recovery. From corals that have grown spectacularly to others that have survived bleaching, breakage or even being turned upside down, this second article in my photographic series celebrates some of the reef's more encouraging stories.

October 24, 2026

The neighbours – six years of watching the same reef

When I began photographing Norfolk Island's reef in 2020, I simply recorded whatever caught my eye. Six years later, those photographs have become a record of individual corals and their changing fortunes – growth, competition, survival and loss. This is the first of six articles looking back at what I've seen, and sometimes what I wish I hadn't.

October 10, 2026

Pulling the rugosity from out of the reef

What do ‘rugosity’, structural complexity and channeling your inner fish have in common? Quite a lot, as it turns out. This week I’m looking at how all the lumps, bumps, branches, holes and ledges on a reef create habitat – with a brief detour into one of my long-standing tricks for navigating tertiary study: write down the jargon until it starts to make sense.

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?

Out on A Swim

References

Side gill sea slugs

Latest Posts