What are rhinophores?
Rhinophores are paired sensory organs on the head of nudibranchs and many other sea slugs. They help detect dissolved chemicals in seawater, including clues from food, mates and the surrounding environment.
Rhinophores are one of the most useful features for identifying nudibranchs and other sea slugs. They are the paired sensory structures on the head, often described by divers as horns or antennae. In reality, they are specialised chemical-sensing organs that help the animal detect food, mates and environmental clues dissolved in seawater.
The shape of a rhinophore matters. Rings, folds, ridges, branches and rolled edges all increase sensory surface area or channel water across the animal's sensory tissue. For divers and underwater photographers, these shapes also provide useful identification clues.
Different field guides and websites sometimes use different terms for rhinophore shapes, so MacroDivers uses five practical groups: horizontal, vertical, smooth, rolled and ornate / complex. These are field-identification categories rather than strict scientific divisions.
Horizontal rhinophores
Horizontal rhinophores have rings, folds, lamellae or leaflets that run around or across the rhinophore. This is one of the most familiar rhinophore styles seen by divers. The folds increase the surface area exposed to moving seawater, helping the animal detect chemical clues more effectively.
Perfoliate rhinophores look like stacked disc-like or leaf-like folds and are common in many dorid nudibranchs. Lamellate rhinophores have fine plates or ridges along the rhinophore. Annulate rhinophores have ring-like bands around the structure and are often seen in aeolid nudibranchs.
Look for: repeated bands, plates or leaflets running across the rhinophore rather than along its length.
Vertical rhinophores
Vertical rhinophores have ridges or grooves running lengthwise up and down the rhinophore. These ridges also increase sensory surface area, but the visual pattern is different from the ringed or stacked look of horizontal rhinophores. Some species have clubbed or bulbous rhinophores, often with a contrasting coloured tip.
Dermatobranchus, Armina and Gymnodoris are useful examples where vertical or clubbed forms may be seen.
Look for: lengthwise grooves, a clubbed shape, or a coloured tip rather than rings around the rhinophore.
Smooth rhinophores
Smooth rhinophores have little or no visible ridging. They often look simple, tapering or finger-like. Many aeolid nudibranchs have smooth or almost smooth rhinophores, and these are often seen alongside oral tentacles, which can confuse beginners. Some smooth rhinophores emerge from a raised collar or sheath.
Oral tentacles are not rhinophores. Rhinophores are usually higher on the head and mainly used for distant chemical sensing, while oral tentacles are closer to the mouth and are more associated with touch and close-range exploration.
Look for: simple, smooth, tapering structures on the top of the head. Check that you are not confusing them with oral tentacles.
Rolled rhinophores
Rolled rhinophores form a tube-like shape, often with a hollow or rolled appearance. This style is not typical of true nudibranchs but is seen in several related sea slug groups, including sacoglossans, sea hares and some side-gilled slugs. It is included here because divers often photograph all of these animals together as "sea slugs", and rolled rhinophores are a useful visual clue that the animal may not be a nudibranch.
Elysia is a sacoglossan sea slug, not a true nudibranch.
Look for: an ear-like or tube-like rolled structure rather than rings, plates or branches.
Ornate and complex rhinophores
Some rhinophores do not fit neatly into the simple horizontal, vertical, smooth or rolled categories. These are grouped here as ornate or complex forms. They may be branched, bumpy, papillate, pocket-shaped or funnel-shaped. These forms can be especially useful in identification because they are often distinctive at genus level.
Papillate rhinophores are bumpy or covered with small rounded projections, seen in genera such as Facelina, Coryphellina, Baeolidia and Hallaxa. Branched rhinophores are tree-like or forked, seen in genera such as Bornella and Dendronotus. Pocket or funnel-shaped rhinophores are folded or cup-like, seen in genera such as Marionia and Trapania.
Look for: branching, bumps, folds, cups or pocket-like structures that do not fit the simpler categories.
Rhinophore Types at a Glance
| Type | What it looks like | Useful ID clue | Example photo |
|---|---|---|---|
| Horizontal | Rings, plates, folds or leaflets across the rhinophore | Often looks banded or stacked | Goniobranchus / Cadlinella / Favorinus |
| Vertical | Lengthwise ridges or clubbed form | Ridges run up and down, not around | Dermatobranchus |
| Smooth | Simple, tapering, little or no ridging | May be confused with oral tentacles | Tenellia / Doto |
| Rolled | Tube-like or ear-like rolled structure | Often suggests a related sea slug rather than true nudibranch | Elysia |
| Ornate / Complex | Branched, bumpy, pocket or funnel-shaped | Often very distinctive for ID | Bornella / Facelina / Hallaxa / Marionia |
How to Photograph Rhinophores for Identification
Rhinophores are small, delicate and often partly hidden, but a good close-up can make identification much easier.
- Try to photograph the head from slightly above and slightly side-on.
- Keep both rhinophores in focus where possible.
- Include enough of the body pattern to help identify the species.
- Watch for oral tentacles; do not label them as rhinophores.
- If the rhinophores are retracted, wait quietly rather than touching or disturbing the animal.
- Take a wider record shot as well as the close-up.
- For database use, crop one rhinophore close-up and one full animal image.
Do not prod or blow water at the animal to make rhinophores appear. Wait, adjust your angle, and avoid stressing the subject.
Rhinophore FAQs
Are rhinophores the same as antennae?
Divers often call them antennae or horns, but rhinophores are specialised sensory organs. They are not true antennae like those found on insects or crustaceans.
Why do rhinophores have rings, folds or ridges?
Rings, folds, lamellae and other surface structures increase the surface area exposed to seawater. This gives the animal more sensory surface for detecting chemical clues.
Can nudibranchs retract their rhinophores?
Many dorid nudibranchs can retract their rhinophores into protective pockets. This helps protect the delicate structures from damage or predators.
What is the difference between rhinophores and oral tentacles?
Rhinophores are usually higher on the head and are mainly used for chemical sensing. Oral tentacles are closer to the mouth and are more involved in touch, taste and close-range exploration.
Are rolled rhinophores found on nudibranchs?
Rolled rhinophores are more typical of related sea slug groups such as sacoglossans and sea hares rather than true nudibranchs. They are included here because divers often encounter and photograph these animals alongside nudibranchs.
Can rhinophores help identify a nudibranch?
Yes. Rhinophore shape, colour, ridging and position can all help with identification, especially when combined with body shape, gills, cerata, colour pattern, food source and location.
Want to improve your nudibranch identification?
Build the full picture by combining rhinophore shape with anatomy, behaviour, food source and location.