Are these official scientific body-shape categories?
No. These are practical MacroDivers database categories. They are designed to make searching and identification easier for divers, photographers and sea slug enthusiasts.
Sea slugs come in a remarkable range of shapes. Some are broad and oval, others are long and slender, some are bulky and muscular, and others have frills, lobes, paddles or visible shells.
For the MacroDivers database, we use a simple set of body-type categories to make identification and searching easier. These categories are not formal scientific classifications. They are practical field-ID groups based on what a diver or photographer can see in a normal underwater photograph.
When choosing a body type, we look at the overall body outline of the animal. Rhinophores, gills and cerata are not counted as part of the body shape. This keeps the database consistent and avoids placing every aeolid nudibranch with cerata into the With Appendages category.
Oval
Oval sea slugs have a broad, rounded or oval body outline. This is the classic dorid-style shape, especially when the mantle spreads over the foot and creates a smooth rounded outline.
Oval is likely to be the most common category in the MacroDivers database because many dorid nudibranchs fit this general form. If the animal is only slightly longer than it is wide, it should usually still be treated as Oval rather than Elongate.
Do not include rhinophores, gills or small mantle details when deciding this category. Look at the overall body outline.
Look for: a broad, rounded animal where the body looks wider and more mantle-like, rather than long, narrow or bulky.
Elongate
Elongate sea slugs appear longer than they are wide. The body is narrow, slender or stretched out, and may taper towards the tail.
This category includes many aeolid nudibranchs and some headshield slugs. Cerata, gills, rhinophores and oral tentacles should not be counted as part of the body outline. For example, a Favorinus or Moridilla may have obvious cerata, but the underlying body shape is still long and narrow, so it belongs in Elongate rather than With Appendages.
Some headshield slugs may also appear elongate. A useful clue is a wider head area and, in some groups, a split or uneven tail.
Look for: a slim animal where the main body is clearly longer than it is wide, even if it has cerata or other structures along the back.
Robust
Robust sea slugs have a heavy-bodied or bulky appearance. They may look thick, muscular, chunky or raised, and some have a domed back. In this category, the animal often appears substantial rather than flat, thin or slender.
Use Robust for animals that look almost as tall as they are wide, or where the body has a strong, bulky appearance. Some dorids and side-gilled slugs may fit here better than Oval because their shape is heavier and more three-dimensional.
Examples include animals such as Pleurobranchus, Reticulidia and some Tambja species.
Look for: a chunky, heavy or domed animal where Oval feels too flat or too delicate.
With Appendages
This category is used when the overall body outline is clearly broken up by large appendages such as frills, lobes, paddles, branches or expanded body structures.
Do not use this category for every animal with cerata. Aeolid cerata should normally be ignored when choosing the body type, because otherwise many elongate aeolids would incorrectly become With Appendages.
Use With Appendages when the appendages are part of the animal's overall silhouette and strongly affect the visible body outline. This may include animals such as Bornella, Melibe, Thecacera and many sacoglossans with prominent parapodia.
Look for: large lobes, paddles, frills, branches or body extensions that change the overall outline of the animal.
With Visible Shell
Some sea slugs have a visible external shell or a partly visible shell-like structure. The shell may look rounded, glossy, bubble-like or partly covered by the soft body.
This category is mainly for sea slugs where the shell is clearly visible in the photograph. It may include bubble-shell type animals and other shelled or partly shelled sea slugs. In the database, this value may appear as With Shell; publicly, we describe it as With Visible Shell.
Sea hares are more difficult because many have a reduced internal shell that is not visible. Where a sea hare has obvious parapodia or no visible shell, choose the most appropriate category rather than automatically using With Shell.
Adult nudibranchs do not have an external shell, so animals in this category are usually related sea slugs rather than true nudibranchs.
Look for: a visible shell, bubble-like shell, glossy shell surface, or a soft body emerging from a shell.
Other
Other is a catch-all category used when it would be misleading to place the animal into one of the main body types.
Use Other when:
- the photo angle is poor
- the animal is partly hidden
- the body shape is unclear
- the animal is damaged or heavily contracted
- the individual is a juvenile and does not yet show the adult body form
- the animal does not fit any of the database categories well enough
This category should not be overused, but it is better to choose Other than to force an uncertain animal into the wrong body type.
Look for: situations where the body shape cannot be judged confidently from the available photograph.
Sea Slug Body Types at a Glance
| Body Type | What it looks like | Database clue | Example genera |
|---|---|---|---|
| Oval | Broad, rounded or oval, often with mantle spreading over the foot | Slightly longer than wide can still be Oval | Glossodoris, Goniobranchus, Phyllidiella |
| Elongate | Long, narrow or slender body | Ignore cerata when judging the main body outline | Chelidonura, Favorinus, Moridilla |
| Robust | Heavy, chunky, bulky or domed | Use when Oval feels too flat or too delicate | Pleurobranchus, Reticulidia, Tambja |
| With Appendages | Body outline broken by large lobes, frills, paddles or branches | Do not use just because an aeolid has cerata | Bornella, Melibe, Thecacera |
| With Visible Shell | Visible or partly visible shell | Usually related sea slugs, not true nudibranchs | Micromelo, Siphopteron, some sea hare examples |
| Other | Shape unclear or does not fit | Use for poor angles, hidden animals or juveniles | Case-by-case |
How to Use Body Type for Identification
Body type is a useful first clue, but it should never be used on its own. Many unrelated sea slugs can share a similar outline, and the same animal may look different depending on angle, age, movement or whether it is stretched, contracted or partly hidden.
- Start with the overall body outline.
- Ignore rhinophores, gills and cerata when choosing the body type.
- Use the widest clear view of the animal if several photos are available.
- Check whether the animal is flat, slender, bulky, shelled or broken up by appendages.
- Use body type together with colour, pattern, gills, rhinophores, cerata, food source and location.
- If the body shape is unclear, choose Other rather than forcing a bad match.
- For database records, keep a full-body photo as well as any close-up detail photos.
Photographing Body Shape for the Database
A good database record needs more than a beautiful close-up. Try to take one full-body identification photo showing the animal's outline, then take detail shots of rhinophores, gills, cerata or patterns.
Photograph the animal from above or slightly side-on where possible, so the overall outline can be judged. If the animal is partly hidden, stretched, contracted or moving, take several angles if you can do so without disturbing it.
Do not move, prod, roll or reposition the animal to improve the photo. Body shape should be recorded as naturally as possible.
Body Type FAQs
Why does this page include sea slugs as well as nudibranchs?
MacroDivers focuses mainly on nudibranchs, but the database also includes related sea slugs that divers commonly encounter on macro dives. Some examples, such as sea hares, headshield slugs and shelled sea slugs, are not true nudibranchs.
Should cerata count as appendages when choosing body type?
Usually no. For the MacroDivers database, cerata are ignored when deciding body type. An aeolid with a long narrow body and cerata is normally classed as Elongate rather than With Appendages.
What is the most common body type?
Oval is likely to be the most common category because many dorid nudibranchs have a broad oval body with the mantle spreading over the foot.
When should I use Robust instead of Oval?
Use Robust when the animal looks heavy, bulky, chunky or domed rather than simply broad and oval. Some animals appear almost as tall as they are wide and fit better in Robust.
Are animals with visible shells nudibranchs?
Adult nudibranchs do not have an external shell. If a sea slug has a visible shell, it is usually a related sea slug rather than a true nudibranch.
When should the Other category be used?
Use Other when the body shape is unclear, the animal is partly hidden, the photo angle is poor, the animal is a juvenile, or none of the other categories would be accurate.
Can body type identify a species on its own?
No. Body type is only one clue. Accurate identification usually also needs rhinophores, gills, cerata, colour pattern, food source, habitat, location and sometimes expert review.