Nudibranch Basics

What Is A Nudibranch?

All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.

Nudibranchs are shell-less marine molluscs. For divers, the most useful starting point is learning the broad body types before trying to identify individual species.

Where nudibranchs fit

Animal KingdomMolluscsGastropodsSea SlugsNudibranchs

Three groups divers see most often

Dorid nudibranch showing a smoother body and visible gill plume

Dorids

Dorids usually have a smoother body and a visible gill plume towards the rear. Many familiar genera, including Chromodoris, Hypselodoris and Nembrotha, are dorids.

Aeolid nudibranch with finger-like cerata on the back

Aeolids and cerata-bearing sea slugs

Aeolid-style body forms have finger-like structures called cerata on the back. These can help with breathing, digestion and defence.

Colourful sea slug often photographed by macro divers

Other colourful sea slugs

Divers also see other nudibranch-like sea slugs and colourful opisthobranchs. Some are not true nudibranchs, but they are still important macro subjects and are often photographed alongside them.

Simple anatomy clues

Rhinophores

The paired sensory structures on the head. These are often the most important detail to keep sharp in a photograph.

Gill plume

Many dorids have a feathery gill plume towards the rear of the body.

Cerata

Finger-like structures on the back of aeolids and similar sea slugs.