Nudibranch Basics
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.
Dorids usually have a smoother body and a visible gill plume towards the rear. Phyllidiidae nudibranchs are also dorids but their gills are hidden under the mantle. Many familiar genera, including Chromodoris, Hypselodoris, Nembrotha and Phyllidia.
Cladobranchs are nudibranchs but do not have a circle of gills on the dorsum. They have different structures that perform gaseous exchange such as cerata. There are three subgroups - Aeolids, Arminina and Dendronotina. Familiar genera include Facelina, Dermatobranchus and Doto.
Not all sea slugs are nudibranchs but they are still fascinating, colourful and make important macro subjects for photographers. Familiar genera include sea hares (Aplysia), head-shield slugs (Chelidonura) and sapsucking slugs (Elysia and Costasiella - the much loved Shaun the Sheep).
The paired sensory structures on the head. These are often the most important detail to keep sharp in a photograph. See the rhinophore guide for the main shapes divers use in field identification.
Many dorids have a feathery gill plume towards the rear of the body.
Finger-like structures on the back of aeolids and similar sea slugs.
A nudibranch is a soft-bodied marine mollusc and a type of sea slug. Most adult nudibranchs have no external shell, and divers usually recognise them by their colourful bodies, rhinophores and exposed gills or cerata.
All nudibranchs are sea slugs, but not all sea slugs are nudibranchs. Divers often use the phrase sea slug loosely, but true nudibranchs are a specific group within the wider sea slug world.
Many nudibranchs have chemical defences that make them distasteful or toxic to predators, and some can store stinging cells or toxins from their food. Divers should never touch or handle them.
Their bright colours can act as warning signals, camouflage or mimicry. Some species advertise chemical defences, while others blend into sponges, hydroids, soft corals or other food sources.
Divers can find nudibranchs on reefs, muck sites, walls, seagrass, sponges, hydroids and soft corals in both tropical and temperate seas. Good macro destinations include the Coral Triangle, Anilao, Lembeh, Ambon, Raja Ampat and cooler-water sites such as the Pacific coast.
Yes. Nudibranchs are excellent macro subjects because they are colourful, detailed and usually slow-moving. They reward careful buoyancy, patient composition and close attention to rhinophores, gills, texture and background.