Order Nudibranchia- Naked-gill sea slugs. Largest of the five Orders of Opisthobranch gastropods. Lack shells, are carnivorous (feed on sessile invertebrates like sponges, sea fans, hydroids, corals... some are predaceous on other nudibranchs.). Lack predators by and large themselves due to unpalatability, toxicity, venom... mainly from recycled molecules via their prey. 

Considering their small size, often bizarre body shapes and dazzling coloration, it's no wonder nudibranchs are often unidentified by divers, aquarists and tide-poolers for what they are; marine snails that lack shells, just like their lowly terrestrial cousins found sliming around under rotting wood and vegetation.

But most of us emote "oohs" and "aahhs" rather than "yecch" when we view these "butterflies of the sea". Unfortunately, few efforts at keeping them in captivity have been successful. Many failures, as you'll see, are aquarist-originated; chemical poisoning, unstable conditions, and most importantly, starvation.

Diversity/Classification:

Nudibranchs are mollusks (Phylum Mollusca, = "Soft-bodied" animals a group including squids, octopi, abalones, Chitons, tusk shells, mussels, scallops, oysters... Further sub classified, they belong to the Class Gastropoda, the "stomach-footed" mollusks (snails and their relatives); which are still further divvied up into the Subclass Prosobranchia- with a mantle cavity and organs located anteriorly in their classically twisted (or torted) shells. Our group is the other Subclass, the Opisthobranchia, shows a reduction of shell and mantle cavity; many are de-torted, that is, secondarily bilaterally symmetrical.

The Order Nudibranchia is one of nine extant Orders in the Subclass; they lack shells and mantle cavities. To review:

Phylum Mollusca

Class Bivalvia, et al.

Class Gastropoda

Subclass Prosobranchia ("ahead gills")

Subclass Opisthobranchia ("behind gills"). Have Rhinophores; sensory organs for chemicals, currents. Below: Rhinophores anteriorly, anal gills posteriorly.

Rhinophores and gills...

Order Aplysiacea, et al. (sea-hares)

Order Nudibranchia- Naked-gill sea slugs, 

    Suborder Doridacea- Dorids

    Suborder Aeolidacea- Aeolids

    Suborder Dendronotidacea- Usually have an oral hood and series of cerata running down their backs. 

    Suborder Arminacea

The nudibranchs comprise four sub-orders, about seventy families and one thousand-plus valid species. Two of the sub-orders are rarely seen in the hobby, the Dendronotacea and Arminacea, but the following information concerning the other two groups applies to them.

Suborder Doridacea, Dorids:

('Door-idz'), (Suborder Doridacea) dwell in rocky intertidal zones worldwide, sometimes in good numbers. Their mantles cover their entire dorsal surface and hang over their foot. Their gill plumes are arranged around a rear anal vent area and are capable of retraction into a specialized pocket. The chemosensory tentacles, or rhinophores at the head end can likewise be retracted. 

Family Chromodorididae: More than 360 species in the Indo-Pacific, Caribbean, Mediterranean... the genus Chromodoris alone with 165... colorful generally. Typified by oval mantles that cover their foot entirely. Their posterior located secondary anal gills are pinnate, one pair of tentacles, rhinophores lamellate.

Genus Anisodoris: 

Genus Cadlinella: 

Genus Ceratosoma: Long-bodied, tailed, highly branched gills. Mantles with prominent lateral extensions. 

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

Genus Chromodoris: Largest genus of Nudibranchs. 

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.