a photographic atlas for the Bass Strait region
Photographic Atlas

About nudibranchs

Nudibranchs and their relatives are also known as sea slugs, a name that does no justice to the beauty and wide appeal of these brightly coloured marine molluscs. The term "nudibranch" is used a little loosely, so it is important to be clear about the taxonomy of these diverse organisms. Slug-like creatures (snails with reduced shells, or with no shells at all) are all members of the Gastropoda (snails) and are classified within the phylum Mollusca. After that, it gets a bit more complicated.

Nudibranchs belong to the Opisthobranchia. But the Nudibranchia (nudibranchs) is only one of six orders within the subclass Opisthobranchia. Bubble shells, bat-winged slugs and other slug-like marine gastropods all belong to other orders within the Opisthobranchia. These others are less frequently seen by divers than nudibranchs, but we would like to include all kinds of opisthobranchs in this atlas project, so please photograph all slug-like marine organisms, whether or not you think they are nudibranchs. As the project develops, we will identify and classify photographs of all species of nudibranchs that we receive. Beginning February 2005, revisit this web site regularly to view updates.

Hoplodoris nodulosa [photo: Sylvia Buchanan]Opisthobranchs are found in most habitats and are almost exclusively marine, although a few species occur in freshwater streams in tropical regions. Nudibranchs, and other opisthobranchs, reach their greatest diversity in tropical seas. However, although the fauna of southern Australia probably comprises only about half as many species as occur in tropical waters, that is still a lot of species! About 300 named species of opisthobranchs occur in the Bass Strait region, but a number of undescribed species also occur here and the total is certainly more than 350 species. Most of these are found nowhere else. Hoplodoris nodulosa (photographed by ReefWatcher Sylvia Buchanan, and illustrated on the left) is just one of about 700 species of nudibranchs found in southern Australia.

A full list of species found in Victoria and the Bass Strait area can be found in Robert Burn's 2006 checklist.

See the resources and contacts page for other sources of information on nudibranchs and other opisthobranchs.

Not all slugs are nudibranchs

A pulmonate: Onchidella patelloides [photo: M. Marmach] Not all gastropod slugs are nudibranchs. Garden slugs, for example, are air-breathing gastropods (classified as subclass Pulmonata) and so is Onchidella nigricans, a small slug which is common in rocky intertidal habitats in southeastern Australia. These slugs are terrestrial (or at least amphibious) organisms, air-breathers, and have lungs (opisthobranchs instead have gills, and must remain underwater).

Onchidella nigricans is pictured in the photograph on the right. Anything found, of its own volition, out of the water is certainly an air-breathing slug, not a nudibranch. Nudibranchs die out of the water.

Marine flatworms also look a little like nudibranchs, but are quite unrelated; they are not even molluscs. The link
about flatworms provides some information on these diverse creatures.