Opisthobranchia

Opisthobranchia Temporal range:
Notodoris minor
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Subclass:	Orthogastropoda
Superorder:	Heterobranchia
Order:	Opisthobranchia Milne-Edwards, 1848
Suborders
Cephalaspidea Sacoglossa Anaspidea Notaspidea Thecosomata Gymnosomata Nudibranchia   Infraorder Anthobranchia   Infraorder Cladobranchia

An opisthobranch (Template:PronEng) is any member of the very large and diverse group of rather specialized, highly evolved marine slugs and snails (marine gastropod mollusks) known as Opisthobranchia, within the Heterobranchia.

This Opisthobranchia include a number of families of bubble snails which have shells, and other headshield slugs in the order Cephalaspidea. In addition it includes many other orders, including the saccoglossans, the anaspidean sea hares, the pelagic sea angels and sea butterflies, and a very large number of different families of nudibranchs.

Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart (from which the group derives its name, Greek opisthen, behind + brankhia, gills).

The name "opisthobranch" was coined by Johannes Thiele in 1931 as a title for one of what were then considered the three subclasses of Gastropoda (the other two being Prosobranchia and Pulmonata).

This group was once a strict taxonomic order, but recent research in molecular phylogeny revealed Opisthobranchia's polyphyly (separate evolutions), and so the group no longer holds any strict taxonomic sense, being divided into nine different orders (and united with the so-called lower heterobranchs).

Opisthobranchia are known from the Carboniferous to the recent periods.

Description

Opisthobranchs are principally soft-bodied marine creatures with a reduced or absent shell and no operculum.

Their bodies have undergone detorsion, an evolutionary reversal of the 180° torsion of their immediate ancestors.

There is no marked distinction between head and mantle. The tentacles, situated close to the mouth, are used for orientation. Behind them you can find the rhinophores, olfactory organs often with complex forms. The middle part of the foot is the sole, used for locomotion. The sides of the foot have evolved into parapodia, fleshy winglike outgrowths. In several suborders, such as the Thecosomata and Gymnosomata, these parapodia are used to move in a swimming motion.

Many have brilliant colors and carry stinging cells, so that predators may learn to avoid them as a food source.

Opisthobranchs and pulmonates

It is speculated that the Opisthobranchia may be paraphyletic (Haszprunar, 1985), having given rise to the Pulmonata, although this is still somewhat disputed.

The Pulmonata may be a sister group to a particular opisthobranch taxon. The Opisthobranchia are not therefore a monophyletic group and are no longer be accepted as a taxon. They are now included in the subclass Orthogastropoda. Note that one can still encounter the old classification in many manuals and on most websites. You can find more about these taxonomic issues at the gastropod page.

One version of the taxonomy

Order Opisthobranchia Milne-Edwards, 1848 (sea slugs)

• Suborder Cephalaspidea P. Fischer, 1883 (headshield slugs)
• Suborder Sacoglossa von Ihering, 1876 (sap-sucking slugs)
• Suborder Anaspidea P. Fischer, 1883 (sea hares)
• Suborder Notaspidea P. Fischer, 1883 (sidegill slugs)
• Suborder Thecosomata Blainville, 1824 (sea butterflies)
• Suborder Gymnosomata Blainville, 1824 (sea angels)
• Suborder Nudibranchia Blainville, 1814 (nudibranchs)
  • Infraorder Anthobranchia Férussac, 1819
  • Infraorder Cladobranchia Willan & Morton, 1984

A new phylogentic study, published in November 2004, has given a new definition to the seven main lineages of the Opisthobranchia.

Images

• 
  A red-lined bubble shell (Bullina lineata) off the coast of Sydney, Australia.
• 
  A sea hare off Morro Bay, California.
• 
  A sea hare off Hawaii Island, Hawaii.
• 
  A blue-spotted sea hare (Bursatella leachi) off Lembeh Strait, North Sulawesi, Indonesia.
• 
  A winged snail (Clione limacina), a type of sea angel.
• 
  A Mediterranean sacoglossan, Oxynoe olivacea.
• 
  A headshield slug (Chelidonura varians) off Tasik Ria, North Sulawesi, Indonesia.
• 
  Another headshield slug (Chelidonura amoena).
• 
  A black headshield slug (Chelidonura inornata).
• 
  Chelidonura punctata.
• 
  A paper bubble shell (Micromelo guamensis) from Fish Rock Cave, South West Rocks, New South Wales.
• 
  A navanax (Navanax inermis) off Santa Catalina Island, California.
• 
  A maximillate pleurobranch (Pleurobranchis maximillatus) off Fly Point, Port Stephens, New South Wales.
• 
  A grand pleurobranch (Pleurobranchus grandis) off Dahab, Egypt.
• 
  An unidentified species of sea butterfly.

References

• The Sea Slug Forum, a resource from the Australian Museum.
• Sea slugs of Hawaii
• Haszprunar G., 1985 The Heterobranchia - a new concept of the phylogeny of the higher Gastropoda. Z. f. zool. Systematik u. Evolutionforschung Bd. 23 H. 1:15-37
• Bieler, R., 1990. Haszprunar's "clado-evolutionary" classification of the Gastropoda -- a critique. Malacologia, 31(2): 371-380, 2 tabs. [28 May; G, Haszprunar's response published in Malacologia, 1990, 32(1): 195-202].
• Bieler, R., 1992. Gastropod phylogeny and systematics. Annual Review of Ecology and Systematics, 23: 311-338.
• Verena Vonnemann, Michael Schrödl, Annette Klussmann-Kolb and Heike Wägele (2005). "Reconstruction of the phylogeny of the Opisthobranchia (Mollusca: Gastropoda) by means of 18s and 28s rRNA gene sequences". Journal of Molluscan Studies. 71 (2): 113–125. doi:10.1093/mollus/eyi014.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

• Born to be Wild: Sea Slugs, 04/30/2008