For information on the anti-aircraft weapons system, see Sea Slug missile

In zoology, the Opisthobranchia (Milne Edwards, 1848) (also known as opisthobranchs) used to be a subclass of gastropods, within the phylum Mollusca, but they are now treated as an order.

They are highly evolved gastropods, characterised by a single gill behind the heart, from which the subclass derives its name (Greek opistho-, behind; brankhia, gills) and two pairs of tentacles.

They are principally soft-bodied marine creatures with a small or absent shell and no operculum. Their bodies have undergone detorsion, an evolutionary reversal of the 180° torsion of their immediate ancestors. They have essentially evolved back to the bilateral linear symmetry of their primitive 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 colours, warning their predators to stay away. These are hard to study, because their presence is so transitory, turning up, sometimes in very large numbers, at unexpected moments.

Members of this order include what are commonly known as sea slugs and more specifically groups such as the canoe shells, sea butterflies, sea hares, and nudibranchs.

The taxonomy of the gastropods, and their phylogenetic understanding has been evolving rapidly in the last few years. The old classification (J. Thiele 1929-1935), with the class Gastropoda divided into three subclasses Prosobranchia, Opisthobranchia and Pulmonata, is no longer accepted.

It is speculated that the Opisthobranchia may be paraphyletic (Haszprunar, 1985), having given rise to the Pulmonata, although evidence is still somewhat disputed. Pulmonata may be the 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, where they have become an order. However, one can still encounter the old classification in many manuals and on most websites. More information is given under the entry Gastropoda.

The term opisthobranch, when not describing the order, can still be used in a descriptive way, meaning "a gastropod with the gills to the right and behind the heart".

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

References

• The Sea Slug Forum, a resource from the Australian Museum.
• 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. 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.

Gastropods

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