Opisthobranchia
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Cephalaspidea |
- 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 shell has undergone detorsion and they have essentially evolved back to the bilateral symmetry of their 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, warning their predators to stay away. These wonderful creatures 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 therefore a non-clade and can no longer be accepted as a taxon. They are now included in the subclass Orthogastropoda, where they have become an order. More information is given under the entry Gastropoda.
However, one can still encounter this old classification in many manuals (older and newer) and on most websites.
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
Reference
- The Sea Slug Forum (http://www.seaslugforum.net), 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. [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.