Opisthobranchia

Euopisthobranchia is a taxon containing a revised collection of opisthobranchs, and that taxon is considered monophyletic. Euopisthobranchia does not include some "traditional" opisthobranchs such as the Sacoglossa and the Acochlidiacea. The subclass Heterobranchia now contains all the species which used to be assigned to Opisthobranchia, plus all the species in the Pulmonata.

The subclass Opisthobranchia included species in the order Cephalaspidea (bubble shells and headshield slugs), the sacoglossans, anaspidean sea hares, pelagic sea angels, sea butterflies, and many families of the Nudibranchia.

Opisthobranch means "gills behind" (and to the right) of the heart. In contrast, Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart. With the lack of a heavily mineralized shell, there has been very little fossil record of the group. However, molecular clock studies have suggested that Opisthobranchia emerged as early as the Carboniferous.

Taxonomy

Under the 1931 classification system drawn up by Johannes Thiele, the class Gastropoda was divided into three subclasses; Prosobranchia, Pulmonata and Opisthobranchia. The latter two were later combined into a single order.

The current classification of the gastropods is more nuanced, with the following subclasses:

• Subclass Caenogastropoda
• Subclass Cocculiniformia
• Subclass Heterobranchia
• Subclass Neomphalina
• Subclass Neritimorpha
• Subclass Patellogastropoda
• Subclass Vetigastropoda
• Subclass Gastropoda incertae sedis

As a non-monophyletic taxon

It was speculated as far back as 1985 that the Opisthobranchia were paraphyletic, based on morphological evidence, and had given rise to the Pulmonata, a group also of subclass rank.

Because the Pulmonata are a sibling group to an opisthobranch taxon, some authors argued that the Opisthobranchia are therefore not a monophyletic group and can no longer be accepted as a valid taxon. The opisthobranchs are now included within the subclass Heterobranchia, although many manuals and websites still use the old classification.

A phylogenetic study published in November 2004, gave new definitions of the seven main lineages of the Opisthobranchia.

However, in 2005, a study of rRNA gene sequences could not resolve monophyly versus paraphyly of the Opisthobranchia Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid clade, reclassifying it as an informal group within the Heterobranchia. Accordingly, articles no longer use the term Opisthobranchia, replacing it with Heterobranchia to emphasise a different concept.

By 2011, the "Opisthobranchia" were declared artificial and obsolete and were replaced by new phylogenetic hypotheses. The concept was abandoned. It now includes including "Lower Heterobranchia", Acteonimorpha, Ringipleura, Umbraculida, Cephalaspidea, Runcinida, Aplysiida, Pteropoda and Sacoglossa.

Linnean taxonomy

Order Opisthobranchia Milne-Edwards, 1848 – sea slugs

• Suborder Cephalaspidea P. Fischer, 1883 – headshield slugs and bubble shells
• Suborder Sacoglossa H. von Ihering, 1876 – sap-sucking slugs and bivalved gastropods
• Suborder Aplysiomorpha P. Fischer, 1883 – sea hares
• Suborder Notaspidea P. Fischer, 1883 – sidegill slugs
• Suborder Thecosomata Blainville, 1824 sea butterflies with shells
• Suborder Gymnosomata Blainville, 1824 – sea angels, no shells
• Suborder Nudibranchia Blainville, 1814 – nudibranchs
  • Infraorder Anthobranchia Férussac, 1819
  • Infraorder Cladobranchia Willan & Morton, 1984