The classification of gastropod molluscs in the traditional three subclasses of prosobranchs (sea snails), opisthobranchs (sea slugs and sea hares, among others) and pulmonates (common snails and slugs) has been widely accepted since the first third of the twentieth century until very few years ago, however it still remains, in most Zoology textbooks.

Modern phylogenetic assessment techniques based on morphological, embryological, molecular and combined have demonstrated that the phylogenies of these groups are not that simple and there has been a significant change in different groups when considering independent evolutive lines (monophyletic).

Some taxa as Prosobranchs subclass render obsolete because they are regarded as paraphyletic while others, such as Opisthobranch and Pulmonates have had their Linnaean taxonomic category changed.

Thus, traditional Prosobranchs would consist of Patellogastropoda, Vetigastropoda, Cocculiniformia, Neritimorpha, Caenogastropoda subclasses and the small group Neomphalina while Opisthobranchs and Pulmonates (sometimes grouped together under the term Euthineures, based on the structure of their nervous system) would become infraclasses of the Heterobranchia subclass, a taxon created in 1985 by Haszprunar.

The basis of this new classification are in the enormous taxonomic work made by Bouchet & Rocroi (2005) and which is followed, with some adjustments, by the World Register Marine Species ( WORMS ) and the Check List of European Marine Mollusca ( CLEMAM ).

On our own website OPK-Opisthobranchs, the taxonomic synopsis of each species fact sheets agrees with these ideas.

The gastropods current taxonomic scheme would result as follows:

  • Class Gastropoda Cuvier, 1795
    • Subclass Caenogastropoda Cox, 1960
    • Subclass Cocculiniformia Haszprunar, 1987
    • Subclass Vetigastropoda Salvini-Plawen, 1980
    • Subclass Patellogastropoda Lindberg, 1986
    • Subclass Neritimorpha Golikov & Starobogatov, 1975
    • Subclass Neomphalina McLean, 1981

The above subclasses correspond to groups of “old Prosobranchs”, mainly marine snails with separate genders, paleal cavity and typical gills.

  • Subclass Heterobranchia   Haszprunar, 1985: gastropods having a gill with a different origin from that of the other gastropods (former Prosobranchs), includes several species of snails and sea slugs, both terrestrial and freshwater.
    • Infraclass “lower Heterobranchia” or basal Heterobranchs (=Architectibranchia; =Heterostropha in part), includes shelled species of marine and freshwater snails, of still uncertain relations, that had been considered traditionally as Opisthobranchs or Prosobranchs.
      • Superfamily   Acteonoidea d’Orbigny, 1843, formerly considered as Cefalaspideans sensu lato or Architectibranchia
        • Family Acteonidae d’Orbigny, 1843
        • Family Aplustridae Gray, 1847
        • Family Bullinidae Gray, 1850
      • Superfamily Pyramidelloidae Gray, 1840, formerly considered as Opisthobranch or Prosobranchs
      • Superfamily Ringiculoidea Philippi, 1853, formerly considered as  Cefalaspideans sensu lato or Architectibranchia
      • plus other superfamilies of “old Prosobranchs”
    • Infraclass Opisthobranchia Milne-Edwards, 1848: includes marine species, hermaphrodites, with outer shell, inner shell or no shell at all in the adult stage, in which the visceral mass has undergone a partial detorsion to the right. In the section concerning each of the different orders is is given a more detailed taxonomic classification.
      • Order Acochlidiacea (= Acochlidia) Odhner, 1937
      • Order Anaspidea Fischer, 1883
      • Order Cephalaspidea P. Fischer, 1883
      • Order Runcinacea Burn, 1963
      • Order Gymnosomata Blainville, 1824 (Pteropods)
      • Order Thecosomata Blainville, 1824 (Pteropods)
      • Order Nudibranchia Cuvier, 1817
      • Order Pleurobranchomorpha Férussac, 1822 (exNotaspideans)
      • Order Umbraculida Dall, 1899 (exNotaspideans)
      • Order Sacoglossa Ihering, 1876
    • Infraclass Pulmonata Cuvier in Blainville, 1814: includes species of snails and slugs, preferably terrestrial or freshwater species, hermaphrodites and whose paleal cavity has become a lung, without gills, where the epithelium performs the gas exchange with the outside.

Recently Jörger et al. (2010) have conducted a phylogenetic analysis of the small group of Acochlidia to glimpse their origin and their relations with the other groups of Heterobranchia.

Also Schrödl et al. (2011) have analyzed Euthineures phylogeny based on molecular analysis and have obtained results that substantially alter the most recent taxonomic and phylogenetic schemes.

The diagram below is the result of these studies.

Euthinerans phylogenetic scheme according to Jörger et al. (2010)

From these studies we can draw the following conclusions:

  • Opisthobranchs are not a monophyletic group.
  • The Acteonoideans along with other basal Heterobranchia are the sister group of Euthineurans.
  • The Nudibranchia along with Pleurobranchomorpha form the monophyletic group of Nudipleura (true sea slugs), which is basal to all other Euthineurans.
  • In Euthineurans there are also two large groups or clades, one that would include most of the traditional opisthobranchs (Umbraculoidea, Runcinacea, Anaspidea, Pteropoda and Cephalaspidea sensu stricto) that they call Euopisthobranchia and one they call Panpulmonata that would house, in addition to the true Pulmonates (Eupulmonata), other groups like the Pyramidelloidae and the traditional opisthobranchs groups of Sacoglossa and Acochlidia.
  • The traditional concept of the taxon Opisthobranchia should be abandoned.

The latest of these studies and the doubts that still exist in some parts of the phylogenetic cladogram states that more molecular analysis are necessary to confirm or not the above statements, particularly those regarding the composition of the two clades of Euopisthobranchia and Panpulmonata.

“Classical” taxonomic classification, considered valid until the last published revision of Iberian opisthobranchs (Cervera et al., 2004) is as follows: