Taxonomy

Classification of the gastropod molluscs in the three traditional subclasses of the Prosobranchs (marine snails), Opisthobranchs (sea slugs and sea hares among others) and Pulmonates (land snails and land slugs) has ruled from the first third of the 20th century until very recently, and still remains in most Zoology textbooks.

Modern phylogenetic evaluation techniques, based on morphological, embryological, molecular and combined characters, have allowed to demonstrate that the phylogenies of these groups are not that simple and there has been a very significant change in different groups when considering evolutionary independent lines (monophyletic).

For these reasons some taxa have been abandoned, like the traditional subclass of the Prosobranchs (abandoned as paraphyletic) that has now been divided into the subclasses Patellogastropoda, Vetigastropoda, Cocculiniformia, Neritimorpha, Caenogastropoda and the small group of Neomphalina.

The concepts of Opisthobranchs and Pulmonates (until now Infraclases of the Heterobranchia, joined under the term Euthyneura based on the structure of their nervous system) have been replaced by the following groupings with category of Subterclass: Acteonimorpha (grouping the traditional Acteonoidea and Rissoelloidea ), Tectipleura (grouping the traditional Euopisthobranchia and Pulmonata) and Ringipleura (grouping Nudipleura and Ringiculimorpha).

The bases of this new classification are in the enormous taxonomic work by Bouchet & Rocroi (2005), recently updated by Bouchet et al. (2017) and followed by World Register of Marine Species (WoRMS).

The taxonomic synopsis of each species descriptive sheets at OPK-Opisthobranchs are basically according to these ideas.

The current taxonomic scheme of the gastropods would be as follows:

  • Class Gastropoda Cuvier, 1795
    • Subclass Caenogastropoda
    • Subclass Cocculiniformia
    • Subclass Neomphalina
    • Subclass Neritimorpha
    • Subclass Patellogastropoda
    • Subclass Vetigastropoda
    • Subclass Gastropoda incertae sedis †
    • Subclass Heterobranchia

Except for Heterobranchia, the previous subclasses correspond to groups of former “Prosobranchs”, preferably marine snails, of separated genders, with pallial cavity and typical gills. The groups marked with the dagger (†) are fossil taxa.

  • Subclass Heterobranchia Burmeister, 1837: Includes the Gastropoda that have a gill origin different from that of the other Gastropods (former Prosobranchs), and comprises species marine, freshwater and terrestrial snails.            
    • Infraclass “lower Heterobranchia: includes shelled species of marine and freshwater snails, of still uncertain relationships, which had traditionally been considered prosobranchs or opisthobranchs. Until recently it also included the superfamilies Acteonoidea, Pyramidelloidea and Ringiculoidea (for some time they have not been considered opisthobranchs) but now these superfamilies are considered again within the Euthyneura and therefore no longer belong to this group, actually formed by:
      • Superfamily Architectonicoidea Gray, 1850
      • Superfamily Cimoidea Warén, 1993
      • Superfamily Mathildoidea Dall, 1889
      • Superfamily Murchisonelloidea T. L. Casey, 1904
      • Superfamily Omalogyroidea G.O. Sars, 1878
      • Superfamily Orbitestelloidea Iredale, 1917
      • Superfamily Rhodopoidea Ihering, 1876
      • Superfamily Valvatoidea Gray, 1840
    • Infraclass [unassigned] Heterobranchia: includes a few groups of the fossil register whose adscription to the other groups is not clear.
      • Superfamily Acteonelloidea Gill, 1871 †
      • Family Dolomitellidae Bandel, 1994 †
      • Family Kuskokwimiidae Frýda & Blodgett, 2001 †
      • Family Misurinellidae Bandel, 1994 †
      • Superfamily Nerineoidea Zittel, 1873 †
      • Superfamily Streptacidoidea Knight, 1931 †
    • Infraclass Euthyneura, includes those that were traditionally considered Opisthobranchs (slugs almost exclusively marine, with or without shell) and Pulmonates (terrestrial and marine slugs and snails).
      • Subterclass Acteonimorpha
        • Superfamily Acteonoidea d’Orbigny, 1843, considered until recently within Cephalaspidea sensu lato or as Architectibranchia.
          • Family Acteonidae d’Orbigny, 1843
          • Family Aplustridae Gray, 1847
          • Family Bullinidae Gray, 1850
          • Family Cylindrobullinidae Wenz, 1938 †
          • Family Tubiferidae Cossmann, 1895 †
          • Family Zardinellidae Bandel, 1994 †
        • Superfamily Rissoelloidea Gray, 1850
          • Family Rissoellidae Gray, 1850
      • Subterclass Ringipleura
        • Superorder Nudipleura, its members share characters of the circulatory system.
        • Superorder Ringiculimorpha, considered until recently within Cephalaspidea sensu lato or as Architectibranchia. The most primitive of the group.
          • Superfamily Ringiculoidea Philippi, 1853
            • Family Ringiculidae Philippi, 1853
      • Subterclass Tectipleura (former Euopisthobranchia + Panpulmonata)
        • Superorder Acochlidiimorpha
          • Superfamily Acochlidioidea Küthe, 1935
          • Superfamily Parhedyloidea Thiele, 1931
        • Order Aplysiida, former Anaspidea (=Aplysiomorpha)
          • Superfamily Akeroidea Mazzarelli, 1891
          • Superfamily Aplysioidea Lamarck, 1809
        • Order Cephalaspidea, former Bullomorpha
          • Superfamily Bulloidea Gray, 1827
          • Superfamily Cylichnoidea H. Adams & A. Adams, 1854
          • Superfamily Haminoeoidea Pilsbry, 1895
          • Superfamily Newnesioidea Moles, Wägele, Schrödl & Avila, 2017
          • Superfamily Philinoidea Gray, 1850 (1815)
        • Order Pteropoda, of pelagic habits, they can swim thanks to body extensions that they use as swimming fins.
          • Suborder Pseudothecosomata, with a cartilaginous shell, feeds on phytoplankton.
          • Suborder Euthecosomata, with a calcified shell, feeds on phytoplankton.
          • Suborder Gymnosomata, without a shell, carnivorous, feeds on Euthecosomata and Pseudothecosomata.
        • Order Runcinida, without differentiation between head and body.
          • Superfamily Runcinoidea H. Adams & A. Adams, 1854
        • Superorder Sacoglossa
        • Order Umbraculida, former Notaspideans
        • Superorder Eupulmonata, this superorder and the following groups includes species of snails and slugs, preferably terrestrial or freshwater, hermaphroditic and whose pallial cavity has become a lung without gills whose epithelium exchanges gases with the outside.
        • Superorder Hygrophila
        • Superorder Pylopulmonata
        • Superorder Siphonarimorpha
        • Order Tectibranchiata
      • Superorder [unassigned] Euthyneura includes those species that could not be clearly assigned to other groups.

Jörger et al. (2010) carried out a phylogenetic analysis of the small group of Acochlidia to investigate their origin and their relationships with the other Heterobranchia groups. After a short time, Schrödl et al. (2011) analyzed the phylogeny of the Euthyneura based on molecular analysis and obtained results that substantially modified the established taxonomic and phylogenetic schemes. These works were the trigger -taking advantage of the great advances in molecular techniques and their popularization- that started the long journey of discovering the relationships among these gastropods in the last decade.

Due to the complexity of the problem, we must emphasize that there are still doubts in many points of the phylogenetic cladogram, more molecular analysis are necessary to confirm or discard previous statements. The latest published works, such as Bouchet et al. (2017) try to synthesize and consolidate all the researcher’s discoveries in this complex matter, of which the “last say” has not been said… yet.

The “classic” taxonomic classification, considered valid until the work by Bouchet et Rocroi (2005) can be found here:

Bibliography