Euopisthobranchia

Tylodina fungina, copyright Ed Bierman.

Belongs within: Heterobranchia.
Contains: Aplysiidae, Akeridae, Gymnosomata, Thecosomata, Runcinidae, Umbraculidae, Cylichnidae, Diaphanidae, Retusidae, Rhizoridae, ActeocinaTornatinaBulla, Atys, Haminoea, Philinidae, Aglajidae, Philinoglossidae, Scaphandridae.

The Euopisthobranchia are a major clade of marine gastropods supported by molecular data, including the bubble shells, sea hares and pteropods. Subgroups include the Aplysiida, sea hares and related taxa, a group of soft-bodied marine gastropods with a reduced shell (external in Akeridae, internal in Aplysiidae) characterised by the presence of defensive opaline and purple glands in the mantle cavity, and an oesophageal gizzard containing an anterior chamber with large chitinous plates and a posterior chamber with numerous fine spines. The Pteropoda are planktonic gastropods that swim by means of broad lateral expansions of the foot (parapodia).

The Cephalaspidea, bubble snails and headshield slugs, are characterised by a distinctive broadening of the head to form a shield used in digging through sand. The shield bears a central pair of light-sensitive organs, and the foot may bear lateral parapodia. The Haminoeidae are a group of bubble snails that primarily graze on algal tissue and/or diatoms; the shell may be partially or entirely covered by lateral outgrowths of the mantle. The shell may be thin and fragile as in the genus Haminoea, or relatively solid and thick as in Atys.

The Umbraculoidea, umbrella shells, have a thin limpet-like shell and plume-like gills on the left side of the body. In Tylodinidae, the animal is yellow, elongate and smooth-skinned, bearing prominent tentacles and rhinophores. The Runcinoidea are minute, flattened slugs (a minute external shell may be present posteriorly) with an undivided notum covering most of the dorsum. Members of the Australasian family Ilbiidae lack a gill or shell remnant.

The beautiful angel of death
Published 14 January 2009

If you’re a tiny zooplanktic animal, that is. This is Clione limacina, the sea angel (photo copyright Alexander Semenov):

An ethereal, delicate little swimmer. Until prey passes by, in which case the tentacles come out (also copyright Alexander Semenov):

Aquatic gastropods are usually members of the benthos, and relatively few groups have made the change to a pelagic lifestyle. Those that have are invariably freakishly bizarre:

Two species of the planktic nudibranch Glaucus (G. atlanticus on the left and G. marginatus on the right). Glaucus is a member of the Aeolidioidea, and like other aeolids it feeds on cnidarians (siphonophores and chondrophores in the case of Glaucus) and sequesters their stinging cells for its own defense. Photo by Gary Cobb.

Even the superficially more normal-looking (albeit attractively coloured) violet snails of the genus Janthina (also predators of planktic hydrozoans) make up for their unassuming appearance with their bohemian lifestyle, hanging upside down from a floating raft constructed from bubbles held together with mucus:

Janthina feeding on a Portuguese man-of-war, Physalia. Photo by Bill Rudman.

The sea angel Clione belongs to one of the larger groups of pelagic gastropods, the Gymnosomata. Together with another group, the Thecosomata, they have been classified into the Pteropoda, characterised by the adaptation of the foot into a pair of ‘wings’ for swimming. Recent authors have disagreed as to whether or not the pteropods form a monophyletic group (with probably the majority favouring “not”), or whether gymnosomates and thecosomates took to the seas independently, but pteropod monophyly was supported by Klussmann-Kolb & Dinapoli (2006). Pteropods have an interesting taxonomic history. When Cuvier first established the group in 1804, he regarded them not as gastropods but as a separate order in their own right. Pteropods seem to have been regarded as a link between gastropods and cephalopods, with some believing them closer to the latter than the former. Even after the modern pteropods were well-established to be gastropods, and ‘Pteropoda’ often abandoned as a formal category, the name maintained a strange shadow existence in palaeontology, with many Palaeozoic fossil problematica such as tentaculitoids, hyoliths or even conulariids (almost meaninglessly) regarded as or compared with “pteropods”.

Within the pteropods, the most obvious difference between Gymnosomata and Thecosomata is that gymnosomates such as Clione completely lack a shell, while thecosomates retain one. And what incredible shells they are—the ultimate in lightweight, translucent construction, some thecosomates are ethereal constructions in blown glass:

Limacina helicina, photographed by Russ Hopcroft.
Systematics of Euopisthobranchia
<==EuopisthobranchiaBR17
    |--Aplysiida [Anaspidea, Aplysiacea, Aplysiina, Aplysiomorpha]BR17
    |    |--AplysiidaeBR05
    |    `--AkeridaeBR05
    |--PteropodaBR17
    |    |  i. s.: Hyalea corneaG20
    |    |--GymnosomataBR17
    |    `--ThecosomataBR17
    |--Runcinoidea [Runcinacea, Runcinida, Runcinidea, Runciniformes]BR17
    |    |--RuncinidaeBR05
    |    `--Ilbia Burn 1963 [Ilbiidae, Ilbiinae]BR05
    |         `--*I. ilbi Burn 1963BR17
    |--Umbraculoidea [Tylodinoidea, Umbraculacea, Umbraculida, Umbraculiformes, Umbraculoidei, Umbraculomorpha]BR17
    |    |--UmbraculidaeTTE93
    |    `--Tylodina Rafinesque 1814 [Tylodinacea, Tylodinana, Tylodinidae, Tylodininae, Tylodinoidei]BR05
    |         |--*T. punctulata Rafinesque 1814BR17 [incl. T. citrinaO27]
    |         |--T. corticalis (Tate 1889)MG-H11
    |         |--T. duebeniN79
    |         |--T. fungina Gabb 1865O27
    |         |--‘Parmophorus’ patelloideusS79
    |         `--T. (Tylodinella) rafinesquei Philippi 1844TTE93
    `--Cephalaspidea [Aspidocephala, Bulliformes, Bulliformii, Bulloidei, Bullomorpha, Ectoconcha]BR17
         |  i. s.: PriscaphanderJB12
         |         Cylindrotruncatum Sohl 1963KB01
         |           |--*C. demersum sohl 1963KB01
         |           `--C. caldera Kiel & Bandel 2001KB01
         |         Goniocylichna Wade 1926KB01
         |           |--*G. bisculpturata Wade 1926KB01
         |           `--G. laeviata Kiel & Bandel 2001KB01
         |--Newnesia Smith 1902 [Newnesiidae, Newnesioidea]BR17
         |    `--*N. antarctica Smith 1902BR17
         |--Cylichnoidea [Diaphanacea, Diaphanida, Diaphanoidea]BR17
         |    |--CylichnidaeBR05
         |    |--DiaphanidaeBR17
         |    |--Colinatys Ortea, Moro & Espinosa 2013 [Colinatydidae]BR17
         |    |    `--*C. alayoi (Espinosa & Ortea 2004) [=Atys alayoi]BR17
         |    |--Diaphanella Thiele 1912BR17 [=Notodiaphana Thiele 1917BR17; NotodiaphanidaeBR05]
         |    |    `--*D. fragilis (Vélain 1877) [=Bulla fragilis, *Notodiaphana fragilis]BR17
         |    |--Eoscaphander Habe 1952 [Eoscaphandridae]BR17
         |    |    `--*E. fragilis Habe 1952BR17
         |    `--Mnestia Adams & Adams 1854 [Mnestiidae]BR17
         |         `--*M. marmorata (Adams 1850) [=Bulla marmorata]BR17
         |--BulloideaBR17
         |    |--RetusidaeBR17
         |    |--RhizoridaeBR17
         |    |--TornatinidaeBR17
         |    |    |--ActeocinaBR17
         |    |    `--TornatinaBR05
         |    `--Bullidae [Bullacea]BR05
         |         |--BullaBR05
         |         `--Quibulla Iredale 1929P61, BR17
         |              |--*Q. botanica (Hedley 1918)BR17 [=Bulla botanicaP61, Bullaria botanicaF26]
         |              `--Q. quoyi (Gray 1843)P61
         |--Haminoeidae [Atyacea, Hamineidae, Hamineina, Haminoeoidea]BR05
         |    |  i. s.: LiloaK90
         |    |           |--L. brevis (Quoy & Gaimard 1833)K90
         |    |           `--L. curta (Adams 1850)MG-H11
         |    |         ‘Cylichna’ incisa Stephenson 1941TTE93
         |    |         Aliculastrum Pilsbry 1896TTE93
         |    |         Cylichnatys campanula Burn 1978K90
         |    |--AtysBR17
         |    |--HaminoeaBR17
         |    |--Bullacta Bergh 1901BR05 [Bullactidae, BullactinaeBR17, Bullactininae]
         |    |    |--*B. caurina (Benson 1856) [=Bullaea caurina]BR17
         |    |    `--B. exarataZLK11
         |    `--Smaragdinellinae [Cryptophthalmidae, Cryptophthalminae, Ophthalmidae, Smaragdinellidae]BR17
         |         |--Smaragdinella Adams 1848BR05
         |         |    `--*S. viridis (Rang in Quoy & Gaimard 1832) [=Bulla viridis]BR17
         |         `--Lathophthalmus Pruvot-Fol 1932 (see below for synonymy)BR05
         |              |--*L. olivaceus (Ehrenberg 1828) [=*Cryptophthalmus olivaceus]BR17
         |              `--‘Cryptophthalmus’ cylindricus Pease 1861K65
         `--Philinoidea [Entoconcha, Philinacea]BR05
              |--PhilinidaeBR05
              |--AglajidaeBR05
              |--PhilinoglossidaeBR05
              |--ScaphandridaeBR17
              |--Alacuppa Oskars, Bouchet & Malaquias 2015 [Alacuppidae]BR17
              |    `--*A. supracancellata (Schepman 1913) [=Atys supracancellata]BR17
              |--Colpodaspis Sars 1870 [Colpodaspididae]BR17
              |    `--*C. pusilla Sars 1870BR17
              |--Laona Adams 1865BR05 [LaonidaeBR17, Laoninae]
              |    `--*L. zonata Adams 1865BR17
              |--Philinorbis Habe 1950 [Philinorbidae]BR17
              |    `--*P. teramachii Habe 1950BR17
              `--Gastropteron Kosse 1813 (see below for synonymy)BR05
                   |--*G. meckeli Blainville 1825BR17
                   |--G. cinereum Dall 1925 [=G. pacificum var. cinereum]O27
                   |--G. coccineumG20
                   |--G. pacificum Bergh 1893O27
                   `--G. rubrumO27

Gastropteron Kosse 1813 [=Gasteropteron (l. c.), Gastropterum Agassiz 1847; Gasteropteridae, Gastropteridae, Gastropterinae, Gastropteroidae]BR05

Lathophthalmus Pruvot-Fol 1932 [=Cryptophthalmus Ehrenberg 1828 non Rafinesque 1814; Lathophthalminae]BR05

*Type species of generic name indicated

References

[BR05] Bouchet, P., & J.-P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia 47 (1–2): 1–397.

[BR17] Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

[F26] Finlay, H. J. 1926. A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320–485.

[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.

[JB12] Johnson, M. E., & B. G. Baarli. 2012. Development of intertidal biotas through Phanerozoic time. In: Talent, J. A. (ed.) Earth and Life: Global biodiversity, extinction intervals and biogeographic perturbations through time pp. 63–128. Springer.

[K65] Kay, E. A. 1965. Marine molluscs in the Cuming collection, British Museum (Natural History) described by William Harper Pease. Bulletin of the British Museum (Natural History): Zoology Supplement 1: 1–96, 14 pls.

[K90] Kendrick, G. W. 1990. A Pleistocene molluscan fauna with Anadara trapezia (Deshayes) (Bivalvia: Arcoida) from the Dampier Limestone of Shark Bay, Western Australia. In: Berry, P. F., S. D. Bradshaw & B. R. Wilson (eds) Research in Shark Bay: Report of the France-Australe Bicentenary Expedition Committee pp. 33–48. Western Australian Museum.

[KB01] Kiel, S., & K. Bandel. 2001. About Heterostropha (Gastropoda) of the Campanian of Torallola, Spain. Journal of the Czech Geological Society 46 (3–4): 319–334.

Klussmann-Kolb, A., & A. Dinapoli. 2006. Systematic position of the pelagic Thecosomata and Gymnosomata within Opisthobranchia (Mollusca, Gastropoda)—revival of the Pteropoda. Journal of Zoological Systematics and Evolutionary Research 44 (2): 118–129.

[MG-H11] McEnnulty, F. R., K. L. Gowlett-Holmes, A. Williams, F. Althaus, J. Fromont, G. C. B. Poore, T. D. O’Hara, L. Marsh, P. Kott, S. Slack-Smith, P. Alderslade & M. V. Kitahara. 2011. The deepwater megabenthic invertebrates on the western continental margin of Australia (100–1100 m depths): composition, distribution and novelty. Records of the Western Australian Museum Supplement 80: 1–191.

[N79] Norman, A. M. 1879. The Mollusca of the fiords near Bergen, Norway. Journal of Conchology 2: 8–77.

[O27] Oldroyd, I. S. 1927. The Marine Shells of the West Coast of North America vol. 2 pt 1. Stanford University Press: Stanford University (California).

[P61] Powell, A. W. B. 1961. Shells of New Zealand: An illustrated handbook 4th ed. Whitcombe and Tombs Limited: Christchurch.

[S79] Smith, E. A. 1879. Notes on the species of the genus Scutus. Journal of Conchology 2: 252–264.

[TTE93] Tracey, S., J. A. Todd & D. H. Erwin. 1993. Mollusca: Gastropoda. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 131–167. Chapman & Hall: London.

[ZLK11] Zou, S., Q. Li & L. Kong. 2011. Additional gene data and increased sampling give new insights into the phylogenetic relationships of Neogastropoda, within the caenogastropod phylogenetic framework. Molecular Phylogenetics and Evolution 61: 425–435.

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