Limacina helicina, copyright Alexander Semenov.

Belongs within: Euopisthobranchia.
Contains: Cavoliniidae.

The Thecosomata, sea butterflies, are a group of pelagic marine gastropods that have the foot modified into a pair of wing-like parapodia used in swimming. Most Thecosomata retain a calcareous shell though the shell is demineralised or lost in some subgroups. Loss of the adult shell is found in the Cymbuliidae and Desmopterus within the Cymbulioidea.

Crystal butterflies of the sea
Published 26 April 2020

All chains of life in the open ocean are ultimately dependent on plankton. Photosynthetic micro-plankton convert the energy of sunlight into their own stores that are in turn commandered by animal plankton through consumption and digestion. Both animal and photosynthetic plankton provide food sources for larger animals, both plankton and nekton, and even deeper dwelling organisms may take their sustenance from the rain of planktonic corpses settling from above. A wide range of animal lineages may be identified among oceanic plankton, one of the most prominent being the gastropod group known as the Thecosomata.

An orthoconch thecosomatan, Clio pyramidata, copyright Russ Hopcroft.

The Thecosomata are small gastropods, rarely exceeding a couple of centimetres in size at their largest. Many marine molluscs will spend at least part of their lives as planktonic larvae but relatively few mollusc groups have taken the route that the Thecosomata have, remaining part of the plankton through their entire life cycle. They maintain their position in the plankton by means of broad expansions of the foot on either side of the mouth, known as parapodia. The appearance and movement of these parapodia have given the Thecosomata the vernacular name of ‘sea butterflies’. They also inspired the name Pteropoda (‘wing-foot’), used for a clade that unites the Thecosomata with another group of planktonic gastropods, the Gymnosomata. Historically, many authors have questioned the association of the pteropods, in part because of the differing dispositions of the parapodia in the two component groups (Gymnosomata, commonly known as ‘sea angels’, have the wings of the parapodia located further back on the body rather than around the mouth). Nevertheless, more recent studies have corroborated pteropod monophyly (Klussmann-Kolb & Dinapoli 2006). The Thecosomata themselves fall between two major sublineages, known as the Euthecosomata and Cymbulioidea (or Pseudothecosomata). Members of the Euthecosomata have well-divided parapodia and the viscera are contained within a delicate, translucent, calcareous shell. In some euthecosomes such as the genus Limacina, this shell is coiled like that of other gastropods, but in others the shell has become straight and bilaterally symmetrical, being conical or globular with lateral projections. Recent phylogenetic analysis suggests that the straight-shelled sea butterflies may form a single monophyletic lineage known as the Orthoconcha* (Corse et al. 2013). In the Cymbulioidea, the parapodia are fused around the front of the animal to form a single swimming plate. Of the three families of cymbulioids, the Peraclidae have a calcareous shell as in the euthecosomes. The Cymbuliidae shed the larval calcareous shell over the course of their development and replace it with a pseudoconch, a hardened gelatinous, slipper-shaped structure that is still secreted by the mantle. In the third family, the Desmopteridae, the shell has been lost entirely.

*In the early days of invertebrate palaeontology, pteropod affinities were suggested for a number of groups of early Palaeozoic conical shells of uncertain affinities, such as the hyoliths and tentaculitoids. It is worth noting that, at the time, the pteropods themselves were often thought to represent a distinct molluscan class independent of the gastropods. Such proposals have long since fallen by the wayside. Not only is there nothing to connect such Palaeozoic forms with modern pteropods but the most superficial of resemblances in overall shape to certain Orthoconcha, but all indications now are that the Orthoconcha themselves did not evolve until some time in the Cenozoic (Corse et al. 2013), leaving a gap of some hundreds of millions of years between them and their erstwhile forebears.

Cymbulia peronii, copyright Vincent Maran.

Live sea butterflies feed on a wide range of organisms, including both micro-algae and other planktonic animals. The ancestral radula is reduced or lost and prey is captured by means of a mucous web, globular in euthecosomes and a funnel-shaped sheet in cymbulioids (Gilmer & Harbison 1986). This web may be absolutely gigantic relative to the animal itself, reaching up to two meters in diameter. While the web is extended, the sea butterfly does not swim actively but hangs suspended in the water column below, drawing food into the mouth by means of tracts of cilia on lobes of the foot. A further mucous array may trail away from the animal containing faecal particles and/or particles rejected as food; this may keep such particles from re-entering the feeding web. Should the animal be disturbed, the mucous web is rapidly ingested or abandoned before swimming away. Sea butterflies have commonly been referred to as suspension feeders but Gilmer & Harbison (1986) noted that a case could potentially be made for considering them as predators. Though micro-algae make up a large proportion of thecosome gut contents, the mucous web allows them to also capture active prey such as copepods that might have otherwise eluded them. It is possible that such prey is in fact more important overall for satisfying the sea butterfly’s nutritional requirements. A number of sea butterflies possess brightly coloured mantle appendages that may lure active prey; the faecal trails may also assist in this way.

Limacina helicina, copyright Russ Hopcroft.

Sadly, any discussion of Thecosomata is forced to end on a tragic note. Recent increases in atmospheric carbon dioxide have lead to oceanic waters becoming more acidic than previously, which in turn reduces the concentration of dissolved carbonate. Because carbonate is a vital component of molluscan shells, ocean acidification compromises shell production. Studies of recent thecosome samples show that their shells have become thinner and more porous as acidification increases (Roger et al. 2012). If this trend continues, we may reach a point where shell secretion becomes impossible for these animals, leading to tragic consequences both for the thecosomes themselves and for the countless other organisms ecologically dependent on them. In recent years, concern has been expressed that ecological degredation may mean that we can no longer see butterflies flying in our gardens; their marine analogues are no less vulnerable.

Systematics of Thecosomata
<==Thecosomata [Campyloconques, Eupteropoda]
    |  i. s.: TiedemanniaBR05
    |         Bovicornu Meyer 1886CR13
    |--Cymbulioidea [Cymbuliacea, Cymbulioidei, Peraclida, Peracliformes, Peracliformii, Peracloidei, Pseudothecosomata]BR05
    |    |--Desmopterus Chun 1889CR13, BR05 [Desmopteridae]
    |    |    |--*D. papilio Chun 1889BR17
    |    |    |--D. gardinieriCR13
    |    |    `--D. pacificus Essenberg 1919O27
    |    `--+--Peraclidae [Peraclacea, Peraclididae]BR05
    |       |    |--Procymbulia Meisenheimer 1905 [Procymbuliidae]BR05
    |       |    |    `--*P. valdiviae Meisenheimer 1905BR17
    |       |    `--Peracle Forbes 1844CR13, BR05
    |       |         |--*P. physoides Forbes 1844BR17
    |       |         |--P. apicifulvaCR13
    |       |         |--P. bispinosaCR13
    |       |         |--P. depressaCR13
    |       |         |--P. moluccensisCR13
    |       |         |--P. reticulataP61
    |       |         |--P. triacanthaCR13
    |       |         `--P. valdiviaeCR13
    |       `--Cymbuliidae [Cymbuliadae, Subtestacea]BR05
    |            |--Cymbulia Péron & Lesueur 1810CR13, BR05 [Cymbuliinae]
    |            |    |--*C. proboscidea Lamarck 1816BR17
    |            |    |--C. parvidentata Pelseneer 1888P61
    |            |    |--C. peroniP61
    |            |    `--C. sibogaeCR13
    |            `--+--Gleba Forskål 1776CR13, BR05 [Glebinae]
    |               |    `--*G. cordata Forskål 1776BR17
    |               `--Corolla Dall 1871CR13, O27
    |                    |--*C. spectabilis Dall 1871O27
    |                    |--C. calceaolarisCR13
    |                    |--C. chrysostictaCR13
    |                    |--C. cupulaCR13
    |                    `--C. ovataCR13
         |--Cavolinioidea [Orthoconcha]BR05
         |    |  i. s.: Camptoceros Wenz 1929CR13
         |    |--CavoliniidaeBR05
         |    |--Sphaerocina Jung 1971 [Sphaerocinidae]BR05
         |    |    `--*S. formae (Audenino 1897) [=Limacina formae]BR17
         |    |--Praecuvierina Janssen 2005 [Praecuvierinidae]BR17
         |    |    `--*P. lura (Hodgkinson 1992) [=Cuvierina lura]BR17
         |    `--Creseis Rang 1828BR05 [CreseidaeBR17, Creseinae]
         |         |--*C. acicula Rang 1828BR17 (see below for synonymy)
         |         |--C. chierchiae (Boas 1886) [=Cleodora chierchiae]R02
         |         |--C. conica Escholtz 1829 [=C. virgula conica]R02
         |         |    |--C. c. conicaR02
         |         |    `--C. c. falciformis Rampal 2002R02
         |         `--C. virgula Rang 1828P61 [=Cleodora virgulaR02, Clio virgulaH09]
         |              |--C. v. virgulaR02
         |              `--C. v. frontieri Rampal 2002R02
         `--Limacinidae [Limacinacea, Limacinoidea, Limacinoidei, Spiratellacea, Spiratellidae, Spiriconcha]BR17
              |--Embolus Jeffreys 1869 nec Haller 1768 (ICBN) nec Batsch 1783 (ICBN)P61
              |    `--*E. inflatus (d’Orbigny 1836)P61 [=Limacina (Thilea) inflataCR13]
              |--Thielea Strebel 1908P61
              |    `--T. helicoides [=Limacina helicoides; incl. *T. procera]P61
              |--Spirialis Eydoux & Souleyet 1840 [Spirialidae]BR05
              |    |--*S. trochiformis (d’Orbigny 1834) [=Atlanta trochiformis]BR17
              |    `--S. mercinensis Watelet & Lefèvre 1885CR13
              `--Limacina Bosc 1817 [=Spiratella Blainville 1817]BR05
                   |--*L. helicina (Phipps 1774)BR17 (see below for synonymy)
                   |    |--L. h. helicinaCR13
                   |    `--L. h. antarticaCR13 [=Spiratella antarcticaO27]
                   |--‘Spiratella’ australis (Eydoux & Souleyet 1840)P61
                   |--L. balea Moeller 1841M05, P61 [=Heterofusus baleaM05, Spiratella baleaP61, Spirialis baleaM05]
                   |--L. bulimoides (d’Orbigny 1836)H09 (see below for synonymy)
                   |--L. lesueurii d’Orbigny 1836CR13, P61 [=Atlanta leseuriH09, L. (Thilea) lesueuriiCR13]
                   |--L. mercinensis (Watelet & Lefèvre 1885)TTE93
                   |--L. pacifica Dall 1871M05, O27 [=Spiratella pacificaO27]
                   |--L. rangiiB26
                   |--L. retroversa (Flemming 1823)R02 (see below for synonymy)
                   `--L. trochiformis (d’Orbigny 1836)R02 [=L. (Munthea) trochiformisCR13]

*Creseis acicula Rang 1828BR17 [=Cleodora aciculaR02, Clio aciculaH09; incl. Creseis acicula clava (Rang 1828)R02]

Limacina bulimoides (d’Orbigny 1836)H09 [=Atlanta bulimoidesH09, L. (Munthea) bulimoidesCR13, Spiratella bulimoidesP61]

*Limacina helicina (Phipps 1774)BR17 [=Clio helicinaBR17, *Spiratella helicinaP61; incl. Argonauta arctica Fabricius 1780M05, Limacina arcticaG20, Spiratella arcticaM05, L. helicialis de Lamarck 1819M05, S. limacina de Blainville 1824M05]

Limacina retroversa (Flemming 1823)R02 [=Fusus retroversusM05, Heterofusus retroversusM05, Spirialis retroversusM05; incl. Heterofusus alexandri Verrill 1872M05, Sp. australis Souleyet 1852M05, Peracle flemingi Thompson 1844M05, Sp. flemingiiM05, Sp. gouldii Stimpson 1854M05, Sp. mcandrei Forbes & Hanley 1849–1853M05, Limacina mcandreiM05, Scaea stenogyra Philippi 1844M05, Spirialis stenogyraM05]

*Type species of generic name indicated


[B26] Bigelow, H. B. 1926. Plankton of the offshore waters of the Gulf of Maine. Bulletin of the Bureau of Fisheries 40 (2): 1–509.

[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.

[CR13] Corse, E., J. Rampal, C. Cuoc, N. Pech, Y. Perez & A. Gilles. 2013. Phylogenetic analysis of Thecosomata Blainville, 1824 (holoplanktonic Opisthobranchia) using morphological and molecular data. PLoS One 8 (4): e59439.

Gilmer, R. W., & G. R. Harbison. 1986. Morphology and field behavior of pteropod molluscs: feeding methods in the families Cavoliniidae, Limacinidae and Peraclididae (Gastropoda: Thecosomata). Marine Biology 91: 47–57.

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

[H09] Hedley, C. 1909. The Marine Fauna of Queensland: Address by the President of Section D. Australasian Association for the Advancement of Science: Brisbane.

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.

[M05] Meisenheimer, J. 1905. Die arktischen Pteropoden. In: Römer, F., & F. Schaudinn (eds) Fauna Arctica. Eine Zusammenstellun der arktischen Tierformen, mit besonder Berücksichtigung des Spitzbergen-Gebietes auf Grund der Ergebnisse der Deutschen Expedition in das Nördliche Eismeer im Jahre 1898 vol. 4 pp. 407–430. Gustav Fischer: Jena.

[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.

[R02] Rampal, J. 2002. Biodiversité et biogéographie chez les Cavoliniidae (Mollusca, Gastropoda, Opisthobranchia, Euthecosomata). Régions faunistiques marines. Zoosystema 24 (2): 209–258.

Roger, L. M., A. J. Richardson, A. D. McKinnon, B. Knott, R. Matear & C. Scadding. 2012. Comparison of the shell structure of two tropical Thecosomata (Creseis acicula and Diacavolinia longirostris) from 1963 to 2009: potential implications of declining aragonite saturation. ICES Journal of Marine Science 69 (3): 465–474.

[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.

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