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Sea butterflies
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
(unranked): clade Heterobranchia
informal group Opisthobranchia
clade Thecosomata
Blainville, 1824


Sea butterflies, also known as Thecosomata or flapping snails, are a taxonomic suborder of small pelagic swimming sea snails. These are holoplanktonic opisthobranch gastropod mollusks in the informal group Opisthobranchia.

This group used to be known as pteropods. This term, however, is no longer scientifically precise, but is still used sometimes as a convenience. The word pteropod applies both to the sea butterflies in the clade Thecosomata and also to the sea angels in the clade Gymnosomata. The Thecosomata have a shell, while the Gymnosomata lack a shell.

"Holoplanktonic" means that these snails spend their whole life in a planktonic form, rather than just being planktonic during the larval stage, as is more commonly the case in many marine gastropods, whose veliger larvae are part of the meroplankton.

This is, geologically-speaking, rather a young group, having evolved from the Late Paleocene in the Cenozoic Era.


Description and life habits

These snails float and swim freely in the water, and are carried along with the currents. This has led to a number of adaptations in their bodies. The shell and the gill have disappeared in several families. Their foot has taken the form of two wing-like lobes, or parapodia, which propel this little animal through the sea by slow flapping movements. At times, they just float along, ventral-side up, with the currents. They are rather difficult to observe, since the shell (when present) is mostly colorless, very fragile and usually less than 1 cm in length. Their calcareous shells are bilaterally symmetric and can vary widely in shape: coiled, needle-like, triangular, globulous.

Little is known about the behaviour of sea butterflies, but they are known to have a peculiar way of feeding. They are mostly passive plankton feeders, but at times they can be real hunters. They entangle planktonic food through a mucous web that can be up to 5 cm wide, many times larger than themselves. If disturbed, they abandon the net and flap slowly away. When descending to deeper water, they hold their wings up. Sometimes, they swarm in large numbers and can be found washed up in flotsam especially along the coast of eastern Australia.

Every day, they migrate vertically in the water column, following their planktonic prey. At night they hunt at the surface and return to deeper water in the morning.

Importance in the food chain

These creatures, which are about the size of a lentil, are eaten by various marine species, including a wide variety of fishes that are, in turn, consumed by penguins and polar bears. Researcher Gretchen Hofmann, calls them the "potato chip" of the ocean. She says that as the ocean becomes more acidic and warmer, these creatures are not able to survive. "It's possible by 2050 they may not be able to make a shell anymore. If we lose these organisms, the impact on the food chain will be catastrophic." [1]

Popular culture

"Sea butterflies" make an appearance in the video games Animal Crossing: Wild World and Animal Crossing: City Folk, where they may be caught in the ocean. However, the organisms shown appear to be sea angels.


Ponder & Lindberg

Order Thecosomata de Blainville, 1824

Bouchet & Rocroi

In the new taxonomy of Bouchet & Rocroi (2005) Thecosomata is treated differently :

Clade Thecosomata : [2]

  • Superfamily Cavolinioidea Gray, 1850 ( = Euthecosomata)
    • Family Cavoliniidae Gray, 1850 (1815)
      • Subfamily Cavoliinae Gray, 1850 (1815) (formerly Hyalaeidae Rafinesque, 1815 )
      • Subfamily Clioinae Jeffreys, 1869 (formerly Cleodoridae Gray, 1840 - nomen oblitum)
      • Subfamily Cuvierininae van der Spoel, 1967 (formerly : Cuvieriidae Gray, 1840 (nom. inv.); Tripteridae Gray, 1850 )
      • Subfamily Creseinae Curry, 1982
    • Family Limacinidae Gray, 1840 (formerly : Spirialidae Chenu, 1859 ; Spiratellidae Dall, 1921 )
    • † Family Sphaerocinidae A. Janssen & Maxwell, 1995
  • Superfamily Cymbulioidea Gray, 1840 ( = Pseudothecosomata)
    • Family Cymbuliidae Gray, 1840
      • Subfamily Cymbuliinae Gray, 1840
      • Subfamily Glebinae van der Spoel, 1976
    • Family Desmopteridae Chun, 1889
    • Family Peraclidae Tesch, 1913 (formerly Procymbuliidae Tesch, 1913

The superfamily Limacinoidea becomes redundant and the family Limacinidae becomes part of the superfamily Cavolinioidea. The families Creseidae and Cuvierinidae become the subfamilies Creseinae and Cuvierininae. The infraorder Pseudothecosomata becomes the superfamily Cymbulioidea. The superfamily Peraclidoidea becomes redundant and the family Peraclididae is included in the superfamily Cymbulioidea as the family Peraclidae.


  1. ^ Climate Change Seen Turning Seas Acidic, 93106, March 3, 2008, University of California Santa Barbara.
  2. ^ van der Spoel, S. (1976). Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht: Bohn, Scheltema & Holkema. pp. 484 pp..  


  • A.W.H. Bé and R.W. Gilmer. 1977. A zoogeographic and taxonomic review of euthecosomatous pteropoda. Pp. 733-808 In: Oceanic Micropaleontology, Vol. 1. A.T.S. Ramsey (ed.). Academic Press, London.
  • S. van der Spoel, 1967. Euthecosomata, a group with remarkable developmental stages (Gastropoda, Pteropoda). Gorinchem (J. Noorduijn)(thesis University of Amsterdam):375 pp., 17 tabs, 366 figs
  • S. van der Spoel, 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht (Bohn, Scheltema & Holkema): 484 pp., 246 figs.
  • Cainozoic Research, 2(1-2): 163-170, 2003: regarding the raising of ranks.

See also

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