The Pseudothecosomata are pelagic, swimmer and holoplanktonic opisthobranchs that spend their entire lives in planktonic phase. Their name refers to having the body protected by a cartilaginous shell or theca (case in Greek). As an adaptation to pelagic life, the pseudothecosomata have foot expansions that allows them to use as swimming fins. For this reason they are commonly called Sea butterflies.

The cartilaginous consistency shell is called pseudoconch and is considered internal because it is covered by a thin epithelium; this pseudoconch is semitransparent and has a curled shape that allows to accommodate the animal’s visceral mass. Some species of the Desmopteridae family lack any kind of shell.

Corolla spectabilis, a typical Pseudothecosomata


The head of the pseudothecosomata has a pair of tentacles, called rhinophores, for olfactory purposes. The mantle cavity is well developed in this group but only a few species, such as genus Peracle, have a gill. Other species can breathe through the mantle epithelium. The radula has 3 to 5 teeth per row and digestive tract has some sort of masticatory plates to grind the eaten food.

Pseudothecosomata feed on plankton captured with the following system: epithelia and mantle fins secrete large amount of mucus forming an extensive mucous network up to 2 m long floating around the animal, food particles are retained in this network and the animal simply collects and swallows the mucous network from time to time.

Pseudothecosomata, as many marine plankton species, usually have a wide geographic distribution that depends largely on ocean currents. They can be found from polar waters to tropical seas but there are frequent changes in size and morphology of many species related to their geographical location, which has led to consider forms or subspecies. On the Catalan coast are mainly cited from empty pseudoconchs stranded on beaches after storms or occasionally collected by commercial trawlers.

Actual classification of Pseudothecosomata, according to the last work by Bouchet et al.(2017) is:

  • Suborder Pseudothecosomata
    • Superfamily Cymbulioidea Gray, 1840
      • Family Cymbuliidae Gray, 1840
        • Subfamily Cymbuliinae Gray, 1840
          • Genus Cymbulia Péron & Lesueur, 1810
        • Subfamily Glebinae van der Spoel, 1976
          • Genus Corolla Dall, 1871
          • Genus Gleba Forsskål in Niebuhr, 1776
      • Family Desmopteridae Chun, 1889
        • Genus Desmopterus Chun, 1889
      • Family Peraclidae Tesch, 1913
        • Genus Peracle Forbes, 1844

Some Pseudothecosomata species cited in the Mediterranean Sea or in the Iberian Peninsula:

Corolla spectabilis

Corolla spectabilis

Cymbulia peronii by Gilles Cavignaux

Cymbulia peronii

Peracle reticulata by Russell Hopcroft

Peracle reticulata


    Bouchet, P., and J. P. Rocroi. 2005. Classification and nomenclator of gastropod families. Malacologia. 47(1-2): 1-397 ISBN 3-925919-72-4.
    Bouchet, P., J. P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl, and E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61(1-2): 1-526.

    Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.