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.
Classification
Actual classification of Pseudothecosomata, according to the last work by Bouchet et al.(2017) is:
Pseudothecosomata Meisenheimer, 1905
- Cymbulioidea Gray, 1840
- Cymbuliidae Gray, 1840
- Cymbuliinae Gray, 1840
- Cymbulia F. Péron & Lesueur, 1810
- Glebinae van der Spoel, 1976
- Corolla Dall, 1871
- Gleba Forsskål, 1776
- Cymbuliinae Gray, 1840
- Desmopteridae Chun, 1889
- Desmopterus Chun, 1889
- Peraclidae Tesch, 1913
- Peracle Forbes, 1844
- Cymbuliidae Gray, 1840
Some Pseudothecosomata species cited in the Mediterranean Sea or in the Iberian Peninsula:
Bibliography
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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.
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Bouchet, P., Rocroi, J. P., Hausdorf, B., Kaim, A., Kano, Y., Nützel, A., Parkhaev, P. et al. (2017). Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia, 61(1-2), 1-526.
