The euthecosomata are pelagic, swimming holoplanktonic opisthobranchs that spend their entire lives in planktonic phase. Their name refers to having the body protected by a calcified cover or theca (case in Greek) . As an adaptation to pelagic life, the euthecosomata have foot expansions that allows them to use as swimming fins. For this reason they are commonly called Sea butterflies.
The calcified shell could be spirallled such as in species of the genus Limacina or shaped like a conical or pyramidal tube, as in the species of the genera Cavolinia and Clio, but the shell is always fragile, with low consistency, to reduce the weight of the animal and facilitate its suspension in water. They may also have a corneous operculum, usually with spines, used to completely enclose and protect the animal inside the shell.
Styliola subula, an Euthecosomata
The head of the euthecosomata has a pair of tentacles for olfactory purposes called rhinophores. The mantle cavity is well developed in this group but generally they don’t have a gill, instead they breathe through the mantle epithelium. The radula has 3 to 5 teeth per row and the digestive tract has some sort of masticatory plates to grind the eaten food.
Biology
The euthecosomata are great swimmers and it has been calculated that they can swim at a speed of up to 45 cm per second. Daily vertical migrations are common in most species of euthecosomata. Some epipelagic species as Clio pyramidata perform daily vertical migrations in the water column of up to 1,000 m, so during the day they live at approximately this depth while every night climb to more superficial levels for feeding. In other species it has been observed that while adults are found in deep waters, juveniles tend to live in shallow waters.
The euthecosomata are microphages that feed on microscopic plankton (dinoflagellates, diatoms, radiolarians, foraminifera, etc.) that are being directed to the mouth with the swimming fins and cilia on the epithelium. Phytoplankton food is retained by mucus, forming a cord that goes into the mouth.
Euthecosomata are hermaphrodites and when two individuals come close together they have a mutual intercourse in which both fertilize each other. After fertilization the eggs are released into the water immersed in gelatinous or mucous substance. The ovigerous capsules hatching may produce shelled or unshelled veliger larvae. The larval stage can last from 45 to 90 days and the adults live between 1 to 2 years.
The progressive acidification of marine waters as a result of increased athmospheric CO2 levels affect very negatively the survival rate of euthecosomata (they can’t grow a proper shell) and there is a theory that because of this reason, the southern seas will soon be uninhabitable for these testacean species if the present trend continues.
Distribution
The Euthecosomata, 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 many forms or subspecies. Euthecosomata of the Catalan coast are mainly cited from empty shells or pseudoconchs stranded on beaches after storms or occasionally collected by commercial trawlers.
Classification
There are many euthecosomata species known worldwide. They are classified mainly by the shape of the shells, while other traits such as the shape of the fins, mantle or oral appendages are secondary.
The actual taxonomy of Euthecosomata as proposed by Bouchet et al. (2017) and published in WoRMS is:
- Suborder Euthecosomata
- Superfamily Cavolinioidea Gray, 1850 (1815)
- Family Cavoliniidae Gray, 1850 (1815)
- Genus Cavolinia Abildgaard, 1791
- Genus Diacavolinia van der Spoel, 1987
- Genus Diacria J.E. Gray, 1840
- Genus Diacrolinia A. W. Janssen, 1995 †
- Genus Gamopleura Bellardi, 1873 †
- Genus Praehyalocylis Korobkov, 1962 †
- Genus Vaginella Daudin, 1800
- Family Cliidae Jeffreys, 1869
- Genus Clio Linnaeus, 1767
- Genus Praehyalocyclis Korobkov & Makarova, 1962
- Family Creseidae Rampal, 1973
- Genus Bovicornu O. Meyer, 1886 †
- Genus Camptoceratops Wenz, 1923 †
- Genus Creseis Rang, 1828
- Genus Euchilotheca P. Fischer, 1882 †
- Genus Hyalocylis Fol, 1875
- Genus Styliola Gray, 1847
- Genus Thecopsella Munier-Chalmas, 1888 †
- Genus Tibiella O. Meyer, 1884 †
- Family Cuvierinidae van der Spoel, 1967
- Genus Cuvierina Boas, 1886
- Genus Ireneia A. W. Janssen, 1995 †
- Family Sphaerocinidae A. W. Janssen & Maxwell, 1995 †
- Genus Sphaerocina Jung, 1971 †
- Superfamily Limacinoidea Gray, 1840
- Family Limacinidae Gray, 1840
- Genus Altaspiratella Korobkov, 1966 †
- Genus Currylimacina A. W. Janssen, 2003 †
- Genus Heliconoides d’Orbigny, 1836
- Genus Limacina Bosc, 1817
- Genus Skaptotion Curry, 1965 †
- Genus Striolimacina A. W. Janssen, 1999 †
- Genus Thielea Strebel, 1908
Some Euthecosomata species cited in the Mediterranean Sea or in the Iberian Peninsula:
Bibliography
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Bouchet P, Rocroi JP. 2005. Classification and nomenclator of gastropod families. Malacologia. 47(1–2):1–397.
Bouchet P, Rocroi JP, Hausdorf B, et al. 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.
Styliola subula, an Euthecosomata
