Prolecithophora is an order consisting of an estimated 300 species of small (typically 0.2 – 12 mm, one species up to 50 mm), active, aquatic flatworms. The order lacks a common English name. Most species are shaped like an elongated, stylized droplet, and are opaque white or yellow; they frequently have contrasting bands or spots in colors, such as purple, yellow, red, or brown. They have no to three (normally two) pairs of pigment-cup eyes, and well-developed tactile and chemoreceptor senses. With few exceptions, species are protandric hermaphrodites with internal fertilization. Egg capsules are, according to species, glued to various hard surfaces; the young hatch as miniature copies of their parents.Karling, T. G. 1940. "Zur Morphologie und Systematik der Alloeocoela Cumulata und Rhabdocoela Lecithophora (Turbellaria)". Acta Zoologica Fennica, 26, 1–260.
Ecology All prolecithophorans are aquatic, with most living in the oceans. Some species, especially those living in freshwater, are predators and scavengers, but many marine species are associated with colonial animals such as bryozoans or live as symbionts on larger animals such as urchins; a few species harbor symbiotic algae. Although most are accomplished swimmers, they normally rarely venture far from the bottom; young specimens are sometimes found in plankton. Many species display positive or negative phototaxis.Noren M. 2002. Phylogeny and classification of prolecithophoran flatworms. Doctoral thesis at Stockholm University. .
Distribution The majority of prolecithophorans live in the oceans. There are also brackish and freshwater species; in Lake Baikal, an adaptive radiation has resulted in a number of endemic species. The order has a cosmopolitan distribution; most described species are from temperate waters, while little is known about tropical or deep-sea species. This is probably due to a sampling artefact, as prolecithophorans are known to be common in the tropics; when Norén & JondeliusNoren M, Jondelius U. 2004. "Four new species of the family Plagiostomidae Graff, 1882, (Prolecithophora, "Platyhelminthes") from Phuket, Thailand". Phuket Marine Biological Center Research Bulletin 65:2 sampled the shore adjacent to Phuket Marine Biological Center, Phuket, Thailand, they found 14 species of prolecithophorans, all of which were new to science.
Families The following families are recognised in the order Prolecithophora: Multipeniatidae Karling, 1940 Multipeniata Nasonov, 1927 Plagiostomidae - mainly marine, a few freshwater species (e.g. Vänern, Lake Biwa and Lake Tanganyika) Acmostomum Schmarda, 1859 Auriculifera Kulinitch, 1973 Hydrolimax Haldeman, 1842 Plagiostomum Schmidt, 1852 Plicastoma Graff, 1904 Puzostoma Marcus, 1950 Torgea Jondelius, 1997 Tuilica Marcus, 1951 Vorticeros Schmidt, 1852 Protomonotresidae Reisinger, 1924 - exclusively freshwater species from Lake Baikal, with one exception Baicalarctiinae Friedman, 1933 Baicalarctia Friedmann, 1926 Friedmaniella Timoshkin & Zabrovskaya, 1985 Porfirievia Timoshkin, 1997 Protomonotresinae Timoshkin, 1986 Acanthiella Rieger & Sterrer, 1975 Archimonotresis Meixner, 1938 Prorogonophora Riedl, 1954 Protomonotresis Reisinger, 1924 Pseudostomidae Graff, 1904-08 - exclusively marine species Allostoma Beneden, 1861 Cylindrostoma Ørsted, 1845 Einarhelmins Karling, 1993 Enterostomula Reisinger, 1926 Euxinia Graff, 1911 Gonostomula Westblad, 1955 Monoophorum Bohmig, 1890 Pregermarium Stirewalt, Ferguson & Kepner, 1942 Pseudostomum Schmidt, 1848 Reisingeria Westblad, 1955 Thallagus Marcus, 1951 Ulianinia Levinsen, 1879 Scleraulophoridae Marcus, 1950 Rosmarium Marcus, 1950 Scleraulophorus Karling, 1940 Urostomatidae
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