The Prolecithophora are an order consisting of an estimated 300 species of small (typically 0.2 – 12 mm, one species up to 50 mm), active, aquatic . 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 Touch and chemoreceptor senses. With few exceptions, species are 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
. Some species, especially those living in freshwater, are
and
scavengers, but many marine species are associated with colonial animals such as
or live as
symbiosis 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
endemism 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 & Jondelius
[Noren 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 Province, 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
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Plagiostomidae - mainly marine, a few freshwater species (e.g. Vänern, Lake Biwa and Lake Tanganyika)
-
Protomonotresidae Reisinger, 1924 - exclusively freshwater species from Lake Baikal, with one exception
-
Pseudostomidae Graff, 1904-08 - exclusively marine species
-
Allostoma Beneden, 1861
-
Cylindrostoma Ørsted, 1845
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Einarhelmins Karling, 1993
-
Enterostomula Reisinger, 1926
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Euxinia Graff, 1911
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Gonostomula Westblad, 1955
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Monoophorum Bohmig, 1890
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Pregermarium Stirewalt, Ferguson & Kepner, 1942
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Pseudostomum Schmidt, 1848
-
Reisingeria Westblad, 1955
-
Thallagus Marcus, 1951
-
Ulianinia Levinsen, 1879
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Scleraulophoridae Marcus, 1950
-
Urostomatidae
