Monoplacophora
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 Neopilina sp.
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Monoplacophora is a class or, more likely, polyphyletic group of shelled mollusks (the name "Monoplacophora" means 'bearing one plate'). Previously, these organisms were known only from the fossil record, and thought to have become extinct in mid-Devonian times, until in April 1952 a living specimen was collected from deep marine sediments in the Middle America Trench off the Pacific coast of Costa Rica. In 1957 it was described and named Neopilina galatheae by its discoverer, Danish biologist Dr. Henning M. Lemche (1904-1977) [1]. So far, more than two dozen living species of Monoplacophora have been discovered; the first to be photographed live was Vema hyalina, at a depth of 400 m off Catalina Island, California, in 1977.[2] All the present species live deep down in the abyssal depths of ocean trenches. An attempt at a common name, gastroverm, has proved unsuccessful.[3]
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[edit] Anatomy, ecology, and systematics
Little is known about the monoplacophora. They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres. The apex of the shell is forward. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism; indeed, with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation, which suggested a "missing link" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary. All known mollusks are thus non-segmented, and a derivation from annelids, which are always segmented, is very unlikely. The ancestors of mollusks were maybe flatworms.
Monoplacophorans move on a rounded foot. Respiration is through five or six pairs of gills on either side of the body. Their reduced head lacks eyes or tentacles. It is presumed that they graze on microscopic organisms in mud or bottom detritus. They are apparently a widespread component of the benthos, having been dredged from depths of between 2000 and 6500 meters in the South Atlantic, the Gulf of Aden, the East Pacific, and the Southern Ocean off Antarctica.
In 2006 a new molecular study [4] on Laevipilina antarctica revealed that Monoplacophora and Polyplacophora form a well-supported clade with the researched Neopilina closest to the chitons. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated gills and eight sets of dorsoventral pedal retractor muscles. This study goes against previous cladistic hypotheses that the Monoplacophora are the sister group to the remainder of the conchiferans [5], [6], [7]
[edit] Taxonomy
Order Tryblidiida
- Family Laevipilinidae
- Genus Laevipilina J. H. McLean, 1979
- Species Laevipilina antarctica Warén & Hain, 1992
- Species Laevipilina cachuchensis Urgorri, García-Alvarez & Luque, 2005
- Species Laevipilina hyalina J. H. McLean, 1979
- Species Laevipilina rolani Warén & Bouchet, 1990
- Species Laevipilina theresae Schrödl, 2006
- Genus Laevipilina J. H. McLean, 1979
- Family Micropilinidae
- Genus Micropilina Warén, 1989
- Species Micropilina arntzi Warén and Hain, 1992
- Species Micropilina minuta Warén, 1989
- Species Micropilina rakiura Marshall, 1998
- Species Micropilina reingi Marshall, 2006
- Species Micropilina tangaroa Marshall, 1992
- Species Micropilina wareni Marshall, 2006
- Genus Micropilina Warén, 1989
- Family Monoplacophoridae
- Genus Monoplacophorus Moskalev, Starobogatov & Filatova, 1983
- Species Monoplacophorus zenkevitchi Moskalev, Starobogatov & Filatova, 1983
- Genus Monoplacophorus Moskalev, Starobogatov & Filatova, 1983
- Family Neopilinidae
- Genus Adenopilina Starobogatov & Moskalev, 1987
- Species Adenopilina adenensis (Tebble, 1967)
- Genus Neopilina H. Lemche, 1957
- Species Neopilina bruuni Menzies, 1968
- Species Neopilina galatheae Lemche, 1957
- Species Neopilina rebainsi Moskalev, Starobogatov & Filatova, 1983
- Genus Rokopella Starobogatov & Moskalev, 1987
- Species Rokopella brummeri Goud and Gittenberger, 1993
- Species Rokopella capulus Marshall, 2006
- Species Rokopella euglypta (Dautzenberg and Fischer, 1897)
- Species Rokopella goesi (Warén, 1988)
- Species Rokopella oligotropha (Rokop, 1972)
- Species Rokopella segonzaci Warén and Bouchet, 2001
- Genus Veleropilina Starobogatov & Moskalev, 1987
- Species Veleropilina reticulata (Seguenza, 1876)
- Species Veleropilina veleronis (Menzies and Layton, 1963)
- Species Veleropilina zografi (Dautzenberg and Fischer, 1896)
- Genus Vema (Clarke & Menzies, 1959)
- Species Vema bacescui (Menzies, 1968)
- Species Vema ewingi (Clarke and Menzies, 1959)
- Species Vema hyalina
- Species Vema levinae Waren, 1996
- Species Vema occidua Marshall, 2006
- Genus Adenopilina Starobogatov & Moskalev, 1987
[edit] Notes
- ^ "New Pilina": Pilina was a monoplacophore that lived during Silurian times. Galathea was the name of the Danish research vessel that recovered it.
- ^ Lynn Margulis and Karlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (third edition (1997), p. 290).
- ^ Sara S. Bretsky, in reviewing R. Tucker Abbott's popularization, Kingdom of the Seashell in The Quarterly Review of Biology (vol. 49.1 (March 1974), p. 85) declared that she found "gastroverm" "singularly unattractive" and, by and large, writers since have tacitly agreed. "Monoplacophore" remains the common usage.
- ^ Gonzalo Giribet, Akiko Okusu, Annie R. Lindgren, Stephanie W. Huff, Michael Schrödl and Michele K. Nishiguch (May 2006). "Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons". Proceeding of the National Academy of Sciences of the United States of America 103 (20).
- ^ Scheltema, A. H. (1993) Biol. Bull 184, 57–78 Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula abstract [1]
- ^ Haszprunar, G. (2000) Am. Malacol. Bull 15, 115–130.
- ^ Salvini-Plawen, L. V. & Steiner, G. (1996) in Origin and Evolutionary Radiation of the Mollusca ed. Taylor, J. D. (Oxford Univ. Press, Oxford), pp. 29–51.
[edit] References
- Horný, Radwan 1963. On the systematic position of cyrtonelloids (Mollusca). Časopsis národního Muzea, oddil přírodovědný, 132: 90–93, Prague.
- Lemche, Henning 1957. A new living deep-sea mollusc of the Cambro-Devonian class Monoplacophora. Nature, 179: 413–416, London.
- Lemche, Henning, in Marie Jenkins. 1972. The Curious Mollusks, New York.
- Rozov, S. N. 1975. A new order of the Monoplacophora. Paleontological Journal, 9: 39–43, Washington.
- Wingstrand, Karl Georg 1985. On the anatomy and relationships of recent Monoplacophora. Galathea Report, 16: 7–94, Leiden & Copenhagen.
- V. Urgorri, O. García-Álvarez and Á. Luque (2005). "Laevipilina Cachuchensis, A New Neopilinid (Mollusca: Tryblidia) From Off North Spain". Journal of Molluscan Studies 71 (1). doi:.
- Michael Schrödl, Katrin Linse and Enrico Schwabe (August 2006). "Review on the distribution and biology of Antarctic Monoplacophora, with first abyssal record of Laevipilina antarctica". Polar Biology 29 (9).
[edit] External links
- Anatomy
- DiscoverLife: Taxonomy of Monoplacophora
- Animal Diversity Web: Monoplacophora
- Washington Nature Mapping Program: Monoplacophora
- Palaeos.com, Monoplacophora (Tryblidiida) Includes pictures and thorough discussion of cladistic issues.
