the encyclopedic entry of Monoplacophora

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Monoplacophora

Monoplacophora, meaning “bearing one plate”, is a class of shelled mollusks. These organisms were known only from the fossil record, ranging from the early Cambrian to the mid-Devonian periods (ca. 550 - 380 million years ago) until April 1952, when a living specimen was collected from deep depths in the Middle America Trench off Costa Rica's Pacific coast. In 1957 that species was described and named Neopilina galatheae by its discoverer, Danish biologist Henning M. Lemche (1904-1977) . An expert in the field has called this discovery "One of the greatest sensations in the [twentieth] century." As of 2008, there were 31 living species known, discovered in waters from 200 meters in depth to hadal depths, or more than 6,000 meters in the deepest ocean trenches. The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977. Scientists believe that the taxon Monoplacophora is probably polyphyletic and have proposed including all the living members in the order Tryblidiida. An attempt at a common name for Monoplacophores, gastroverm, proved unsuccessful, the term being called "singularly unattractive." By and large, writers since have tacitly agreed; "Monoplacophores" remaining the common usage.

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.

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 200 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 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 ,

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

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

Family Monoplacophoridae

Genus ''Monoplacophorus Moskalev, Starobogatov & Filatova, 1983

Species Monoplacophorus zenkevitchi 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

Notes

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): 59–66.
Michael Schrödl, Katrin Linse and Enrico Schwabe (2006). "Review on the distribution and biology of Antarctic Monoplacophora, with first abyssal record of Laevipilina antarctica". Polar Biology 29 (9): 721–727.