Nothing is known.
CONSERVATION STATUS Not listed by the IUCN.
SIGNIFICANCE TO HUMANS None known. ♦
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Tatiana Amabile de Campos, MSc
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Evolution and systematics
Monoplacophorans are a small group of Cambro-Silurian fossils in the family Tryblidiidae. The single modern genus is Neopilina. In 1952, the Galathea expedition dredged 10 living specimens of Neopilina galathea from a deep trench of the Pacific Ocean. The specimens were found at a depth of 11,700 ft (3,570 m) in a bottom of dark, muddy clay off the west coast of Mexico. Later, four specimens of Neopilina ewingi were collected from the Peru-Chile Trench at a depth of slightly more than 9,840 ft (3,000 m). The monopla-cophorans had been classified with the chitons or the gastropods, and it was only on examination of the soft parts of living Neopilina specimens that it was recognized that a new class was needed for this genus and for the fossil genera Pilina, Scenella, Stenothecoides, Tryblidium, Archaeophiala, Drahomira, Proplina, and Bipulvina. All these fossil genera share a peculiar and distinctive feature: the undersurface of the shell has three to eight muscle scars that must be interpreted as homologous, segmentally repeated structures also present in the living genus Neopilina. This segmental internal structure is thought to show a relation between mollusks and annelids.
The survival of Neopilina species undoubtedly correlates with adaptation to life at great depths, and they are perhaps considerably more specialized than other members of the class. Fossil species appear to have evolved along two lines. In one group (subclass Cyclomya), there was an increase in the dorsoventral axis of the body, leading to a planospiral shell and a reduction of gills and retractor muscles. Although they disappeared from the fossil record in the Devonian, this group may have been ancestral to the gastropods. The other line (subclass Tergomya) retained a flattened shell with five to eight retractor muscles. Although this group also disappeared from the fossil record in the Devonian, it is believed to have survived in the genus Neopilina.
Monoplacophorans comprise one order (Tryblidioidea), one family (Neopilininae), four genera, and 17 recent species.
Monoplacophorans have a single symmetrical shell, which varies in shape from a flattened shieldlike plate to a short cone. The shell is large and bilaterally symmetric and has a single depressed, limpet-shaped valve. The apex of the shell lies almost vertically above the anterior margin in the median line. The margin of the shell is almost circular, being slightly elongated in the sagittal plane. The shell is relatively thin, has three layers, and is slightly thicker toward the margin than in the center.
A pallial groove (the mantle cavity) separates the edge of the foot from the mantle on each side. The mantle cavity forms a shallow gutter that entirely surrounds the animal. The cavity is delimited internally by the walls of the foot and laterally by the pallial fold that underlies the margin of the shell. Anteriorly, the mantle cavity contains the mouth, which is surrounded by the anterior velar ridge of a lateral fold and a pair of tentacle ridges. Laterally, the mantle cavity contains five or six pairs of gills, each of which is suspended from the roof of the mantle cavity by a slender base. The foot is a short circular column. The flat ventral wall of the foot is thin, transparent, relatively lacking in muscular support, and covered by ciliated epithelium, and it forms a creeping sole. A pedal gland, which lies along the anterior borders of the foot, may aid creeping movement by supplying mucus. Monoplacophorans are 0.25 in (3 mm) to a little more than 1.25 in (3 cm) long and externally resemble a combination of gastropod and chiton. The head is small. The mouth is in front of the foot, and the anus is in the pallial groove at the posterior end of the body, behind the foot. In front of the mouth is a preoral fold, or velum, which extends on each side as a rather large ciliated palplike structure. Another fold lies behind the mouth and projects to each side as a mass of postoral tentacles.
Deep seas (624-22,980 ft [190-7,000 m]) in various parts of the world, including the South Atlantic Ocean, Gulf of Aden, and a number of localities in the eastern Pacific Ocean
Feeding ecology and diet
Monoplacophorans feed on detritus.
The sexes are separate. There are no copulatory organs, no indication of sexual dimorphism, and no traces of sperm in the female reproductive system. It has been suggested that genital products (eggs and sperm) are discharged into the water column and that fertilization occurs externally.
Monoplacophorans live at great depths in the sea, crawling on radiolarians, attached to rocks, and in debris collected from the bottom.
Nothing is known.
No species are listed by the IUCN.
Significance to humans
Species in this taxonomic group are used for scientific research.
1. Micropilina arntzi; 2. Laevipilina antarctica. (Illustration by Bruce Worden)
No common name
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