Monoplacophora

Monoplacophorans
Fossil range: early Cambrian–mid Devonian
Neopilina sp. Head region is on the right.
Neopilina sp. Head region is on the right.
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Monoplacophora
Odhner, 1940
Included groups
  • Cyrtonellida
  • Tryblidiida
  • Pelagiellida

Monoplacophora, meaning "bearing one plate", is a class of mollusks with a cap-like shell, living on the bottom of deep sea. They have been known as a recent class since 1952; previously they were known from the fossil record.

Contents

History of discoveries

Before 1952, 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).[1] The first captured living monoplacophoran was Veleropilina zografi in 1896, but it was considered to be an archaeogastropod. This species was revealed as monoplacophoran 87 years later in 1983.

In April 1952, a living specimen was collected from deep depths in the Middle America Trench off Costa Rica's Pacific coast.[2] In 1957 that species was described and named Neopilina galatheae by its discoverer, Danish biologist Henning M. Lemche (1904-1977).[3] An expert in the field has called this discovery "one of the greatest sensations in the [twentieth] century."[4] 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.[4][5]

The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977.[6] Scientists believe that the taxon Monoplacophora is probably polyphyletic and have proposed including all the living members in the order Tryblidiida.[4] An attempt at a common name for Monoplacophores, gastroverm, proved unsuccessful, the term being called "singularly unattractive".[7] By and large, writers since have tacitly agreed; "Monoplacophores" remains the common usage.

In 1989, fossils in Italy from the middle Pleistocene were described which appear to be identical with the living species Micropilina minuta.[8]

Anatomy

Drawing of the shell of Pilina unguis. Head region is on the left.
Ventral view of the (fossil) shell of Tryblidium reticulatum Lindström, 1880. There are visible muscular attachment scars. Head region is on the upper part of the drawing. The shell length is up to 43 mm.
Dorsal view of the shell of Tryblidium reticulatum.

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 (in recent species). 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.

Monoplacophorans move on a rounded foot. Their reduced head lacks eyes or tentacles.

The mantle cavity forms a horseshoe-shaped groove running around the muscular foot, in a similar fashion to that of the chitons, and contains five or six gills on either side. The mouth opens on the underside between the ends of the groove, while the anus opens into the hindmost part. Like chitons, monoplacophorans possess a sensory subradula organ, as well as a rasping radula. A fold of ciliated tissue surrounds the mouth to the front and sides, while a smaller fold, bearing a number of tentacles, lies just behind it. The stomach contains a style, projecting from a diverticulum, or "style sac".

The heart is divided into two equal halves, each with its own auricle, ventricle and aorta. The left and right aorta fuse shortly after leaving the heart, and supply blood to the open circulatory system. There are six pairs of nephridial excretory organs, which empty into the mantle cavity.

The nervous system has small ganglia around the oesophagus from which two pairs of main nerve cords run through the body; one pair supplying the foot, and the other the visceral organs. As in the chitons, these main nerve cords are connected by a series of lateral nerves, giving the layout of the nervous system an appearance somewhat like a ladder.

There are two pairs of gonads, which release gametes into the water through one of the pairs of nephridia. The sexes are separate, and fertilisation is external.[9]

Distribution

Ecology

Habitat

Monoplacophora are a geographically widespread component of the benthos. Most are known from deep water (1800 – 6500 meters), although several species are found in shallower waters ranging up to 200 meters.[10]

Feeding habits

It is presumed that they graze on microscopic organisms in mud or bottom detritus.

Phylogenetic position

In 2006 a molecular study on Laevipilina antarctica suggested that Monoplacophora and Polyplacophora form a well-supported clade with the researched Neopilina closest to the chitons.[11] 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 contradicts the fossil evidence, which suggests that the Monoplacophora are the sister group to the remainder of the conchiferans,[12][13][14] and that the cephalopods (squid and octopus) arose from within the monoplacophoran lineage.[15] However, some authors dispute this view and do not necessarily see modern monoplacophora as related to their fossil ancestors.[16]

The fossil record does indicate that the ancestral mollusc was monoplacophoran-like and that the polyplacophora arose from within the monoplacophora – not the other way round;[17] this could be reconciled if a secondary loss of shells caused a monoplacophoran body form to reappear secondarily. This is plausible: modern monoplacophorans are not closely related to vent-dwelling representatives from the Silurian, at least.[18]

Cambrian monoplacophoran Knightoconus antarcticus is thought to be an ancestor to the cephalopods.

Fossil species

Families:

Tryblidiida

ordo ?

The taxonomy of the Gastropoda by Bouchet & Rocroi, 2005[19] also contains Paleozoic molluscs of uncertain systematic position. It is not known whether these were gastropods or monoplacophorans.

Overview of taxonomy of recent species

Order Tryblidiida

References

  1. ↑ Heinz A. Lowenstam and Stephen Weiner, On Biomineralization, Oxford University Press, New York, p. 89 (1989)
  2. ↑ Enrico Schwabe, A summary of reports of abyssal and hadal Monoplacophora and Polyplacophora (Mollusca), Zootaxa 1866: 2005-222 (2008).
  3. ↑ "New Pilina": Pilina was a monoplacophore that lived during Silurian times. Galathea was the name of the Danish research vessel that recovered it.
  4. ↑ 4.0 4.1 4.2 Schwabe, above, at p. 205.
  5. ↑ A list of species names is in WoRMS World Register of Marine Species
  6. ↑ Lynn Margulis and Karlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (third edition (1997), p. 290).
  7. ↑ 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)
  8. ↑ doi:10.1111/j.1502-3931.1990.tb01361.x
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  9. ↑ Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 379–381. ISBN 0-03-056747-5. 
  10. ↑ Wingstrand, KG (1985). "On the anatomy and relationships of Recent Monoplacophora" (Link to free full text + plates). Galathea Report 16: 7–94. http://www.zmuc.dk/inverweb/Galathea/Galathea_p5.html.  Leiden & Copenhagen.
  11. ↑ doi:10.1073/pnas.0602578103
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  12. ↑ Scheltema, A. H. (February 1, 1993). "(1993) Biol. Bull 184, 57–78 Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula". The Biological Bulletin 184 (1): 57. doi:10.2307/1542380. http://www.biolbull.org/cgi/content/abstract/184/1/57?ijkey=d45e9547a760e1cb9921f81243d05ae05773204c&keytype2=tf_ipsecsh. 
  13. ↑ Haszprunar, G. (2000) Am. Malacol. Bull 15, 115–130.
  14. ↑ 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.
  15. ↑ Clarke, M.R.; Trueman, E.R., ed (1988). "Main features of cephalopod evolution". The Mollusca. 12: Palaeontology and Neontology of Cephalopods. Orlando, Fla.: Acad. Pr.. ISBN 0127514120. 
  16. ↑ doi:10.1017/S000632310000548X
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  17. ↑ doi:10.1126/science.186.4161.311
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  18. ↑ doi:10.1016/j.tree.2003.08.009
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  19. ↑ Bouchet P. & Rocroi J.-P. (Ed.); Frýda J., Hausdorf B., Ponder W., Valdés Á. & Warén A. 2005. Classification and nomenclator of gastropod families. Malacologia: International Journal of Malacology, 47(1-2). ConchBooks: Hackenheim, Germany. ISBN 3-925919-72-4. ISSN 0076-2997. 397 pp. http://www.vliz.be/Vmdcdata/imis2/ref.php?refid=78278
  20. ↑ 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. doi:10.1093/mollus/eyi008. 

Further reading

External links