Lancelet

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Not to be confused with Lancet or Lancelot.
Lancelets
Branchiostoma lanceolatum.jpg
Lancelet (Branchiostoma lanceolatum)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Cephalochordata Owen 1846
Class: Leptocardii
Müller, 1845
Order: Amphioxiformes[1]
Families

Asymmetronidae
Branchiostomidae

The lancelets (from "lancet") — also known as amphioxi (singular, amphioxus) — comprise some 32 species[2] of fish-like marine chordates in the order Amphioxiformes, with a global distribution in shallow temperate (as far north as Scotland)[3] and tropical seas, usually found half-buried in sand. They are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates.[4] In Asia, they are harvested commercially as food for humans and domesticated animals. They are an important object of study in zoology as they provide indications about the evolutionary origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions.[5] They are regarded as similar to the archetypal vertebrate form.

The first representative organism of the group to be described was Branchiostoma lanceolatum. It was described by Peter Simon Pallas in 1774 as molluscan slugs in the genus Limax. It was not until 1834 that Gabriel Costa brought the phylogenetic position of the group closer to the agnathan vertebrates (hagfish and lampreys), including it in the new genus Branchiostoma (from the Greek, branchio = "gills", stoma = "mouth").[6] In 1836, Yarrel renamed the genus as Amphioxus (from the Greek: "pointed on both sides"), now considered an obsolete synonym of the genus Branchiostoma. Today, the term "amphioxus" is still used as a common name for the Amphioxiformes, along with "lancelet", especially in the English language.

The genome of the Florida lancelet (Branchiostoma floridae) has been sequenced.[7]

Physical features[edit]

1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill's slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum 20. swim bladder 21. lateral line
Anatomy of a Branchiostoma lancelet
Anterior external anatomy
A: buccal cirri, B: wheel organ, C: velum, D: rostrum, E: notochord extending beyond nerve cord, F: nerve cord, G: Hatschek's pit, H: fin rays, I: gill bar, J: buccal cavity (vestibule)
Cross section through the pharyngeal region
A: fin box, B: nerve cord, C: notochord,
D: myomeres, E: epibranchial groove,
F: endostyle, G: secondary gill bar,
H: primary gill bar, I: pharyngeal cavity,
J: metapleural fold, K: epidermis.

Lancelets are typically 5 centimetres (2.0 in) long, or 7 centimetres (2.8 in) at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, so they are not especially good swimmers. While they do possess some cartilage-like material stiffening the gill slits, mouth, and tail, they have no true skeleton.[8]

In common with vertebrates, lancelets have a hollow nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres.

Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed to form a toughened rod. The lancelet notochord, unlike the vertebrate spine, extends into the head. This gives the subphylum its name (cephalo- meaning 'relating to the head'). The nerve cord is only slightly larger in the head region than in the rest of the body, so that lancelets cannot be said to possess a true brain.

Lancelets also have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body. Water passes from the mouth into the large pharynx, which is lined by numerous gill-slits. The ventral surface of the pharynx contains a groove, called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus. Ciliary action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so. The mucus is collected in a second, dorsal, groove, and passed back to the rest of the digestive tract. Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.[8]

The remainder of the digestive system consists of a simple tube running from the pharynx to the anus. A single blind-ending caecum branches off from the underside of the gut, with a lining able to phagocytize the food particles, a feature never found in vertebrates. It used to be thought that this hepatic caecum might be homologous to the liver of vertebrates, but this is now thought to be less likely.[8]

Lancelets have no respiratory system, breathing solely through their skin, which consists of a simple epithelium. Despite the name, little if any respiration occurs in the gill slits, which are solely devoted to feeding. The circulatory system does resemble that of primitive fish in its general layout, but is much simpler, and does not include a heart. There are no blood cells, and no haemoglobin.[8]

The excretory system consists of segmented "kidneys" containing protonephridia instead of nephrons, and quite unlike those of vertebrates. Also unlike vertebrates, there are numerous, segmented gonads.[8]

Taxonomy[edit]

The lancelet is a small, translucent, fish-like animal that is the closest living invertebrate relative of the vertebrates.[9][10]

The Cephalochordata is traditionally seen as a sister subphylum to the vertebrates, with which it is grouped together into a clade (sometimes called Notochordata) which in turn is the sister group to the simpler still Urochordata. Newer research suggests this may not be the case. The Cephalochordata may be the most basal subphylum of the chordates,[11] while the sister group of the vertebrates may be the tunicates, previously known as the urochordates.[7] However, other recent molecular studies (cit. in Benton 2005: 8) place cephalochordates nearer to vertebrates, and "[m]ost authors regard amphioxus as the closest relative of the Vertebrata on the basis of 10–15 [morphological] features that are not seen in tunicates".[12]

The asymmetric nature of juveniles is unique to the cephalochordates and demonstrates (as do certain other features, including the seriated gonads) that lancelets are more derived than would be expected. This is a reminder that the "living fossil" representative of a basal clade has as long an evolutionary history as any other living thing, and thus is more derived than the actual primitive ancestor it otherwise so closely resembles. The following are the species recognised by ITIS. Other sources, for instance Tudge,[13] show that there might be up to thirty species.[verification needed] [14]

References[edit]

  1. ^ Classification of Class: Leptocardii - Ocean Biogeographic Information System: Canadian Museum of Nature (OBIS Canada)
  2. ^ ""Cephalochordata: EoL species"". Retrieved 2014-06-12. 
  3. ^ "Year of discovery". Scotland.gov.uk. 2011-12-29. Retrieved 2012-08-13. 
  4. ^ Webb, J. E. (1958). "The Ecology of Lagos Lagoon. III. The Life History of Branchiostoma nigeriense Webb". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (The Royal Society) 241 (683): 335–353. doi:10.2307/92531. JSTOR 92531.  edit
  5. ^ Worm-like Marine Animal Providing Fresh Clues About Human Evolution Newswise, Retrieved on July 8, 2008.
  6. ^ Garcia‐Fernàndez, J., & Benito‐Gutiérrez, È. (2009). It's a long way from amphioxus: descendants of the earliest chordate. Bioessays, 31(6), 665-675, [1].
  7. ^ a b Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.; Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.; Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.; Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton, A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.; Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.; Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T. (Jun 2008). "The amphioxus genome and the evolution of the chordate karyotype". Nature 453 (7198): 1064–1071. Bibcode:2008Natur.453.1064P. doi:10.1038/nature06967. ISSN 0028-0836. PMID 18563158.  edit
  8. ^ a b c d e Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 18–21. ISBN 0-03-910284-X. 
  9. ^ Gewin, V. (2005). "Functional genomics thickens the biological plot". PLoS biology, 3 (6), e219. doi:10.1371/journal.pbio.0030219
  10. ^ Lancelet (amphioxus) genome and the origin of vertebrates Ars Technica, 19 June 2008.
  11. ^ Henry Gee (2008). "Evolutionary biology: The amphioxus unleashed". Nature 453: 999-1000.
  12. ^ Michael J. Benton (2005). Vertebrate Palaeontology, Third Edition 8. Oxford: Blackwell Publishing. ISBN 0-632-05637-1.
  13. ^ Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0198604262. 
  14. ^ WoRMS Editorial Board (2013). "World Register of Marine Species- Cephalochordates species list". Retrieved 2013-10-22. 
  15. ^ WoRMS: Epigonichthys
  16. ^ URMO taxon details: Branchiostoma mortonense
  17. ^ WoRMS: Branchiostoma mortonense

Further reading[edit]

  • Stach, T. G. (2004). Cephalochordata (Lancelets). In M. Hutchins, R. W. Garrison, V. Geist, P. V. Loiselle, N. Schlager, M. C. McDade, ...W. E. Duellman (Eds.), Grzimek's Animal Life Encyclopedia (2nd ed., Vol. 1, pp. 485-493). Detroit: Gale.

External links[edit]