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Comprehensive Description

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The Gnathostomulida are translucent microscopic, interstitial (i.e., living between sediment particles) marine worms, with a muscular pharynx armed with distinctive forceps-like jaws in nearly all species. Gnathostomulids feed on bacteria, fungi, and protists. The elongate body, which is typically less than 2 mm long (range 0.3 to 3.5 mm), usually has distinct head, trunk, and tail regions. Outer epithelial cells each have single cilia which collectively help the animal glide between sand grains. The ventral mouth leads to a blind gut (i.e., the gut has just a single opening, rather than a separate mouth and anus), but undigested solid waste may leave not only through the mouth but also via a temporary anus or temporary doraal connection to the epidermis. Gnathostomulids are hermaphrodites, either simultaneously functioning as male and female or as male first. One egg matures at a time and fertilization is apparently internal; in at least some species, sperm are apparently injected by an individual beneath the skin of its mate. Fertilized eggs are believed to be deposited singly. Development is direct, with no larval stage.

Gnathostomulids are found worldwide, but the best studied areas are western Europe, the Mediterranean Sea, the U.S. coastline, the Caribbean Sea, and a few localities in the Pacific Ocean. They are found mainly in medium or fine-grained sand mixed with detritus and are often very common in sand from seagrass beds. They may occur at high densities in anoxic, sulfide-rich conditions (population densities of more than 6000 individuals per liter of sediment have been reported). They occur from the intertidal zone to depths of hundreds of meters. There are close to 100 described gnathostomulid species, but many more times that number likely await discovery. Gnathostomulids were first described, in 1956, as turbellarian Platyhelminthes (free-living flatworms), but were given phylum rank in 1969. Recent work has supported the recognition of a monophyletic clade Gnathifera consisting of the clade Syndermata (Rotifera + Acanthocephala) plus the Gnathostomulida (Witek et al. 2009 and references therein). Sørensen et al. (2006) investigated relationships within the Gnathostomulida.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)

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Distribution

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Gnathostomulids are found worldwide.

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Habitat

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Gnathostomulids are found mainly in medium or fine-grained sand mixed with detritus and are often very common in sand from seagrass beds. They may occur at high densities in anoxic, sulfide-rich conditions (population densities of more than 6000 individuals per liter of sediment have been reported). They occur from the intertidal zone to depths of hundreds of meters.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)

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Morphology

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The elongate body of a gnathostomulid, which is typically less than 2 mm long (range 0.3 to 3.5 mm), usually has distinct head, trunk, and tail regions. Outer epithelial cells each have single cilia which collectively help the animal glide between sand grains. The ventral mouth leads to a blind gut (i.e., the gut has just a single opening, rather than a separate mouth and anus), but undigested solid waste may leave not only through the mouth but also via a temporary anus or temporary dorsal connection to the epidermis.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)

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Reproduction

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Gnathostomulids are hermaphrodites, either simultaneously functioning as male and female or as male first. One egg matures at a time and fertilization is apparently internal; in at least some species, sperm are apparently injected by an individual beneath the skin of its mate. Fertilized eggs are believed to be deposited singly. Development is direct, with no larval stage.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)

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Shapiro, Leo
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Systematics and Taxonomy

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There are close to 100 described gnathostomulid species, but many more times that number likely await discovery. Gnathostomulids were first described, in 1956, as turbellarian Platyhelminthes (free-living flatworms), but were given phylum rank in 1969. Recent work has supported the recognition of a monophyletic clade Gnathifera consisting of the clade Syndermata (Rotifera + Acanthocephala) plus the Gnathostomulida (Witek et al. 2009 and references therein). Sørensen et al. (2006) investigated relationships within the Gnathostomulida.

(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)

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Shapiro, Leo
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Gnathostomulid

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Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.[1]

Anatomy

Most gnathostomulids measure 0.5 to 1 millimetre (0.02 to 0.04 in) in length. They are often slender to thread-like worms, with a generally transparent body. In many Bursovaginoidea, one of the major group of gnathostomulids, the neck region is slightly narrower than the rest of the body, giving them a distinct head.[2]

Like flatworms they have a ciliated epidermis, but in contrast to flatworms, they have one cilium per cell.[3] The cilia allow the worms to glide along in the water between sand grains, although they also use muscles, allowing the body to twist or contract, for movement.

They have no body cavity, and no circulatory or respiratory system. The nervous system is simple, and restricted to the outer layers of the body wall. The only sense organs are modified cilia, which are especially common in the head region.[2]

The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present. The basal plate is used to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat.[4] This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera. The ultrastructure of the jaws made of rods with electron dense core in transmission electron microscopy sections also support their close relation with Rotifera and Micrognathozoa. The mouth opens into a blind-ending tube in which digestion takes place; there is no true anus.[2] However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore.[5]

Reproduction

Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary and one or two testes. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature version of the adult, without a larval stage.[2]

Taxonomy

There are approximately 100 described species[6] and certainly many more as yet undescribed. The known species are grouped in two orders. The filospermoids are very long and are characterized by an elongate rostrum. The bursovaginoids have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.[4]

Gnathostomulids have no known fossil record, though there are (debatable) similarities between the jaws of modern gnathostomulids and certain conodont elements. (Ochietti & Cailleux, 1969; Durden et al, 1969)[7]

They appear to be a sister clade to the Syndermata.[2] [8]

References

  1. ^ a b Ax, P. (1956). "Die Gnathostomulida, eine rätselhafte Wurmgruppe aus dem Meeressand". Abhandl. Akad. Wiss. U. Lit. Mainz, Math. - Naturwiss. 8: 1–32.
  2. ^ a b c d e Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 311–312. ISBN 0-03-056747-5.
  3. ^ Ruppert, Edward E., Fox, Richard S., Barnes, Robert D. (2004) Invertebrate Zoology (7th edition). Brooks/Cole-Thomson Learning, Belmont, US
  4. ^ a b Barnes, R.F.K. et al. (2001). The Invertebrates: A Synthesis. Oxford: Blackwell Science.
  5. ^ Knauss, Elizabeth (December 1979). "Indication of an Anal Pore in Gnathostomulida". Zoologica Scripta. 8 (1–4): 181–6. doi:10.1111/j.1463-6409.1979.tb00630.x.
  6. ^ Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12. doi:10.11646/zootaxa.3148.1.3.
  7. ^ Cited in chapter 2 , p192 of Andreas Schmidt-Rhaesa'a 2015 Gastrotricha and Gnathifera, VOl 3 of Gast---, Cycloneuralia and Gnath---.
  8. ^ Golombek, A.; Tobergte, S.; Struck, T.H. (May 2015). "Elucidating the phylogenetic position of Gnathostomulida and first mitochondrial genomes of Gnathostomulida, Gastrotricha and Polycladida (Platyhelminthes)". Mol Phylogenet Evol. 86: 49–63. doi:10.1016/j.ympev.2015.02.013. PMID 25796325.
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Gnathostomulid: Brief Summary

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Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.

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