Gnathostomulida (Unsegmented Marine Worms)


First described in 1956, Gnathostomulida is a phylum of microscopic, free‐living, unsegmented and hermaphroditic marine worms. Uniquely distinguished by a monociliated epidermis (that is, a body surface where each cell carries only one cilium) and possession of complex cuticular mouthparts consisting of paired jaws and an unpaired basal plate, they are most likely related to Rotifera, Acanthocephala and Micrognathozoa. There are two orders, Filospermoidea (with a pointed anterior devoid of compound sensory bristles; without a bursa copulatrix; and filiform sperm, with one flagellum) and Bursovaginoidea (with a rounded anterior with compound sensory bristles; with a bursa; and rounded or cone‐shaped sperm, aflagellate). Approximately 100 species have been recorded to date, of which many are with global or circumtropical distribution. Gnathostomulids may numerically dominate the invertebrate meiobenthos in detritus‐rich, sulfide‐rich and oxygen‐poor shallow sandy bottoms (0–25 m, rarely to 400 m) where they typically occur, seemingly feeding on microbes that they scrape off sand grains.

Key Concepts:

  • Gnathostomulida is a small phylum of microscopic, free‐living, unsegmented and hermaphroditic marine worms.

  • Gnathostomulids are unique among animals in having an entirely monociliated epidermis.

  • Gnathostomulids have complex cuticular mouthparts consisting of paired jaws and an unpaired basal plate.

  • Gnathostomulids are most likely related to Rotifera, Acanthocephala and Micrognathozoa, with which they can be united in the superphylum Gnathozoa.

  • Gnathostomulids typically occur interstitially in detritus‐ and sulfide‐rich but oxygen‐poor shallow sandy bottoms.

Keywords: marine; interstitial; meiobenthos; pseudocoelomate; worms; monociliated epidermis; Rotifera; Acanthocephala; Micrognathozoa

Figure 1.

Representatives of the three major taxa of Gnathostomulida. (a) Filospermoidea (Haplognathia rosea); (b) Scleroperalia (Gnathostomula peregrina); and (c) Conophoralia (Austrognathia microconulifera). Adapted from Sterrer .

Figure 2.

Mouth parts and sperm in Austrognathia. (a) Austrognathia hymanae Kirsteuer, basal plate (width 20 μm) and jaws; (b) Austrognathia riedli Sterrer, sperm (conuli; length 35 μm). Phase contrast micrographs of live specimens.



Ax P (1956) Die Gnathostomulida, eine rätselhafte Wurmgruppe aus dem Meeressand. Abhandlungen der Akademie für Wissenschaft und Literatur Mainz, Mathematisch‐Naturwissenschaftliche Klasse 8: 1–32.

Riedl RJ (1969) Gnathostomulida from America. Science 163: 445–452.

Rieger RM and Tyler S (1995) Sister‐group relationship of Gnathostomulida and Rotifera‐Acanthocephala. Invertebrate Biology 114(2): 186–188.

Sterrer W (1972) Systematics and evolution within the Gnathostomulida. Systematic Zoology 21(2): 151–173.

Sterrer W (1986) Marine Fauna and Flora of Bermuda, pp. 742. New York: John Wiley & Sons.

Sterrer W (1998) Gnathostomulida from the (sub)tropical northwestern Atlantic. Studies on the Natural History of the Caribbean Region 74: 1–178.

Witek A, Herlyn H, Ebersberger I, Welch DBM and Hankeln T (2009) Support for the monophyletic origin of Gnathifera from phylogenomics. Molecular Phylogenetics and Evolution 53: 1037–1041.

Further Reading

Giribet G, Distel D, Polz M, Sterrer W and Wheeler WC (2000) Triploblastic relationships with emphasis on the position of Gnathostomulida, Cycliophora, Platyhelminthes and Chaetognatha: a combined approach of 18S rDNA sequences and morphology. Systematic Biology 49(3): 539–562.

Sørensen MV and Sterrer W (2002) New characters in the gnathostomulid mouth parts revealed by scanning electron microscopy. Journal of Morphology 253: 310–334.

Sterrer W, Rieger RM and Mainitz M (1985) Gnathostomulida: enigmatic as ever. In: Conway Morris S, George JD, Gibson R and Platt HM (eds) The Origins and Relationships of Lower Invertebrates, pp. 183–199. Oxford: Clarendon Press.

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How to Cite close
Sterrer, Wolfgang(Feb 2013) Gnathostomulida (Unsegmented Marine Worms). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001587.pub3]