Gastrotrichahairy back worms


Phylum Gastrotricha, or hairy-bellied worms, includes approximately 790 currently known species of small, bilaterally symmetrical, acoelomate organisms found in marine, brackish, and fresh waters worldwide. They may constitute 1 to 8% of benthic meiofaunal organisms in marine waters, and can reach densities of over 150 individuals per 10 cm^2 in freshwaters, making them one of the most abundant organism groups in both environments. Gastrotrichs may also be semi-terrestrial. There are over 300 species in order Macrodasyida, all but two of which are marine or estuarine, and over 400 species in order Chaetonotida, three-quarters of which are freshwater, the remainder are marine or semi-terrestrial. Both groups are distinguished by the shape and orientation of their pharyngeal lumen. Likewise, macrodasyids have two pores in their pharynx, which expels excess water during feeding. Freshwater species are generally benthic or periphytic, marine species tend to be found in interstitial spaces in loose sediments, a few aquatic species are planktonic, and semi-terrestrial gastrotrichs are found in water films around soil particles. Many species have posterior adhesive tubes that form a pair of projections from the end of their body, while a few have a single, elongated “tail” instead. (Appeltans, et al., 2012; Brusca and Brusca, 1990; Strayer, et al., 2010; Todaro, et al., 2006; Todaro, 2013; Zhang, 2011)

Geographic Range

Gastrotrichs are found globally in freshwater, marine, and semi-terrestrial environments, although some genera and species have limited local distributions. (Strayer, et al., 2010)


Many gastrotrich species are found in vegetated areas or in surface sediment. They may be planktonic or benthic, and are found in marine and freshwater environments, including lakes, ponds, and wetlands. Some species are semi-terrestrial, living in water films on land. Most marine species live interstitially, and may even be found in anoxic environments. (Strayer, et al., 2010; Todaro, 2013)

Systematic and Taxonomic History

The evolutionary relationship of gastrotrichs to other metazoan phyla is unclear. Early morphological studies suggested a relationship to phylum Nematoda or Gnathostomulida (jaw worms). A recent, exclusively molecular phylogenetic study indicated their inclusion within the clade known as Spiralia, which includes Gnathostomulida, Platyhelminthes (flatworms), Syndermata (rotifers and spiny-headed worms), and Lophotrochozoa (nemerteans, phoronids, bryozoans, brachiopods, molluscs, and annelids). However, a study combining morphology, developmental characters, genetic data, and ecological characteristics indicated a possible relationship with Ecdysozoa (arthropods, nematodes, and assorted smaller phyla). In both studies, the relationships of gastrotrichs to members of these larger groups have been ambiguous. Further studies are therefore needed to evaluate these competing hypotheses and to determine their position in the clade to which they belong. (Paps and Riutort, 2012; Petrov, et al., 2007; Todaro, et al., 2006; Zrzavý, et al., 1998; Zrzavý, 2002)

Studies of the internal phylogenetic relationships of gastrotrichs are scarce. A morphological study of macrodasyid species found strong evidence for the monophyly of this order. A recent molecular analysis supported this finding, and confirmed the monophyly of order Chaetonotida. (Hochberg and Litvaitis, 2001; Paps and Riutort, 2012)

  • Synapomorphies
    • bilayered cuticle with basal fibrous/striated zone and outer lamellar zone of multiple membranes
    • cuticle-covered locomotory and sensory cilia
    • tubular duo-gland adhesive organs

Physical Description

Gastrotrichs are small, 50 to 800 micrometers in length, bilaterally symmetrical, acoelomate organisms, with transparent bodies divided into head and trunk regions. The head bears sensory cilia. Many species have posterior adhesive tubes that form a pair of projections from the end of the body, while a few have a single, elongated “tail”. Species lacking adhesive tubes are planktonic, while those with adhesive tubes use their secretions to temporarily anchor themselves to various substrates. The body, which is spindle or tenpin-shaped and ventrally flattened, is covered with a cuticle that may be composed of a single layer or many layers. The body can be covered with spines, scales, or plates depending on the species, which are derived from the fibrous lower layers of the cuticle. Outer cuticular layers are composed of membrane-like structures. The epidermis is partially syncytial (multiple nuclei without membranes) and partially cellular. The ventral epidermal layer is ciliated, giving members of this phylum their common name, "hairy-bellied worms." (Brusca and Brusca, 1990; Ruppert, et al., 2004; Strayer, et al., 2010)

Gastrotrichs generally feed by generating currents that draw food particles into the mouth, using either pumping actions of their muscular pharynx or ciliary currents. The pharynx leads into the intestine, where enzymes secreted by glandular cells digest the food, and nutrients are absorbed via diffusion; in some species, the pharynx also has multiple tubes that connect the pharyngeal lumen to the outside of the organism, allowing excess water taken in during feeding to be pumped out. Solid wastes and undigested food are passed through a dorsally placed anus, while nitrogenous and other soluble wastes passively diffuse across the body wall. Circulation and gas exchange occur by passive diffusion across the body wall, without any specialized organs to aid in the process. Osmoregulation is aided by one or several pairs of protonephridia, depending on the order the species belongs to, which release excess ions through excretory nephridiopores at the ventral surface of the organism, usually in the mid-body area. (Brusca and Brusca, 1990; Ruppert, et al., 2004)

  • Sexual Dimorphism
  • sexes alike


Embryonic cleavage is holoblastic and apparently spiral. After the formation of a coeleoblastula, gastrulation occurs, with two cells from the ventral surface moving into the blastocoel. This eventually leads to the formation of the entoderm and midgut. Additional invaginations form, connecting with the midgut, and two additional surface cells drop to the interior and contribute to the development of germ cells and gametes. Juveniles hatch from their egg capsules and show direct development, reaching sexual maturity in a few days. Gastrotrich species are eutelic, with development proceeding to a particular number of cells, with further size increases from increases in individual cell size rather than the production of new cells, although some gastrotrich species are capable of regeneration in response to damage or loss of tissue. (Brusca and Brusca, 1990)


Gastrotich females are simultaneous hermaphroditic or parthenogenetic. Male reproductive systems consist of one or two testes, with associated ducts leading to a single pore on the ventral surface, a few species have paired pores. A caudal copulatory organ is present in a few species, including members of genus Macrodasys. The female portion of the reproductive system is composed of one or two ovaries, which lie directly behind the testes in hermaphroditic species. Eggs are produced and released into a uterine space that is bound by sperm ducts, and is associated with tissues that produce yolk for developing eggs (also called a vitellarium). From here, they are moved to a sac-like area, called the X-organ, which connects to the female gonopore. Reciprocal cross fertilization occurs when two gastrotrichs meet, while internal fertilization occurs after sperm are transferred to the female gonopore. Fertilized eggs are released via a rupture of the body wall. (Brusca and Brusca, 1990)

Newly hatched juveniles already contain developing parthenogenetic eggs which, under favorable conditions, may be laid within a day of the mother's hatching. Typically, four partheogenetic eggs will be laid over a four day period. Tachyblastic eggs begin to develop immediately and hatch within a day. Opsiblastic eggs are thick-shelled and very resistant to drying and freezing; these are produced when conditions are unfavorable, such as when they are overcrowded. After laying their parthenogenetic eggs, most gastrotrichs develop into simultaneous hermaphrodites, although some species remain parthenogenetic for their entire lives, most common in freshwater species. Fertilization is thought to be internal, as sperm are nonmotile and reciprocal cross-fertilization appears to be the most common mode. (Brusca and Brusca, 1990; Ramel, 2012; Strayer, et al., 2010)

Gastrotrichs do not exhibit any parental investment beyond the production of gametes and parthenogenetic eggs. (Brusca and Brusca, 1990)

  • Parental Investment
  • no parental involvement


Gastrotrichs have very short lives of 3 to 21 days. (Brunson, 1963; Ramel, 2012)


Gastrotrichs move in a gliding fashion, using their ventral cilia. Species with adhesive tubes may use these in locomotion as well, moving in a leech-like way. Some species are known to swim, though most are relatively sessile, living interstitially in sediments or attached to the substrate with the aid of their adhesive tubes. Although population levels may be quite high locally, they are considered solitary animals. Their activity does not appear to be affected by the presence or absence of light. ("Gastrotrichs", 2012; Armonies, 1988; Strayer, et al., 2010)

Communication and Perception

A large, bi-lobed cerebral ganglion is located above the pharynx, with each lobe giving rise to a longitudinal nerve cord that extends to the posterior end of the body. The spines and bristles found around the outside of the body serve as tactile receptors, some species may have ciliated chemosensory pits on the sides of the head, or photosensitive pigment ocelli in the cerebral ganglion. (Brusca and Brusca, 1990; Ruppert, et al., 2004; Strayer, et al., 2010)

Food Habits

Gastrotrichs generally feed by generating currents using pumping actions of their muscular pharynx or ciliary currents, which draw food particles into the mouth. The primary component of most of their diets is bacteria; they also consume algae, protozoans, detritus and inorganic particles. Some species may use a tactile chemical sense to distinguish between food types. (Brusca and Brusca, 1990; Strayer, et al., 2010)


As very small organisms, gastrotrichs play an important part in their ecosystem's food chain and are probably a food source to most benthic invertebrate predators. Known predators include turbellarians, heliozoan and sarcodine amoebae, cnidarians, and tanypodine midges. ("Gastrotrichs", 2012; Brunson, 1963)

  • Known Predators
    • Amoeba spumosa (Superclass Sarcodina, Kingdom Protozoa)
    • Heliozoan amoebas (Subkingdom Hacrobia, Kingdom Protozoa)
    • Tanypodine midges (Order Diptera, Class Insecta)
    • Turbellarians (Class Turbellaria, Phylum Platyhelminthes)
    • Comb jellies (Phylum Cnidaria)

Ecosystem Roles

Gastrotrichs play an important part in the food chains of their environments. Little research has been conducted regarding parasites of these animals and no currently recognized gastrotrichs are known to be parasitic. (Brunson, 1963; Brusca and Brusca, 1990; Manylov, 1999)

Commensal/Parasitic Species
  • Microsporidia sp. (Phylum Microspora, Kingdom Fungi)

Economic Importance for Humans: Positive

Beyond the potential for scientific research, there are no known positive effects of gastrotrichs on humans. As they can reproduce at incredibly fast rates, it is possible that these animals play an important role in the bacterial population levels of their environment, but this has yet to be studied in detail. (Strayer, et al., 2010)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of gastrotrichs on humans. (Strayer, et al., 2010)

Conservation Status

No members of this phylum are currently recognized as threatened or endangered. (Strayer, et al., 2010)

  • IUCN Red List [Link]
    Not Evaluated


Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.



lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.


active at dawn and dusk


an animal that mainly eats decomposed plants and/or animals

  1. active during the day, 2. lasting for one day.

union of egg and spermatozoan


mainly lives in water that is not salty.


An animal that eats mainly plants or parts of plants.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

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Found in northern North America and northern Europe or Asia.

internal fertilization

fertilization takes place within the female's body


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


marshes are wetland areas often dominated by grasses and reeds.


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.


found in the oriental region of the world. In other words, India and southeast Asia.

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reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


development takes place in an unfertilized egg


an animal that mainly eats plankton


the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate

year-round breeding

breeding takes place throughout the year


2012. "Gastrotrichs" (On-line). Accessed April 29, 2013 at

Appeltans, W., P. Bouchet, G. Boxshall, C. De Broyer, N. de Voogd, D. Gordon, B. Hoeksema, T. Horton, M. Kennedy, J. Mees, G. Poore, G. Read, S. Stöhr, T. Walter, M. Costello. 2012. "WoRMS - World Register of Marine Species" (On-line). Accessed February 23, 2013 at

Armonies, W. 1988. Physical factors influencing active emergence of meiofauna from boreal intertidal sediment. Marine Ecology Progress Series, 49: 277-286. Accessed April 29, 2013 at

Brunson, R. 1963. Aspects of the natural history and ecology of the gastrotricha. Pp. 473-485 in E Dougherty, Z Brown, E Hanson, W Hartman, eds. The Lower Metazoa: Comparative Biology and Phylogeny. Berkeley, CA: University of California Press.

Brusca, R., G. Brusca. 1990. Invertebrates (2nd Edition). Sunderland, MA: Sinauer Associates.

Hochberg, R., M. Litvaitis. 2001. Macrodasyida (Gastrotricha): A cladistic analysis of morphology. Invertebrate Biology, 120/2: 124-135.

Manylov, O. 1995. Regeneration in Gastrotricha. I. Light microscopal observations on the regeneration in Turbanella sp. Acta Zoologica, 76/1: 1-6.

Manylov, O. 1999. First finding of a microsporidian parasite in the gastrotrich, Turbanella lutheri (Gastrotricha: Macrodasyida). Protistology, 1: 17-19. Accessed April 29, 2013 at

Paps, J., M. Riutort. 2012. Molecular phylogeny of the phylum Gastrotricha: New data brings together molecules and morphology. Molecular Phylogenetics and Evolution, 63: 208-212.

Petrov, N., A. Pegova, O. Manylov, N. Vladychenskia, N. Miuge, W. Aleshin. 2007. Molecular phylogeny of gastrotricha based on 18S rRNA genes comparison: rejection of hypothesis of relatedness with nematodes. Molekulyarnaya biologiya, 41/3: 499-507.

Ramel, G. 2012. "The Gastrotrichs (Phylum Gastrotricha)" (On-line). Earthlife. Accessed April 29, 2013 at

Ruppert, E., R. Fox, R. Barnes. 2004. Invertebrate zoology: A functional evolutionary approach (7th Edition). Belmont, CA: Thomson-Brooks/Cole.

Strayer, D., W. Hummon, R. Hochberg. 2010. Gastrotricha. Pp. 163-172 in J Thorp, A Covich, eds. Ecology and Classification of North American Freshwater Invertebrates, 3rd Edition. London, England: Academic Press. Accessed April 26, 2013 at

Todaro, M. 2013. "Gastrotricha World Portal" (On-line). Accessed April 26, 2013 at

Todaro, M., M. Telford, A. Lockyer, D. Littlewood. 2006. Interrelationships of the Metazoa inferred from 18S rRNA genes. Zoologica Scripta, 35: 251-259.

Zhang, Z. 2011. Animal biodiversity: an introduction to higher-level classification and taxonomic richness. Zootaxa, 3148: 7-12.

Zrzavý, J. 2002. Gastrotricha and metazoan phylogeny. Zoologica Scripta, 32/1: 61-81.

Zrzavý, J., S. Mihulka, P. Kepka, A. Bezděk, D. Tietz. 1998. Phylogeny of the Metazoa Based on Morphological and 18S Ribosomal DNA Evidence. Cladistics, 14/3: 249-285.