page_title: Gastrotrich
section_title: Distribution and habitat
hierarchical_section_title: Gastrotrich / Distribution and habitat
caption_attribution_description: English: Gastrotricha from St. Lucia beach (KwaZulu-Natal, South Africa). The microphotographs were obtained using differential interference contrast microscopy. A – Dactylopodola australiensis (Macrodasyida), ventral view. B – Pseudostomella sp. (Macrodasyida), twisted juvenile. C – Halichaetonotus sp. (Chaetonotida), dorsal view. D – Halichaetonotus sp. (Chaetonotida), ventral view. E – Heteroxenotrichula squamosa (Chaetonotida), ventral view. F – Xenotrichula sp. (Chaetonotida), dorsal view.
context_page_description: The gastrotrichs, commonly referred to as hairybellies or hairybacks, are a group of microscopic, worm-like, pseudocoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine, and the Chaetonotida, some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described.
  Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs, and a trunk with a simple gut and the reproductive organs. They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx.
context_section_description: Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land. They are also found in stagnant pools and anaerobic mud, where they thrive even in the presence of hydrogen sulfide. When pools dry up they can survive periods of desiccation as eggs, and some species are capable of forming cysts in harsh conditions.  In marine sediments they have been known to reach 364 individuals per 10 cm² (1.6 sq in) making them the third most common invertebrate in the sediment after nematodes and harpacticoid copepods. In freshwater they may reach a density of 158 individuals per 10 cm² (1.6 sq in) and are the fifth most abundant group of invertebrates in the sediment.
caption: A & B = Macrodasyida
  C, D, E & F = Chaetonotida
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