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|Darkfield photograph of a gastrotrich|
|Darkfield photograph of a gastrotrich|
Gastrotricha is a phylum of microscopic (0.06-3.0 mm), pseudocoelomate animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles, while terrestrial species live in the water films around grains of soil.
The name "gastrotrich" comes from the Greek γαστήρ gaster, meaning "belly", and θρίξ thrix, meaning "hair". The common name "hairy back" apparently arises from a mistranslation of "gastrotrich" with a better common name for all gastrotrichs being "hairy belly", as this refers to the cilia present in most species on the ventral surface. The term "hairy back" should be limited to the large genus Chaetonotus, whose members usually have dorsal surfaces covered with hair-like spines.
The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the Rotifera, or the Nematoda. On the other hand genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa. About 790 species have been described.
Gastrotrichs are bilaterally symmetric, with a transparent strap-shaped or bowling pin-shaped body, arched dorsally and flattened ventrally. The anterior end is not clearly defined as a head but contains the sense organs, brain and pharynx. Cilia are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the posterior end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body.
The body wall consists of a cuticle, an epidermis and longitudinal and circular bands of muscle fibres. In some primitive species, each epidermal cell has a single cilium, a feature shared only by the gnathostomulans. The whole ventral surface of the animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no coelom (body cavity) and the interior of the animal is filled with poorly differentiated connective tissue. In the macrodasyans, Y-shaped cells each containing a vacuole surround the gut and may function as a hydrostatic skeleton.
The mouth is at the anterior end, and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen, lined by myoepithelial cells. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The anus is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to the ventral surface; these contain valves and may allow egestion of any excess water swallowed while feeding.
In the chaetonotidans the excretory system consists of a single pair of protonephridia, which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyans there are several pairs of these opening along the side of the body. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation. Unusually, the protonephridia do not take the form of flame cells, but instead the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum. These cells, termed cyrtocytes, connect to a single outlet cell which passes the excreted material into the protonephridial duct.
As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two ganglia, one on either side of the pharynx, connected by a commisure. From these lead a pair of nerve cords which run along either side of the body beside the longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface. There are also ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors. Some gastrotrichs also possess photosensitive cells within the brain that function as primitive ocelli.
Gastrotrichs demonstrate eutely, each species having an invariant genetically-fixed number of cells. Cell division ceases at the end of embryonic development and further growth is by cell enlargement.
Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments and the surface film of water surrounding soil particles on land. 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.
In marine and freshwater environments gastrotrichs form part of the benthic community. They are detrivores and are microphagous, sucking dead or living organic material, diatoms, bacteria and small protozoa into their mouths by the muscular action of the pharynx. They are themselves eaten by turbellarians and other small macrofauna.
Like many microscopic animals, gastrolich locomotion is primarily powered by hydrostatics. Movement takes place in Chaetonotus by means of a smooth gliding motion when the whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. By contrast, Macrodasys moves with a creeping action using the adhesive glands; the body is elongated, the anterior end is anchored, the body is shortened, the posterior end is anchored, the anterior end is released and the body elongated again. In response to a threat, the head and trunk can be rapidly retracted, or the creeping movement can be reversed. Muscular action is important in turning actions and in copulation when two individuals twine around each other. In the pelagic Stylochaeta, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body.
Gastrotrichs are simultaneous hermaphrodites, possessing both male and female sex organs. There is generally a single pair of gonads, the anterior portion of which contains sperm-producing cells and the posterior part ova. Sperm are released through male gonopores that open, often temporarily, on the underside of the animal roughly two-thirds of the way along the body. A copulatory organ on the tail collects the sperm and transfers it to the partner's seminal receptacle through the female gonopore. Fertilisation is internal, and the eggs are released by rupture of the body wall.
Many species of chaetotonid gastrotrichs reproduce entirely by parthenogenesis. In these species the male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. The eggs have a diameter of less than 50 µm which is very large in comparison with the animal's size. Some species are capable of laying eggs that can remain dormant during times of desiccation or cold temperatures; these species, however, also produce regular eggs when environmental conditions are more favourable, which hatch in one to four days. The eggs undergo direct development and hatch into miniature versions of the adult. The young typically reach sexual maturity in about three days. In the laboratory, Lepidodermella squamatum has lived for up to forty days, producing four or five eggs during the first ten days of life.
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