The gastrotrichs (

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s, and are widely distributed and abundant in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

and marine environments. They are mostly

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

and live within the periphyton, the layer of tiny

organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pr ...

and

detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...

that is found on the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

and the beds of other water bodies. The majority live on and between particles of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

. Gastrotrichs are divided into two orders, the

Macrodasyida Macrodasyida is an order (biology), order of gastrotrichs. Members of this order are somewhat worm-like in form, and not more than 1 to 1.5 mm in length. Macrodasyids are almost in entirely marine and live in the sediment in marine or brack ...

which are marine (except for two species), 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 The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

and

sensory organs A sense is a biological system used by an organism for sensation, the process of gathering information about the surroundings through the detection of stimuli. Although, in some cultures, five human senses were traditionally identified as s ...

, 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 The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

with which they move around. They feed on detritus, sucking up organic particles with their muscular

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

. They are

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s, the marine species producing eggs which develop directly into miniature adults. The freshwater species are

parthenogenetic Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, producing unfertilised eggs, and at least one species is viviparous. Gastrotrichs mature with great rapidity and have lifespans of only a few days.

Etymology and taxonomy

The name ''gastrotrich'' comes from Greek γαστήρ, ''gaster'' 'stomach' and θρίξ, ''thrix'' 'hair'. The name was coined by the Russian zoologist

Élie Metchnikoff Ilya Ilyich Mechnikov (; – 15 July 1916), also spelled Élie Metchnikoff, was a zoologist from the Russian Empire of Moldavian noble ancestry and alshereat archive.org best known for his research in immunology (study of immune systems) and ...

in 1865. The common name ''hairyback'' apparently arises from a mistranslation of ''gastrotrich''. The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the

Rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...

a, or the

Nematoda The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitism, parasitic. Parasitic ...

. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...

or the Lophotrochozoa. As of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the

and the Chaetonotida. Edward Ruppert ''et al.'' report that the Macrodasyida are wholly marine, but two rare and poorly known species, ''Marinellina flagellata'' and ''Redudasys fornerise'', are known from fresh water. The Chaetonotida comprises both marine and freshwater species.

Anatomy

Gastrotrichs vary in size from about in body length. They are bilaterally symmetrical, with a transparent strap-shaped or bowling pin-shaped body, arched dorsally and flattened ventrally. The

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

end is not clearly defined as a head but contains the sense organs, brain and pharynx.

Cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

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. Diplodasys rothei legends

The body wall consists of a

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

, an

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

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 Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...

. In the macrodasyidans, Y-shaped cells, each containing a

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

, surround the gut and may function as a

hydrostatic skeleton A hydrostatic skeleton or hydroskeleton is a type of skeleton supported by hydrostatic fluid pressure or liquid, common among soft-bodied organism, soft-bodied invertebrate animals colloquially referred to as "worms". While more advanced organisms ...

. The mouth is at the anterior end and opens into an elongated muscular

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 macrodasyidans, 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

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

ic 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 A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...

, one on either side of the pharynx, connected by a

commissure A commissure () is the location at which two objects wikt:abut#Verb, abut or are joined. The term is used especially in the fields of anatomy and biology. * The most common usage of the term refers to the brain's commissures, of which there are at ...

. 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 which function as mechanoreceptors. There are also ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors.

Distribution and habitat

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 Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

. 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 making them the third most common invertebrate in the sediment after

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s and harpacticoid copepods. In freshwater they may reach a density of 158 individuals per and are the fifth most abundant group of invertebrates in the sediment.

Behaviour and ecology

In marine and freshwater environments, gastrotrichs form part of the benthic community. They are detritivores and are microphagous: they feed by sucking small dead or living organic materials, diatoms,

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

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, gastrotrich locomotion is primarily powered by hydrostatics, but movement occurs through different methods in different members of the group. Chaetonotids only have adhesive glands at the back and, in them, locomotion typically proceeds in a smooth gliding manner; the whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. In the

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

chaetonotid genus '' Stylochaeta'', however, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body. By contrast, with chaetonotids, macrodasyidans typically have multiple adhesive glands and move forward with a creeping action similar to that of a "looper" caterpillar. In response to a threat, the head and trunk can be rapidly pulled backwards, or the creeping movement can be reversed. Muscular action is important when the animal turns sideways and during copulation, when two individuals twine around each other.

Reproduction and lifespan

Gastrotrich reproduction and reproductive behaviour has been little studied. That of macrodasiyds probably most represents that of the ancestral lineage and these more primitive gastrotrichs are simultaneous

s, possessing both male and female sex organs. There is generally a single pair of

gonad A gonad, sex gland, or reproductive gland is a Heterocrine gland, mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gon ...

s, the anterior portion of which contains

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm ...

-producing cells and the posterior portion producing ova. The sperm is sometimes packaged in spermatophores and is 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. Details of the process and the behaviour involved vary with the species, and there is a range of different accessory reproductive organs. During copulation, the "male" individual uses his copulatory organ to transfer sperm to his partner's gonopore and fertilisation is internal. The fertilised eggs are released by rupture of the body wall which afterwards repairs itself. As is the case in most protostomes, development of the embryo is determinate, with each cell destined to become a specific part of the animal's body. At least one species of gastrotrich, '' Urodasys viviparus'', is viviparous. Many species of chaetotonid gastrotrichs reproduce entirely by

parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

. In these species, the male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. Though the eggs have a diameter of less than 50 μm, they are still very large in comparison with the animals' size. Some species are capable of laying eggs that remain dormant during times of

desiccation Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The ...

or low temperatures; these species, however, are also able to produce regular eggs, which hatch in one to four days, when environmental conditions are more favourable. The eggs of all gastrotrichs 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. Gastrotrichs demonstrate eutely, each species having an invariant genetically fixed number of cells as adults. Cell division ceases at the end of embryonic development and further growth is solely due to cell enlargement.

Classification

Gastrotricha is divided into two orders and a number of families: Order

Remane, 1925 ao and Clausen, 1970/small> *Family Cephalodasyidae Hummon & Todaro, 2010 ::*Genus '' Cephalodasys'' Remane, 1926 ::*Genus '' Dolichodasys'' Gagne, 1977 ::*Genus '' Megadasys'' Schmidt, 1974 ::*Genus '' Mesodasys'' Remane, 1951 ::*Genus '' Paradasys'' Remane, 1934 ::*Genus '' Pleurodasys'' Remane, 1927 *Family Dactylopodolidae Strand, 1929 ::*Genus '' Dactylopodola'' Strand, 1929 ::*Genus '' Dendrodasys'' Wilke, 1954 ::*Genus '' Dendropodola'' Hummon, Todaro & Tongiorgi, 1992 *Family Lepidodasyidae Remane, 1927 ::*Genus '' Lepidodasys'' Remane, 1926 *Family Macrodasyidae Remane, 1926 ::*Genus '' Macrodasys'' Remane, 1924 ::*Genus '' Urodasys'' Remane, 1926 *Family Planodasyidae Rao & Clausen, 1970 ::*Genus '' Crasiella'' Clausen, 1968 ::*Genus '' Planodasys'' Rao & Clausen, 1970 *Family Redudasyidae Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012 ::*Genus '' Anandrodasys'' Todaro, Dal Zotto, Jondelius, Hochberg et al., 2012 ::*Genus '' Redudasys'' Kisielewski, 1987 *Family Thaumastodermatidae Remane, 1927 **Subfamily Diplodasyinae Ruppert, 1978 ***Genus '' Acanthodasys'' Remane, 1927 ***Genus '' Diplodasys'' Remane, 1927 **Subfamily Thaumastodermatinae Remane, 1927 ***Genus '' Hemidasys'' Claparède, 1867 ***Genus '' Oregodasys'' Hummon, 2008 =(Platydasys Remane, 1927) ***Genus '' Pseudostomella'' Swedmark, 1956 ***Genus '' Ptychostomella'' Remane, 1926 ***Genus '' Tetranchyroderma'' Remane, 1926 ***Genus '' Thaumastoderma'' Remane, 1926 *Family Turbanellidae Remane, 1927 ::*Genus '' Desmodasys'' Clausen, 1965 ::*Genus '' Dinodasys'' Remane, 1927 ::*Genus '' Paraturbanella'' Remane, 1927 ::*Genus '' Prostobuccantia'' Evans & Hummon, 1991 ::*Genus '' Pseudoturbanella'' d'Hondt, 1968 ::*Genus '' Turbanella'' Schultze, 1853 *Family Xenodasyidae Todaro, Guidi, Leasi & Tongiorgi, 2006 ::*Genus '' Chordodasiopsis'' Todaro, Guidi, Leasi & Tongiorgi, 2006 ::*Genus '' Xenodasys'' Swedmark, 1967 *''

Incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'' ::*Genus '' Marinellina'' Ruttner-Kolisko, 1955 Order Chaetonotida Remane, 1925 ao and Clausen, 1970/small> Suborder Multitubulatina d'Hondt, 1971 *Family Neodasyidae Remane, 1929 ::*Genus '' Neodasys'' Remane, 1927 Suborder Paucitubulatina d'Hondt, 1971 *Family Chaetonotidae Gosse, 1864 **Subfamily Chaetonotinae Kisielewski, 1991 ***Genus '' Arenotus'' Kisielewski, 1987 ***Genus '' Aspidiophorus'' Voigt, 1903 ***Genus '' Caudichthydium'' Schwank, 1990 ***Genus '' Chaetonotus'' Ehrenberg, 1830 ***Genus '' Fluxiderma'' d'Hondt, 1974 ***Genus '' Ichthydium'' Ehrenberg, 1830 ***Genus '' Halichaetonotus'' Remane, 1936 ***Genus '' Heterolepidoderma'' Remane, 1927 ***Genus '' Lepidochaetus'' Kisielewski 1991 ***Genus '' Lepidodermella'' Blake, 1933 ***Genus '' Polymerurus'' Remane, 1927 ***Genus '' Rhomballichthys'' Schwank, 1990 **Subfamily Undulinae Kisielewski 1991 ***Genus '' Undula'' Kisielewski 1991 *Family Dasydytidae Daday, 1905 ::*Genus '' Anacanthoderma'' Marcolongo, 1910 ::*Genus '' Chitonodytes'' Remane, 1936 ::*Genus '' Dasydytes'' Gosse, 1851 ::*Genus '' Haltidytes'' Remane 1936 ::*Genus '' Ornamentula'' Kisielewski 1991 ::*Genus '' Setopus'' Grünspan, 1908 ::*Genus '' Stylochaeta'' Hlava, 1905 *Family Dichaeturidae Remane, 1927 ::*Genus '' Dichaetura'' Lauterborn, 1913 *Family Muselliferidae Leasi & Todaro, 2008 ::*Genus '' Diuronotus'' Todaro, Kristensen & Balsamo, 2005 ::*Genus '' Musellifer'' Hummon, 1969 *Family Neogosseidae Remane, 1927 ::*Genus '' Neogossea'' Remane, 1927 ::*Genus '' Kijanebalola'' Beauchamp, 1932 *Family Proichthydiidae Remane, 1927 ::*Genus '' Proichthydium'' Cordero, 1918 ::*Genus '' Proichthydioides'' Sudzuki, 1971 *Family Xenotrichulidae Remane, 1927 **Subfamily Draculiciterinae Ruppert, 1979 ***Genus '' Draculiciteria'' Hummon, 1974 **Subfamily Xenotrichulinae Remane, 1927 ***Genus '' Heteroxenotrichula'' Wilke, 1954 ***Genus '' Xenotrichula'' Remane, 1927

References

External links

Gastrotrichs
in the Encyclopedia of Life {{Authority control