Gastrotrich
Encyclopedia
The gastrotrichs are a phylum of microscopic (0.06-3.0 mm) animal
s 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
.
Gastrotrichs demonstrate eutely
, with development proceeding to a particular number of cells, and further growth coming only from an increase in cell size. Originally they were thought to have a body cavity
(pseudocoel), but this was an artifact created by preservation methods, and they are now known to be acoelomate.
The mouth is at the anterior end, and opens into an elongated pharynx
lined by myoepithelial cells
. In some species, the mouth includes an eversible capsule, often bearing teeth formed from the outer cuticle
of the body wall. The pharynx opens into the intestine, which is lined with glandular and digestive cells. The anus
is located close to the hindmost part of the body. In some species, there are pores in the pharynx opening to the ventral surface; these may allow egestion of any excess water swallowed while feeding.
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. Unusually, the protonephridia do not take the form of flame cell
s, but instead the excretory cells consist of a skirt surrounding a series of cytoplasm
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. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation
.
The nervous system is relatively simple. The brain consists of two ganglia
, one on either side of the pharynx, connected by a commisure. Each ganglion gives rise to a single nerve cord, which runs the length of the body and includes further, smaller ganglia.
The primary sensory organs are the bristles and ciliated tufts of the body surface. Some gastrotrichs also possess photosensitive cells within the brain that function as primitive ocelli.
s, possessing both male and female sex organs. There is generally a single pair of gonad
s, including sperm-producing cells anteriorly, and producing ova
from the posterior part. Sperm are released through ducts that open on the underside of the animal roughly two-thirds of the way along the body. Once the sperm are produced, they are picked up by an organ on the tail that functions as a penis
to transfer the sperm to the partner. 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. 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 during good environmental conditions, which hatch in one to four days.
The eggs hatch into miniature versions of the adult. The young typically reach sexual maturity in about three days, and gastrotrichs can live up to ten days under laboratory conditions.
a, the Rotifer
a, or the Nematoda
. On the other hand genetic studies place them as close relatives of the Platyhelminthes
, the Ecdysozoa
or the Lophotrochozoa
. About 700 species have been described.
The phylum contains a single class, divided into two orders: the Macrodasyida
and
Chaetonotida
.
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s abundant in fresh water and marine environments. Most fresh water species are part of the periphyton
Periphyton
Periphyton are a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus that are attached to submerged surfaces in most aquatic ecosystems. It serves as an important food source for invertebrates, tadpoles, and some fish. It can also absorb contaminants; removing them from...
and benthos
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
. Marine species are found mostly interstitially in between sediment particles, while terrestrial species live in the water films around grains of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
.
Anatomy
Gastrotrichs are bilaterally symmetric, with a transparent body and a flat underside. Many species have a pair of short projections at the posterior end. The body is covered with cilia, especially about the mouth and on the ventral surface, and has two terminal 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 to sever the connection. Like many microscopic animals, their locomotion is primarily powered by hydrostatics.Gastrotrichs demonstrate eutely
Eutely
Eutelic organisms have a fixed number of cells when they reach maturity, the exact number being constant for any one species. Development proceeds by cell division until maturity; further growth occurs via cell enlargement only....
, with development proceeding to a particular number of cells, and further growth coming only from an increase in cell size. Originally they were thought to have a body cavity
Body cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
(pseudocoel), but this was an artifact created by preservation methods, and they are now known to be acoelomate.
The mouth is at the anterior end, and opens into an elongated pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
lined by myoepithelial cells
Myoepithelium
Myoepithelial cells are cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the lumenal cells. These may be positive for alpha smooth muscle actin and can contract and expel the secretions of exocrine glands...
. In some species, the mouth includes an eversible capsule, often bearing teeth formed from the outer cuticle
Cuticle
A cuticle , or cuticula, is a term used for 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 "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...
of the body wall. The pharynx opens into the intestine, which is lined with glandular and digestive cells. The anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
is located close to the hindmost part of the body. In some species, there are pores in the pharynx opening to the ventral surface; these may allow egestion of any excess water swallowed while feeding.
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. Unusually, the protonephridia do not take the form of flame cell
Flame cell
right|frameA flame cell is a specialized excretory cell found in the simplest freshwater invertebrates, including flatworms , rotifers and nemerteans; these are the simplest animals to have a dedicated excretory system. Flame cells function like a kidney, removing waste materials...
s, but instead the excretory cells consist of a skirt surrounding a series of cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
ic rods that in turn enclose a central flagellum
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
. 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. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation
Osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move...
.
The nervous system is relatively simple. The brain consists of two ganglia
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
, one on either side of the pharynx, connected by a commisure. Each ganglion gives rise to a single nerve cord, which runs the length of the body and includes further, smaller ganglia.
The primary sensory organs are the bristles and ciliated tufts of the body surface. Some gastrotrichs also possess photosensitive cells within the brain that function as primitive ocelli.
Reproduction
Gastrotrichs are simultaneous hermaphroditeHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s, possessing both male and female sex organs. There is generally a single pair of gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s, including sperm-producing cells anteriorly, and producing ova
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
from the posterior part. Sperm are released through ducts that open on the underside of the animal roughly two-thirds of the way along the body. Once the sperm are produced, they are picked up by an organ on the tail that functions as a penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
to transfer the sperm to the partner. Fertilisation is internal, and the eggs are released by rupture of the body wall.
Many species of chaetotonid gastrotrichs reproduce entirely by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
. In these species the male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. 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 during good environmental conditions, which hatch in one to four days.
The eggs hatch into miniature versions of the adult. The young typically reach sexual maturity in about three days, and gastrotrichs can live up to ten days under laboratory conditions.
Taxonomy
The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the GnathostomulidGnathostomulid
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956....
a, the Rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
a, or the Nematoda
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
. On the other hand genetic studies place them as close relatives of the Platyhelminthes
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
, the Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
or the Lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
. About 700 species have been described.
The phylum contains a single class, divided into two orders: the Macrodasyida
Macrodasyida
The Macrodasyida is an order of gastrotrichs. They are somewhat work-like in form, and not more than 1 to 1.5 mm in size.Macrodasyids live in marine or brackish water, and can be distinguished from other gastrotrichs by the presence of two pores in the pharynx, that allow excess water to be...
and
Chaetonotida
Chaetonotida
The Chaetonotida is an order of gastrotrichs. They generally have a tenpin or bottle-like shape.Chaetonotids inhabit both freshwater and marine environments. They can be distinguished from other gastrotrichs by the absence of pores in the pharynx, and by the presence of adhesive glands at the...
.