Gastrotrich

The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate 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 (except for two species), and the Chaetonotida, some of which are marine and some freshwater. As of 2019, more than 860 species of gastrotrichs 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. They are hermaphrodites, the marine species producing eggs which develop directly into miniature adults. The freshwater species are parthenogenetic, producing unfertilised eggs, and at least one species is viviparous. Gastrotrichs mature with great rapidity and have lifespans of only a few days.

The name gastrotrich comes from Greek γαστήρ, gaster 'stomach' and θρίξ, thrix 'hair'. The name was coined by the Russian zoologist Élie Metchnikoff 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 Rotifera, or the Nematoda. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa. As of 2019, more than 860 species have been described. The phylum contains a single class, divided into two orders: the Macrodasyida 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.

Gastrotrichs vary in size from about 0.06 to 3 mm (0.002 to 0.118 in) in body length. They are bilaterally symmetrical, 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 macrodasyidans, 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 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 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.