gastrotrich

n.
Any of various minute aquatic animals of the phylum Gastrotricha, having a wormlike, ciliated body.

[From New Latin Gastrotricha, phylum name : GASTRO- + Greek -trikhos, -trichous.]

(Gastrotrichs)

Phylum: Gastrotricha

Number of families: 13

Thumbnail description
Microscopic, aquatic, strap-shaped, and tenpinshaped ciliated worms with cuticular adhesive tubes

Evolution and systematics

The phylum Gastrotricha is divided into two orders, Macrodasyida and Chaetonotida. The order Macrodasyida contains six families, 31 genera, and approximately 210 marine species. One species occurs in freshwater. The six families are Dactylopodolidae, Lepidodasyidae, Macrodasyidae, Planodasyidae, Turbanellidae, Thaumastodermatidae. The order Chaetonotida contains two suborders, seven families, 29 genera, and approximately 400 marine and freshwater species. The seven families are Neodasyidae, Chaetonotidae, Dasydytidae, Dichaeteuridae, Neogosseidae, Proichthydidae, Xenotrichulidae.

Evolutionary relationships within the phylum are not well known. There is no fossil record. Within the Macrodasyida, the Dactylopodolidae is the most primitive family. Relationships among the five remaining families are unknown; Lepidodasyidae is probably a polyphyletic taxon. The Chaetonotida is divided into two suborders, the Multitubulatina and Paucitubulatina. The Multitubulatina contains a single family, Neodasyidae, and is basal within the Chaetonotida. Species of Neodasyidae are superficially similar to macrodasyidans but possess a chaetonotidan-type pharynx. The remaining six families of Chaetonotida make up the suborder Paucitubulatina. Most members of Paucitubulatina have tenpin-shaped bodies, sculptured cuticles, and a combination of hermaphroditic and parthenogenetic reproduction. The largest family, Chaetonotidae, may be an unnatural taxon.

Physical characteristics

Gastrotrichs are aquatic, strap-shaped to tenpin-shaped worms, 0.002–0.14 in (0.05–3.5 mm) long. The body is flat ventrally and arched dorsally. A multilayered, translucent cuticle covers the entire body. The ventral epidermis is ciliated; the cilia are covered with a thin layer of epicuticle. Epidermal cells may be monociliated or multiciliated. The body generally is divided into head and trunk regions. The head bears a terminal mouth, anterior myoepithelial pharynx, and sometimes eyes or tentacles or both. The trunk contains a straight tubular intestine, at least one pair of protonephridia, reproductive organs, and a ventral anus. There is no body cavity. Cuticular duo-gland adhesive tubes may occur on the head or trunk. Muscles are present in circular, longitudinal, and helical orientations; they may be cross-striated, obliquely striated, or, rarely, smooth.

The order Macrodasyida contains strap-shaped animals, 0.006–0.14 in (0.15–3.5 mm) long. The pharynx has an inverted Y-shaped lumen and pores connecting it to the outside. Pharyngeal pores are absent in Lepidodasys. The ventral epidermal cells may be monociliated or multiciliated. Epidermal glands generally are present. Adhesive tubes often are numerous and occur anteriorly behind the mouth and posteriorly; adhesive tubes also may be present in lateral, dorsolateral, and ventral positions. The cuticle is smooth in most species, except for the species of Thaumastodermatidae and a few others, where the cuticle forms scales, spines, or hooks. Macrodasyida are simultaneous or sequential hermaphrodites with complex male and female reproductive organs.

The order Chaetonotida contains vermiform and tenpinshaped animals, 0.002–0.04 in (0.05–0.9 mm) long. The pharynx has a Y-shaped lumen and no pharyngeal pores. There is a pharyngeal plug at the junction between the pharynx and the intestine. Adhesive tubes typically are present only on the posterior caudal furca. Some species lack adhesive tubes (e.g., Dasydytes), while species of Neodasys (suborder Multitubulatina) possess papilla-like lateral adhesive tubes. The cuticle often bears scales or spines or both, except in species of Neodasys and the Proichthydidae. The epidermis is monociliated in Multitubulatina and multiciliated in Paucitubulatina. Cross-striated muscles occur in Neodasys, and obliquely striated muscles are seen in all other species. Chaetonotida are hermaphroditic; several species of Paucitubulatina also are parthenogenetic. An anomalous reproductive organ, the X-organ, usually is present in Paucitubulatina.

Distribution

Gastrotrichs are found in all tropical, subtropical, and temperate waters worldwide. Families and genera are cosmopolitan. Several species are transoceanic; Dactylopodola baltica, for example, occurs on both sides of the Atlantic. Other species have a more restricted distribution, but this may be a result of inadequate sampling. Several freshwater gastrotrichs are cosmopolitan.

Habitat

All gastrotrichs are aquatic. Approximately half the known species are marine, found living in the voids between sand grains (interstitial spaces) on coastal beaches and continental shelves. Some species are known from the deep sea. Marine gastrotrichs generally prefer well-oxygenated sediments, although some species are present in low-oxygen and even dysoxic (oxygen-free) sediments. Grain size and the consolidation of the sediment also may be important factors. Freshwater gastrotrichs typically live on submerged or floating vegetation; some species are semiplanktonic and others may be interstitial.

Behavior

Little is known about gastrotrich behavior. Locomotion relies entirely on the ventral cilia, and muscles are used to change direction during episodes of ciliary gliding. Marine species are thigmotactic (i.e., move along/toward solid objects such as sand or ground) and adhere to the substratum with their adhesive tubes. Creeping, inchworm-like movements are known for some species. Most gastrotrichs show some form of negative phototaxis (orientation away from light). Chemotaxis (orientation toward the source of a chemical stimulus) may play a significant role in mating and in the general distribution of gastrotrichs. Copulation often involves active flexion and contact between partners.

Feeding ecology and diet

Marine gastrotrichs generally feed on diatoms, foraminiferans, bacteria, and minute protists. Freshwater gastrotrichs probably are bacteriovores but also may consume microalgae and organic detritus. It is thought that gastrotrichs are preyed upon by larger macrofauna.

Reproductive biology

Gastrotrichs are primitively hermaphroditic, with frequent protandry (male organs develop first). Most species possess paired testes and ovaries. Loss or reduction of the testes is common in several lineages. (Some Thaumastodermatidae have lost the left testis, for example, and many Chaenotonotida have reduced testes). The ovary is single in species of Lepidodasyidae. Fertilization is via indirect transfer of sperm or spermatophores. Complex reproductive organs may facilitate transfer of sperm in some species. Development is direct with no larval stage. Freshwater chaetonotidans often are parthenogenetic, with a later hermaphroditic phase.

Conservation status

No species of Gastrotricha is listed by the IUCN. One species, Hemidasys agaso, a facultative ectocommensal (does not require its host for survival) on the annelid Nereilepas cau-data, is thought to be extinct.

Significance to humans

The importance of gastrotrichs remains undetermined. As bacteriovores and detritrovores, gastrotrichs may contribute to the aesthetics of coastal beaches by consuming washed-up debris, preventing its decay and associated odor. The study of gastrotrichs also may be used to augment our knowledge of animal origins, evolution, and relationships. Lepidodermella squamata is a commercially available freshwater gastrotrich.

Species accounts

Lepidodermella squamata
Dactylopodola baltica

Resources

Books:

Hummon, William D. "Gastrotricha." In Synopsis and Classification of Living Organisms, edited by S. P. Parker. New York: McGraw-Hill, 1982.

Ruppert, Edward E. "Gastrotricha." In Introduction to the Study of Meiofauna, edited by Robert P. Higgins and Hjalmar Thiel. Washington, DC: Smithsonian Institution Press, 1988. ——. "Gastrotricha." In Microscopic Anatomy of Invertebrates. Vol. 4, Aschelminthes, edited by Fredrick W. Harrison and Edward E. Ruppert. New York: Wiley-Liss, 1991.

Periodicals:

Hummon, W. D. "The Marine and Brackish-water Gastrotricha in Perspective." Contributions to Zoology 76 (1971): 21–23.

[Article by: Rick Hochberg, PhD]

A phylum of minute metazoan animals (formerly placed in the aschelminth group) numbering 500 described species worldwide. Some 300 species have been reported from the marine habitat, with new ones being described every year.

Gatrotrichs comprise two orders, the Macrodasyida and the Chaetonotida. The term Gastrotricha refers to the ventral locomotor cilia by which the animals glide gracefully over the substratum or through its interstices; unlike many other ciliated animals, they cannot move in reverse. Gastrotrichs have a complete digestive tract, with a sucking pharynx, a simple intestine with a wall only a single cell thick, and an anus. They appear to be selective feeders on bacteria, very small protozoa, and yeasts. Most have protonephridia, accounting in part for their broad salinity tolerances.

Gastrotrichs appear to be regionally cosmopolitan, with 20–30% having broad distributions within continents, and 10–15% between continents; endemism probably does not exceed 20%.

The phylum Gastrotricha is the most primitive in the aschelminth group of phyla. Gastrotrichs and nematodes probably share a common ancestor, which in turn was descended from a stock that included gnathostomulids and turbellarianoid animals. See also Chaetonotida; Gnathostomulida; Macrodasyida; Nemata; Turbellaria.

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