Gastrotricha

(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

List of Species

No common name

Lepidodermella squamata

order

Chaetonotida

family

Chaetonotidae

taxonomy

Lepidodermella squamata (Dujardin, 1841), River Seine, Paris, France.

other common names

None known.

physical characteristics

A short, tenpin-shaped gastrotrich that grows to 0.007 in (0.19 mm) in length. Distinct, five-lobed head separated from the body by a short neck. Trunk has posterior caudal furca and two adhesive tubes. Cuticle consists of scales without ridges or spines. Cilia present on the lateral margin of the head and ventrally in two rows.

distribution

Freshwater bodies across the United States (Arkansas, Ohio, Michigan, New Hampshire, and North Carolina), Brazil, Uruguay, Japan, and much of Europe. Probably distributed worldwide.

habitat

Found on aquatic vegetation in lakes, ponds, swamps, and streams. Also may occur interstitially in sandy sediments.

behavior

Slow ciliary glider, with spectral sensitivity to blue light.

feeding ecology and diet

Diet consists of microalgae, bacteria, and organic detritus.

reproductive biology

The life cycle begins with parthenogenetic reproduction and the deposition of up to four eggs. Eggs usually are opsiblastic (slow developing) and can survive desiccation and freezing; some eggs are tachyblastic (fast developing). The parthenogenetic phase is complete within a few days, after which the animal becomes a simultaneous hermaphrodite.

conservation status

Not listed by the IUCN.

significance to humans

Commercially available for laboratory study.

No common name

Dactylopodola baltica

order

Macrodasyida

family

Dactylopodolidae

taxonomy

Dactylopodola baltica (Remane, 1926), Kiel, Germany.

other common names

None known.

physical characteristics

Body reaches a length of 0.01 in (0.3 mm), with a well-defined head, paired eyespots, and a bifid posterior. Adhesive tubes are present anteriorly, laterally, and posteriorly. Epidermal cells are monociliated, and the muscles are cross-striated.

distribution

Atlantic coast of the United States. Also known from northern Ireland, Wales, the Scilly Isles, and the Baltic Sea to the west coast of France and the Mediterranean Sea.

habitat

Marine coastal beaches in a variety of sediment types; middle to low intertidal zone.

behavior

A slow ciliary glider; common but generally not found in abundance.

feeding ecology and diet

Feeds on diatoms.

reproductive biology

Sequential protandric hermaphrodite with paired ovaries and testes. Spermatophores are passed indirectly to partners. Reproductive activity is greatest during the summer.

conservation status

Not listed by the IUCN.

significance to humans

None known.

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.

Rick Hochberg, PhD