Gastrotricha are a group of minute metazoan (many-celled) animals numbering 500 species worldwide. The term Gastrotricha refers to the ventral locomotor cilia (filaments) by which the animals glide gracefully. Unlike many other ciliated animals, they cannot move in reverse. These microscopic aschelminths (soft-body worms) are less than 1 mm long. Despite their microscopic size, 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, organic waste, and yeasts.1

Gastrotrichs are common members of benthic (bottom-dwelling) communities, both freshwater and marine, sometimes occurring in concentrations of up to 100,000 individuals per m2. Most gastrotrichs live in the spaces between sediment particles. The head bears a number of sensory bristles, and the body has numerous spines. Despite their superficial resemblance to rotifers, gastrotrichs are clearly beasts of a different group: In particular, they lack the characteristic rotifer corona and mastax (the pharynx of a rotifer).

Gstrotrichs also can form temporary attachments to solid surfaces, as can rotifers. More like the free-living flatworms, however, they possess a double-gland system in which one gland secretes the glue and the other secretes a de-adhesive to release the attachment. They cling so tightly to particles that zoologists must first anesthesize them with magnesium chloride (MgCl2) to dislodge them for further study.2

Freshwater gastrotrich
Micrograph by Larysa Johnston


  1. McGRAW-HILL Concise Encyclopedia of Science and Technology, 5th Ed.;
  2. Biology of the Invertebrates. Jan A. Pechenik

Micrograph by Larysa Johnston

Home Contact RSS