Gastrotricha

This group of small-sized metazoans (body length: between 50 and 800 μm) is commonly associated with protozoans, rotifers, and nematodes. They are colourless animals, the body is spindle-shaped as well as ventrally flattened1,2. Their external features include a distinct head, a trunk and a furca. The body is sometimes ornamented with spines or scales of various shapes and lengths. The furca contains distally adhesive tubes that allow the animals to attach themselves to surfaces. The gastrotrichs have ventral cilia that allows them the characteristic smooth, graceful gliding type of locomotion1,2.

• Approximately 70 known species of gastrotrichs occur in freshwater sediments and among aquatic vegetation; roughly half of them are found in streams or rivers. There is a paucity of information concerning their true diversity. Most recent genetic analysis grouped gastrotrichs loosely with the micrognathozoans and rotifers among freshwater groups3.
• Although gastrotrichs are commonly found in streambed sediments reaching densities up to 105 individuals m-2, they are rarely considered in riverine studies4,5.
• Their food consists of algae, bacteria, other microorganisms as well as detritus. Nevertheless, little is still known about their functional role and importance in food-web dynamics of freshwater systems.
• Gastrotrichs have a remarkable life cycle combining a parthenogenetic with a hermaphroditic phase. They can lay parthenogenetic eggs and at the end of the life cycle, depending on the level of environmental stress, they lay one resting egg. After the production of parthenogenetic eggs, animals develop into hermaphrodites.
• Preferably, living animals should be used for observation and identification. However, essential for species identification are the scales, plates and spines of gastrotrichs viewed in oil immersion, which can be aided using diluted acetic acid.