Gastrotricha

Abstract

Gastrotricha is a clade of aquatic microscopic animals that are among the smallest metazoans. Gastrotrichs mainly live in the marine and freshwater interstitial environment and on the surface of aquatic plants. There are around 700 described species of gastrotrichs, which are divided into the marine taxon Macrodasyida and the marine and freshwater taxon Chaetonotida. So far, no fossils have been assigned to the taxon Gastrotricha. All gastrotrichs are direct developers, and the adults measure between 60 and 600 μm, with some exceptions growing to a size of several millimeters. The acoelomate body is wormlike, bilaterally symmetric, and sometimes covered with scales and a cuticle (Fig. 2.1; Ruppert 1991). The ventral side is covered with locomotory cilia (“gaster,” belly; “thrix,” hair). The animals have a brain that is mainly composed of an elaborated dorsal commissure, whereas a ventral commissure is present in some species (Schmidt-Rhaesa 2015). The brain is connected via lateral axon tracts or neurite bundles to the posterior end of the body (Schmidt-Rhaesa 2015). The alimentary canal has an anterior mouth with a muscular pharynx, a midgut that is composed of less than 100 cells and ends with a posterior anal opening (Ruppert 1991). Most species lack an ectodermal hindgut and some even lack an anal opening (Urodasys). No specialized hemal or respiratory system is present in gastrotrichs, but the excretory system is composed of one to six pairs of protonephridia (Ruppert 1991). Gastrotrichs possess characteristic adhesive tubes that are composed of two to three gland cells that secrete adhesive substances (“dual-gland system”). Gastrotrichs can be dioecious or protandric hermaphrodites, and some obligate parthenogenetic species have been described (Hummon 1974). Fertilization is internal and eggs are deposited through a female opening or body rupture. The phylogenetic position of Gastrotricha is still under debate, and affiliations with the Platyzoa, a suggested monophylum within the Spiralia, have been hypothesized based on the results of large-scale phylogenomic analyses (Dunn et al. 2008; Hejnol et al. 2009).

Chapter vignette artwork by Brigitte Baldrian.© Brigitte Baldrian and Andreas Wanninger.