The musculature of three species of gastrotrichs surveyed with confocal laser scanning microscopy (CLSM)

The muscular system of gastrotrichs consists of circular, longitudinal and helicoidal bands that when analysed with confocal laser scanning microscopy, provide new insights into their functional organization and phylogenetic importance. We therefore undertook a comparative study of the muscle organization in three species of Gastrotricha from the orders Macrodasyida (Paradasys sp., Lepidodasyidae; Turbanella sp., Turbanellidae) and Chaetonotida (Polymerurus nodicaudus, Chaetonotidae). The general muscle organization of the marine interstitial macrodasyidans, Paradasys and Turbanella, not only confirms earlier observation on other species but also adds new details concerning the organization and number of helicoidal, longitudinal and other muscle bands (e.g. semicircular band). The freshwater, epibenthic–epiphytic chaetonotid, Polymerurus nodicaudus, has a similar muscular organization to other species of Chaetonotidae, especially species of Chaetonotus, Halichaetonotus and Lepidodermella. Perhaps unique to Polymerurus is the combined presence of an unbranched Rückenhautmuskel (also in Halichaetonotus and Lepidodermella) and a specialized dorsoventral caudal muscle, which flank the splanchnic component of the longitudinal muscles (only in Chaetonotus and Lepidodermella). This combination, together with the presence of splanchnic dorsoventral muscles, known only in Xenotrichulidae, implies a unique phylogenetic position for Polymerurus, and indicates a potential basal position of this taxon among the Chaetonotidae studied so far (i.e. Aspidiophorus, Chaetonotus, Halichaetonotus and Lepidodermella).