We interpret reduction of the claws on the fourth pair of legs as a convergent adaptation of interstitial soil-dwelling tardigrades. Tardigradrdigrades Hexapodibius degenerans, Macrobiotus pseudohufelandi, and M. xerophilus are re-examined and H. degenerans re-described. Due to the absence of true claws on the fourth pair of legs and to the symmetry of its claws on the first three pairs of legs, H. degenerans is attributed to a new genus, Pseudohexapodibius, belonging to the family Macrobiotidae. On the basis of the peculiar type of claws, without a distinct peduncle, M. pseudohufelandi and M. xerophilus are attributed to another new genus of Macrobiotidae, Xerobiotus. Additional key words: Tardigrada, Pseudohexapodibius degenerans, Xerobiotus pseudohufelandi, Xerobiotus xerophilus Many animals that colonize interstitial soil habitats and lack organs suitable for digging possess appendages that are appreciably smaller than those found in their surface dwelling counterparts. A striking example is seen in the Collembola, whose members living on the soil surface (epiedaphic) have long legs and antennae and a strongly developed abdominal furca, whereas those always living underground among the interstices (euedaphic) have short antennae, short legs, and a vestigial furca limited to anal spines that are no longer suitable for jumping (for review see Schaller 1970). Tardigrades are almost always interstitial animals, but the space they can exploit differs when they colonize marine or freshwater sediments, mosses and lichens on tree-trunks or rocks, leaf litter, or soils covered by moss or grass turf but rich in inorganic material. Tardigrades interstitial in soil rich in inorganic material have been considered only recently (Fleeger & Hummon 1975; Manicardi & Bertolani 1987), except for a first and exceptional paper by Ramazzotti (1959), and remain poorly known to date. However, in this kind of habitat various genera and species of eutardigrades have been identified that differ from those living in freshwater, mosses, lichens, and leaf litter, in that they are characterized by very short legs and especially claws, which may sometimes even be absent a Corresponding author: Roberto Bertolani, Dipartimento di Biologia Animale, Universita di Modena, Via Universita, 4, 1-41100 Modena, Italy. Phone: ++39 59 225067; fax: ++39 59 581069; e-mail: bertolan@c220.unimo.it from some or all of the legs. Among these is the genus Hexapodibius, represented by 5 species, including Hexapodibius degenerans BISEROV 1990. When this species was described (Biserov 1990), it was compared with Parhexapodibius xerophilus DASTYCH 1978, the most similar species, and therefore attributed to the family Calohypsibiidae. However, Dastych & Alberti (1990) re-analyzed P. xerophilus by SEM and more correctly attributed the species to the family Macrobiotidae and the genus Macrobiotus. That conclusion and the strong similarity between the two species in many characters prompted us to investigate H. degenerans, M. xerophilus, and also another related species, Macrobiotus pseudohufelandi IHAROS 1966, in greater depth. Based on the symmetry of the claws on each pair of legs-the claws on either side of the leg axis are mirror-images of each other-we concluded that H. degenerans also belongs to the family Macrobiotidae, and not to Calohypsibiidae. Moreover, H. degenerans (on the first three pairs of legs), M. pseudohufelandi, and M. xerophilus show a type of claw peculiar among Macrobiotidae, characterized by the absence of a septum separating the basal part from the rest of the claw, and therefore without a distinct peduncle. As the absence of claws on the hind legs distinguishes Hexapodibius and Haplohexapodibius from other genera in Calohypsibiidae, the same character must be useful at genus level also for Macrobiotidae. For H. degenerans, which almost entirely lacks claws on the hind legs, we erect the new genus PseuThis content downloaded from 207.46.13.110 on Fri, 09 Dec 2016 05:43:58 UTC All use subject to http://about.jstor.org/terms