Description of Milnesium pentapapillatum sp. nov., with an amendment of the diagnosis of the order Apochela and abolition of the class Apotardigrada (Tardigrada)

Abstract

Tardigrades constitute a phylum of cosmopolitan, microscopic invertebrates, capable of withstanding harsh environmental conditions thanks to cryptobiosis. These tiny animals are taxonomically challenging due to the limited number of meaningful morphological traits and morphological stasis. One of the most extreme examples is the order Apochela, in which the morphology is even more simplified and conserved than in the majority of other tardigrades. Despite this, apochelans are currently popular among researchers, as almost half of the 45 known species have been described only in the last five years. During the examination of samples from Kazakhstan we encountered specimens belonging to the genus Milnesium Doyère, 1840, certainly representing a species new to science. Milnesium pentapapillatumsp. nov. is described in the integrative taxonomy framework, which includes morphometrics, imaging both in light and scanning electron microscopy, ontogenetic variability testing, and DNA sequencing of four standard markers, which allowed for pinpointing its phylogenetic position. Although this species, with a [3-3]-[3-3] claw configuration and smooth cuticle, represents the most common morphotype found in Milnesium, it can be easily differentiated from all other congeners by the presence of five (instead of six) peribuccal papillae. Given that to date all apochelans were characterised by six peribuccal papillae, the discovery of M. pentapapillatumsp. nov. entails a change in the diagnosis of the order Apochela, family Milnesiidae, and the genus Milnesium. Moreover, we propose to suppress the recently erected class Apotardigrada and restore the suppressed order Parachela, since grouping the Apochela and Parachela under Eutardigrada reflects tardigrade phylogeny.