Experimental taxonomy exposes ontogenetic variability and elucidates the taxonomic value of claw configuration in MilnesiumDoyère, 1840 (Tardigrada: Eutardigrada: Apochela)

Abstract

Key words: barcoding, diagnostic key, development tracking, in vitro culture, M. berladnicorum, M. variefidum sp. nov. AbstractIn this paper we describe a new apochelan species, Milnesium variefidum sp. nov. from Scotland and provide novel morphological and molecular data for Milnesium berladnicorum Ciobanu et al., 2014. The new species differs from the most similar M. berladnicorum by the presence of developmental dimorphism in claw configuration, absent or weakly developed cuticular bars under claws I-III, a different arrangement of cuticular pseudoplates, and by differences in the sequences of three nuclear DNA fragments: 18S rRNA (p-distance: 0.6%), 28S rRNA (2.0%), ITS-2 (9.3%), and on mitochondrial gene COI (12.4%). Although ontogenetic claw configuration change was suspected to occur in some Milnesium species, we are the first to document it through the combined use of traditional, molecular and experimental methodologies. We discuss the implications of the observed phenomenon for the taxonomy of the genus and propose a new diagnostic key to all Milnesium species described up to the end of 2015. We also review other traits used for species differentiation in the genus and offer recommendations to improve the quality of future descriptions as well as suggest a need for integrative redescriptions of the known species. Finally, we propose to suppress M. dujiangensis and M. tardigradum trispinosum and suggest that M. alpigenum and M. quadrifidum are valid species that require thorough redescriptions.

External morphogenesis of the tardigradeHypsibius dujardinias revealed by scanning electron microscopy

“…It should be noted that “pseudopores” have been described from several species of Milnesium , showing that they are typically more numerous in adults than in juveniles (Kaczmarek, Jakubowska, & Michalczyk, ; Morek, Gąsiorek, Stec, Blagden, & Michalczyk, ). However, these pseudopores are actually intracuticular cavities, and their distribution pattern does not correspond to muscle attachment sites (Morek et al, ). Therefore, these characters are likely a property of the cuticle itself rather than any underlying structures and should not be homologized with the pores we describe herein.…”

Milnesium tardigradum Doyère, 1840: The first integrative study of interpopulation variability in a tardigrade species

“…, as in M. tardigradum , see below) and between the second and the third instar (e.g. , as in M. variefidum Morek, Gąsiorek, Stec, Blagden, & Michalczyk, ), we suggest to use distinct and intuitive single‐word names to describe the two life stages: the hatchling and the juvenile for the first and the second instar, respectively. From the third instar onwards, individuals are sexually mature; thus, they should be referred to as adults .…”

“…Moreover, in order to distinguish the time of the claw configuration change, we propose the terms early and late claw configuration change to describe the change occurring between the hatchling and the juvenile, and between the juvenile and the adult, respectively. Furthermore, as the number of points may increase (e.g., as in M. variefidum Morek, Gąsiorek, et al., ) or decrease (e.g., as in M. tardigradum , see below) during ontogenesis, we postulate to distinguish the direction of the claw configuration change by naming the increase as the positive , and the decrease as the negative claw configuration change . If needed, the position of the claw (i.e.…”

“…In order to test for the potential CC change and to establish the developmental stage at which it occurs, we applied two of the three detection methods proposed by Morek, Gąsiorek, et al. (): hatching and development tracking. Hatching, that is , incubating an exuvia with eggs until hatched, then mounting hatchlings alongside the empty maternal exuvia on a permanent slide, allows comparison of CCs of the mother and first instar offspring (hatchlings).…”

Tardigrada of Ireland: a review of records and an updated checklist of species including a new addition to the Irish fauna

“…Unfortunately, no further details for the Milnesium specimens were included in his description, yet on the basis of the claw configuration it can be concluded that Murray’s Irish specimens are not Milnesium tardigradum sensu stricto or its subspecies and so this record must be listed in our Irish checklist as Milnesium sp. sensu Michalczyk et al (2012a, 2012b) and Morek et al (2016).…”