Abstract
The Mesobiotus harmsworthi group has a global distribution, with localities in polar, temperate and tropical zones. Since the first species of the harmsworthi group was described in the beginning of the 20th century, tens of new species within the group were found and named. However, the diagnosis of the nominal Mesobiotus harmsworthi is insufficient and enigmatic, thus it can be is a serious obstacle in solving the taxonomy of this group. Here, we integratively redescribe the nominal species for the genus Mesobiotus, i.e., Mesobiotus harmsworthi and clarify taxonomic statuses of the two subspecies: M. harmsworthi harmsworthi and M. harmsworthi obscurus that have been recognised as distinct taxa for more than three decades. Traditionally, egg chorion in M. harmsworthi was considered almost smooth and without any traces of areolation, however here we report many misunderstandings that accumulated across decades and we show that, in fact, the chorion in this species exhibits a partially developed areolation. We present an integrative (morphological, morphometric and molecular) diagnosis of the nominal taxon and we confirm that it differs from other species of the harmsworthi group by morphological characters of both animals and eggs. Additionally, we describe two new species of the genus Mesobiotus: M. skorackii sp. nov. from the Kyrgyz Republic (using classical morphological description) and M. occultatus sp. nov. from Svalbard Archipelago (by means of integrative taxonomy). Finally, we also provide the first genetic phylogeny of the genus Mesobiotus based on COI sequences which, together with molecular species delimitation, independently confirms the validity of the analysed taxa.
Highlights
The phylum Tardigrada comprises over 1,200 species [1, 2, 3] that inhabit both aquatic and terrestrial environments throughout the world, from the deepest seas to the highest mountain peaks [4, 5, 6, 7]
In order to clarify the taxonomy of the harmsworthi group, we integratively redescribe M. h. harmsworthi based on typical material of M. h. obscurus and we describe two new species of the harmsworthi group: M. occultatus sp. nov. from Spitsbergen and M. skorackii sp. nov. from the Kyrgyz Republic
Two unidentified Mesobiotus species, one of the furciger group collected from continental Norway (Lyngen Alps, moraine of Steindalen glacier; 69 ̊23’44.04"N, 19 ̊55’0.84"E) and the other of the harmsworthi group collected from Russia (Irkuck; 52 ̊16’42.3’’N, 104 ̊17’22.1’’E), were extracted from moss samples and used in phylogenetic analysis
Summary
The phylum Tardigrada comprises over 1,200 species [1, 2, 3] that inhabit both aquatic (freshwater, brackish, marine) and terrestrial environments throughout the world, from the deepest seas to the highest mountain peaks [4, 5, 6, 7]. Tardigrades have been investigated for over two hundred years, but because of old descriptions and insufficient morphometric data, many species currently need revision and redescription, especially those representing nominal taxa for cosmopolitan groups or genera. One of such taxa is Mesobiotus Vecchi, Cesari, Bertolani, Jonsson, Rebecchi & Guidetti, 2016 [8], a cosmopolitan genus comprising ca. Species of the harmsworthi group are characterised by three clearly separated macroplacoids in the shape of short, rounded rods and a distinct microplacoid situated very close to them, as well as by conical or hemispherical egg processes [10]. Without DNA sequences, it is not possible to verify whether all specimens constituted a single or multiple cryptic or pseudocryptic species
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