Mesobiotus, Vecchi, Cesari, Bertolani, Jonsson, Rebecchi and Guidetti, 2016

Stec, Daniel, 2022, An Integrative Description of Two New Species (Tardigrada: Eutardigrada: Macrobiotidae) with Updated Genus Phylogeny., Zoological Studies 61 (85), pp. 1-30 : 20-22

Mesobiotus phylogeny and species composition

Similarly to other macrobiotid genera, the genus Mesobiotus also exhibits a rather stable and conservative animal morphology. Interestingly, egg morphotypes known in the genus exhibit one of the most drastic examples of morphological diversity in egg ornamentation ( Kaczmarek et al. 2020; Stec et al. 2021). This is in line with the observation that chorion ornamentation evolves faster than animal morphology ( Guidetti et al. 2013) which was also confirmed by experimental findings about the congruence between genetic and morphological divergence ( Stec et al. 2016b). The increase in tempo in the morphological divergence of the egg chorion in tardigrades could be explained by two alternative hypotheses with strong or relaxed natural selection, respectively. In the first scenario, different morphotypes might be shaped by biotic and abiotic constraints and as such constitute adaptations to oviposition in different microhabitats that potentially increase protection and/or attachment properties but also ease dispersion as the empty processes make the egg lighter. Alternatively, if specific ornamentations do not have any adaptive value, relaxed natural selection would enable unconstrained and more flexible evolution that would derive a plethora of morphotypes. Currently, there is a lack of sufficient data to test these hypotheses properly.

As mentioned in the Introduction, Mesobiotus was established by integrative analysis of two former species complexes in the genus Macrobiotus , the harmsworthi and furciger groups ( Vecchi et al. 2016), and the monophyly of the genus was subsequently confirmed by other studies ( Guil et al. 2019; Stec et al. 2021). However, these informal species groups could not be accommodated into any subgeneric rank, as their representatives do not form monophyletic clades and are scattered in different places in the genus phylogeny ( Kaczmarek et al. 2018 2020; Stec 2021, Stec et al. 2021 2022; Short et al. 2022; this study). The two informal groups within the genus are recognized elusively by egg morphology and were recently criticized by Short et al. (2022), who demonstrated large divergence between Antarctic and non-Antarctic Mesobiotus taxa and proposed to abandon the usage of informal groups within the genus. The authors reasoned that the groups have no systematic value as they are not monophyletic clades and as such, they hide evolutionary relationships and biogeographical patterns. However, I argue that (i) given the extreme morphological diversity within the genus, informal groups have a tremendous practical value for both taxonomists and name-users in aiding navigation, identification, and communication regarding taxa, and (ii) the confusion regarding these informal groups results most likely from their elusive working definitions. Therefore, to clarify the distinction between the aforementioned informal species groups, I here propose explicit criteria coming from egg morphological characters that should be met to include Mesobiotus taxa to species morpho-groups. This action resulted in the creation of the third informal taxonomic group of species that greatly differ from the furciger and harmsworthi egg morphotype ( Fig. 14 ). I propose to use the term “morpho-groups” when referring to those divisions that, according to Stec et al. (2021), should represent nonmonophyletic sets of phenotypically similar taxa. Mesobiotus morpho-groups are as follows:

Mesobiotus furciger morpho-group

Egg processes in the shape of branched cones, processes with smooth walls (without labyrinthine layer that is seen as reticulation) or with light refracting areas.

Group composition

Mesobiotus furciger ( Murray, 1907b) , Mesobiotus pilatoi ( Binda and Rebecchi, 1992) , Mesobiotus fiedleri Kaczmarek, Bartylak, Stec, Kulpa, M. Kepel, A. Kepel and Roszkowska, 2020 , Mesobiotus marmoreus Stec, 2021 , Mesobiotus siamensis ( Tumanov, 2006) , Mesobiotus divergens ( Binda, Pilato and Lisi, 2005) , Mesobiotus dilimanensis Itang, Stec, Mapalo, Mirano-Bascos and Michalczyk, 2020 , Mesobiotus creber ( Pilato & Lisi, 2009) , Mesobiotus orcadensis ( Murray, 1907c) , Mesobiotus aradasi ( Binda, Pilato and Lisi, 2005) , Mesobiotus sicheli ( Binda, Pilato and Lisi, 2005) .

Mesobiotus montanus morpho-group

Egg processes in the shape of hemispherical or mammillate-like domes.

Group composition

Mesobiotus montanus ( Murray, 1910) , Mesobiotus mottai ( Binda and Pilato, 1994) , Mesobiotus peterseni ( Maucci, 1991) , Mesobiotus lusitanicus ( Maucci and Durante Pasa, 1984) *.

*Remarks: Mesobitous lusitanicus exhibits a considerable variation in the morphology of the egg processes. However, it is included in the M. montanus morpho-group as the typical form of the processes is mammillate-like domes. The abnormal form of egg reported in the original description may actually belong to a different Mesobiotus species, which could have also been present in the analysed samples.

Mesobiotus harmsworthi morpho-group

Egg processes in the shape of cones with diverse morphology of process endings (long slender endings, long slender endings with filaments, sharp endings, endings with flexible filaments, truncated endings).

Groups composition

Mesobiotus altitudinalis ( Biserov, 1997 /98), Mesobiotus anastasiae Tumanov, 2020 , Mesobiotus arguei ( Pilato and Sperlinga, 1975) , Mesobiotus armatus ( Pilato and Binda, 1996) [nomen inquirendum], Mesobiotus australis ( Pilato and D'Urso, 1976) , Mesobiotus baltatus ( McInnes, 1991) , Mesobiotus barabanovi ( Tumanov, 2005) , Mesobiotus barbarae ( Kaczmarek, Michalczyk and Degma, 2007) , Mesobiotus binieki ( Kaczmarek, Gołdyn, Prokop and Michalczyk, 2011) , Mesobiotus blocki ( Dastych, 1984) , Mesobiotus contii ( Pilato and Lisi, 2006b) , Mesobiotus coronatus (de Barros, 1942) , Mesobiotus datanlanicus Stec, 2019 , Mesobiotus diffusus ( Binda and Pilato, 1987) , Mesobiotus diguensis ( Pilato and Lisi, 2009) , Mesobiotus dimentmani ( Pilato, Lisi and Binda, 2010) , Mesobiotus emiliae Massa, Guidetti, Cesari, Rebecchi and Jönsson, 2021 , Mesobiotus erminiae ( Binda and Pilato, 1999) , Mesobiotus ethiopicus Stec and Kristensen, 2017 , Mesobiotus harmsworthi ( Murray, 1907a) , Mesobiotus helenae Tumanov and Pilato, 2019 , Mesobiotus hieronimi ( Pilato and Claxton, 1988) , Mesobiotus hilariae Vecchi, Cesari, Bertolani, Jönsson, Rebecchi and Guidetti, 2016 , Mesobiotus imperialis Stec, 2021 , Mesobiotus insanis Mapalo, Stec, Mirano-Bascos and Michalczyk, 2017 , Mesobiotus insuetus ( Pilato, Sabella and Lisi, 2014) , Mesobiotus joenssoni Guidetti, Gneuss, Cesari, Altiero and Schill, 2020 , Mesobiotus kovalevi ( Tumanov, 2004) , Mesobiotus krynauwi ( Dastych and Harris, 1995) , Mesobiotus liviae ( Ramazzotti, 1962) , Mesobiotus mauccii ( Pilato, 1974) , Mesobiotus meridionalis ( Richters, 1909) [nomen inquirendum], Mesobiotus neuquensis ( Rossi, Claps and Ardohain, 2009) , Mesobiotus nikolaevae Tumanov, 2018 , Mesobiotus nuragicus ( Pilato and Sperlinga, 1975) , Mesobiotus occultatus Kaczmarek, Zawierucha, Buda, Stec, Gawlak, Michalczyk and Roszkowska, 2018 , Mesobiotus ovostriatus ( Pilato and Patanè, 1998) , Mesobiotus patiens ( Pilato, Binda, Napolitano and Moncada, 2000) , Mesobiotus perfidus ( Pilato and Lisi, 2009) , Mesobiotus philippinicus Mapalo, Stec, Mirano-Bascos and Michalczyk, 2016 , Mesobiotus polaris ( Murray, 1910) [nomen inquirendum], Mesobiotus p s e u d o b l o c k i Roszkowska, Stec, Ciobanu and Kaczmarek, 2016, Mesobiotus pseudocoronatus ( Pilato, Binda and Lisi, 2006) , Mesobiotus pseudoliviae ( Pilato and Binda, 1996) , Mesobiotus pseudonuragicus ( Pilato, Binda and Lisi, 2004) , Mesobiotus pseudopatiens Kaczmarek and Roszkowska, 2016 , Mesobiotus radiatus ( Pilato, Binda and Catanzaro, 1991) , Mesobiotus reinhardti ( Michalczyk and Kaczmarek, 2003) , Mesobiotus rigidus ( Pilato and Lisi, 2006a) , Mesobiotus romani Roszkowska, Stec, Gawlak and Kaczmarek, 2018 , Mesobiotus simulans ( Pilato, Binda, Napolitano and Moncada, 2000) , Mesobiotus skorackii Kaczmarek, Zawierucha, Buda, Stec, Gawlak, Michalczyk and Roszkowska, 2018 , Mesobiotus snaresensis ( Horning, Schuster and Grigarick, 1978) , Mesobiotus stellaris ( du Bois-Reymond Marcus, 1944) [nomen inquirendum], Mesobiotus szeptyckii ( Kaczmarek and Michalczyk, 2009) , Mesobiotus tehuelchensis ( Rossi, Claps and Ardohain, 2009) , Mesobiotus wuzhishanensis (Yin, L. Wang and X. Li, 2011) , Mesobiotus zhejiangensis (Yin, L. Wang and X. Li, 2011) .

Figures

Fig. 14. Mesobiotus peterseni (Maucci, 1991) from Greenaland – PCM images of the egg. (A–B) entire egg with egg processes midsections seen on the egg circumference; (C–E) egg surface. Scale bars in μm.