Data from new taxa inferIsoechiniscoidesgen. nov. and increase the phylogenetic and evolutionary understanding of echiniscoidid tardigrades (Echiniscoidea: Tardigrada)

Abstract

The marine heterotardigrades belonging to Echiniscoididae inhabit a wide range of substrates and represent the dominant tardigrade group in intertidal zones worldwide. Notably, the evolution of this family is interesting as it holds key evidence as to how heterotardigrades colonized terrestrial and freshwater environments. We inferred the phylogeny of Echiniscoididae using 28S and COI sequences from GenBank and from new specimens collected at Roscoff, France. Morphological characterizations of several major clades, inferred from the molecular data, are currently lacking. However, one of the clades is erected as a new taxon, Isoechiniscoides gen. nov., for the former Echiniscoides higginsi-group, which comprises species with isonych claws, pillars in the epicuticle and exceptionally long sensory appendages. The type species for the new genus is described here as Isoechiniscoides sifae nov. gen., sp. nov. Isoechiniscoides comprises six clawed interstitial species, whereas Echiniscoides taxa possess a higher claw number and primarily inhabit algae, barnacles and lichens in tidal zones. The new genus, Isoechiniscoides, together with morphologically undetermined specimens from British Columbia (Canada), is sister-group to all other Echiniscoididae taxa. Consequently, we propose to establish Isoechiniscoidinae subfam. nov. to accommodate the new genus. Isoechiniscoidinae provides an evolutionary link between Echiniscoidea and Arthrotardigrada. The latter comprises exclusively marine and mostly interstitial species.

An integrated study of the biodiversity within the Pseudechiniscus suillus–facettalis group (Heterotardigrada: Echiniscidae)

“…Even though some criticisms have recently arisen about the cryptic status of these molecularly identified species (Stec et al, 2018a), the fact that taxonomically important morphological characters are scarce in tardigrades must not be underrated. Actually, the application of an integrated approach has allowed the definition of many tardigrade species (Cesari et al, 2009(Cesari et al, , 20112016b;Bertolani et al, 2011a, b;Guidetti et al, 2013Guidetti et al, , 2014Vicente et al, 2013;Stec et al, 2015Stec et al, , 2017aStec et al, , b, c, 2018bGąsiorek et al, 2016Gąsiorek et al, , 2017aGąsiorek et al, , b, c, 2018bMøbjerg et al, 2016;Morek et al, 2016;Zawierucha et al, 2016Zawierucha et al, , 2018Roszkowska et al 2017Roszkowska et al , 2018Buda et al, 2018;Kaczmarek et al, 2018;Nowak & Stec, 2018;Perry et al, 2018), but it also pointed out that many difficult taxonomic situations still exist in both Heterotardigrada and Eutardigrada, and it evidenced the surprisingly large presence of putative cryptic species in the phylum (Guidetti et al, 2009(Guidetti et al, , 2016(Guidetti et al, , 2019 ; Bertolani et al, 2011b;Faurby et al, 2011Faurby et al, , 2012Faurby & Barber, 2015;Cesari et al, 2016a;Stec et al, 2018a;Morek et al, 2019). Presently, the most sensible solution would be to place species difficult to distinguish into the category of cryptic or semicryptic species (complexes that may display minor, but still detectable, morphological differences;…”

An upgraded comprehensive multilocus phylogeny of the Tardigrada tree of life

“…Heterotardigrada internal classification has been problematic since the first molecular phylogenies, as they did not support the classical classification based on morphological similarities (Bertolani et al, 2014;Fujimoto, Jørgensen, & Hansen, 2017;Guil & Giribet, 2012;Guil, Machordom, et al, 2013;Jørgensen et al, 2011). Few attempts to organize the heterotardigrade classification have been done (Møbjerg, Kristensen, & Jørgensen, 2016) despite to recent phylogenies that contradicted arthrotardigrade and echiniscoidean classifications (Fujimoto et al, 2017;Guil, Machordom, et al, 2013;Jørgensen, Faurby, Hansen, Møbjerg, & Kristensen, 2010). Our results supported five phylogenetic lineages ((a) Hypechiniscus, Testechiniscus, Diploechiniscus and Echiniscus; (b) Bryodelphax and Bryochoerus; (c) Acanthechiniscus, Cornechiniscus and Proechiniscus; (d) Pseudechiniscus with Mopsechiniscus; and (e) Parechiniscus; Figure 6), also found by other authors with morphological and/or molecular information (Guil & Giribet, 2012;Guil, Machordom, et al, 2013;Jørgensen, 1999;Jørgensen et al, 2011;Kristensen, 1987;Vecchi et al, 2016).…”

Osmotic stress tolerance in semi-terrestrial tardigrades

“…A significant difference between marine and limnic/semi‐terrestrial habitats is the amount of salts present in the surrounding water. Thus, ionic and osmotic stress tolerance may represent a key driver in the evolution of tardigrades (Møbjerg, Kristensen & Jørgensen, ). Regardless of this possible significance, only a limited number of studies have been performed on salt and osmotic stress tolerance in tardigrades, mainly including marine species (Halberg et al ., ; Jørgensen & Møbjerg, ; Hygum et al ., ).…”