Too many spines, or an integrative foundation of the Echiniscus spinulosus morphogroup (Heterotardigrada: Echiniscidae)

Abstract

AbstractHistorical taxonomic bias has started to be effectively removed in tardigrade taxonomy in the last two decades by the application of modern statistical, genetic and morphological tools. Many of the “old” tardigrade taxa have been redefined, and their succinct original diagnoses—refined, finally resulting in making them properly identifiable and less enigmatic. At the same time, many of the “old” species were synonymised and perished in the taxonomic wastebasket. Herein, we address the species distinction between Echiniscus spiniger Richters, 1904 and Echiniscus spinulosus (Doyère, 1840) by means of integrative analyses of 11 European populations collected in the Mediterranean, the Carpathians and the Baltic Sea islands of Öland and Gotland, covering the southern distribution zone and northernmost known localities within trustworthy geographic range of these species. We disclose that, as previously hypothesised, E. spiniger is conspecific with E. spinulosus, and synonymised accordingly. We deliver morphological evidence supporting the triphyly of the morphogroup, with all tropical and subtropical species truly lacking intracuticular pillars, whereas the Palaearctic lineage corresponding solely to E. spinulosus and Afrotropical lineage represented by E. belloporus Gąsiorek and Kristensen, 2018—occasionally exhibiting pillars in some specimens. A synopsis and key to all representatives of the E. spinulosus morphogroup are provided, pinpointing to some problems that must be taken into consideration during identification and solved in the future.