Echiniscus spinulosus (Doyere, 1840)

Gąsiorek, Piotr & Sørensen, Martin V., 2025, Too many spines, or an integrative foundation of the Echiniscus spinulosus morphogroup (Heterotardigrada: Echiniscidae), Organisms Diversity & Evolution 25 (1), pp. 223-247 : 234-235

The Echiniscus spinulosus complex

Our analyses emphasize the accurateness of Cuénot’s hesitation to accept E. spiniger and E. spinulosus as two distinct species. Both Cuénot (1932) and Dastych (1988) asserted that the lengths of lateral spines varied in E. spinulosus , approaching the length of dorsal spines (i.e. the condition characteristic for E. spiniger ). Peculiarly, although Ramazzotti and Maucci (1983) dismissed so early expressed criticism of one of the major early tardigradologists, they actually delivered evidence corroborating the hypothesis of conspecificity ( Cuénot, 1932). The specimens observed by Ramazzotti and Maucci (1983) exhibited variability in the development of dorsal spines, claimed to be absent in some individuals. The cogency of this probability values are provided as first at the nodes, followed by bootstrap values after slash. The asterisk (*) indicates maximal support, pound sign (#) indicates no support, and SAS =the South African- spinulosus morphogroup clade with 32 spp. (see Supplementary Material 2). Diploechiniscus oihonnae and Testechiniscus spitsbergensis constitute the outgroup statement is questionable given that both authors examined various populations identified as E. spiniger , but amongst them, some extra-European, thus representing various, sometimes completely unrelated taxa. A perfect example of this were animals from New Zealand ( Horning et al., 1978; Nelson & Horning, 1979), which several decades later were properly reassigned to the endemic E. elaeinae ( Pilato et al., 2005) . Echiniscus elaeinae has a completely different dorsal sculpturing of the blumi-canadensis type. Furthermore, Maucci (1986) argued that pores of E. spiniger are smaller and more numerous than those of E. spinulosus .

All new populations from Northern Europe, previously analysed populations from Central and Southern Europe ( Gąsiorek & Vončina, 2023), and re-analysed historical material from the Ramazzotti & Maucci collection stay in contrast with the previous communications. Specifically, we did not notice the presence of extensive morphological variability within populations, which always conformed either with the morphotype of E. spiniger , or that of E. spinulosus . The only trunk appendage showing constant chaetotaxy variability is spine B, which is typical for the vast majority of Echiniscus spp. ( Gąsiorek et al., 2022). The two types of pores and their respective size ( E. spinulosus allegedly has larger and more sparsely arranged pores than E. spiniger ; Maucci, 1986), the heteromorphy of primary spurs, the relatively long cirri A, the shape and development of pedal plates or teeth in the dentate collar are identical for E. spiniger and E. spinulosus . In other words, our morphological PSH is that two spp. could be present in Western Palaearctic, yet this is only supported by only one feature: the difference between lengths of lateral vs dorsal spines.

When genetic evidence is included in considerations, the picture shifts decisively. Echiniscus spiniger does not form a monophyletic group, separate from E. spinulosus ( Fig. 6 ). Out of nine molecular PSH, only one suggested the presence of two spp., which did not adhere to detected morphological variability, but reflected the divergence of one population within the more conservative ITS-1 alignment (Fig. 7). Therefore, by integrating all lines of evidence, our secondary species hypothesis ( SSH) is that E. spiniger and E. spinulosus are one species; thus, the former is established a younger synonym of E. spinulosus . The larvae have a variable chaetotaxy since Cuénot (1932) reported the formula A-C d -D d -E, but larvae from Ismantorp have almost fully developed chaetotaxy A-C-C d -D-D d -E. It has already been hypothesised that, based on the similarity of chaetotaxy and sculpturing, E. dikenli from Turkey could also belong in the spinulosus complex ( Gąsiorek & Vončina, 2023; Maucci, 1973). It is of interest that the short-spiniform morphotype of E. spinulosus is more common in the Mediterranean, whereas the long-spiniform morphotype generally prefers cooler climate ( Gąsiorek & Vončina, 2023). Both morphotypes are rare and found chiefly in mosses on calcareous rocks ( Dastych, 1988). Echiniscus spinulosus and E. scabrospinosus are the only members of the spinulosus morphogroup that belong in the continental European fauna. The historical records of E. spiniger and E. spinulosus ( McInnes, 1994) from numerous non-Palaearctic localities are erroneous and were not treated here in detail.

Figures

Fig. 6 Updated phylogeny of Echiniscus in the Bayesian and maximum likelihood tree based on nuclear markers (almost identical topologies; see the text and Supplementary Materials for details). Echiniscus spiniger from the topotypic population given in bold (see Table 2). The scale refers to the Bayesian tree and represents substitutions per position; posterior