Green armoured tardigrades (Echiniscidae: Viridiscus), including a new species from the Southern Nearctic, exemplify problems with tardigrade variability research

Abstract

Ranges of tardigrade intraspecific and interspecific variability are not precisely defined, both in terms of morphology and genetics, rendering descriptions of new taxa a cumbersome task. This contribution enhances the morphological and molecular dataset available for the heterotardigrade genus Viridiscus by supplying new information on Southern Nearctic populations of V. perviridis, V. viridianus, and a new species from Tennessee. We demonstrate that, putting aside already well-documented cases of significant variability in chaetotaxy, the dorsal plate sculpturing and other useful diagnostic characters, such as morphology of clavae and pedal platelets, may also be more phenotypically plastic characters at the species level than previously assumed. As a result of our integrative analyses, V. viridianus is redescribed, V. celatus sp. nov. described, and V. clavispinosus designated as nomen inquirendum, and its junior synonymy with regard to V. viridianus suggested. Morphs of three Viridiscus species (V. perviridis, V. viridianus, and V. viridissimus) are depicted, and the implications for general echiniscid taxonomy are drawn. We emphasise that taxonomic conclusions reached solely through morphological or molecular analyses lead to a distorted view on tardigrade α-diversity.

Figure 1

Two main morphotypes present in Viridiscus (PCM): (A) pores present, epicuticular granules typically reduced: the usual morph of V. viridissimus (Tennessee, dorsal view); (B) an aberrant specimen of V. viridissimus (Tennessee, dorsolateral view, the first median plate merged with the first paired segmental plate); (C) epicuticular granules dominant, pores present only in larval stage: V. viridianus (Alabama, dorsolateral view). Scale bars = 50 μm.

Figure 2

Two main morphotypes of dorsal plate sculpturing present in Viridiscus (SEM): pores dominant, only V. viridissimus (the population from Vietnam): (A) a fragment of the scapular plate; (B) close up of the posterior portion of the second paired segmental plate; epicuticular granules dominant, micropores visible only in SEM, all remaining Viridiscus species (the population of V. perviridis from Vietnam shown): (C) a fragment of the scapular plate; (D) close up of the central portion of the scapular plate. Scale bars in μm.

Figure 3

Intrageneric and intraspecific variability in Viridiscus (PCM): (A) V. viridissimus (Tennessee), the typical morph; (B) V. viridissimus (Tennessee), the atypical morph with well-developed epicuticular granules; (C) V. perviridis (Madeira), the typical morph with well-developed epicuticular granules; (D) V. perviridis (Alabama), the atypical morph with poorly delineated epicuticular granules; (E) V. viridianus (Alabama), the typical morph with well-developed epicuticular granules; (F) V. viridianus (Alabama), the atypical morph (male) with epicuticular granules absent (arrowhead points out conoid primary clava); (G,H) V. celatus sp. nov. (Tennessee), the only morph observed. Scale bars = 20 μm.

Figure 4

Larva of V. viridianus (PCM, dorsolateral view). Scale bar = 20 μm.

Figure 5

Phylogenetic relationships of the genus Viridiscus: Bayesian tree based on the concatenated ITS-1 + ITS-2 dataset (1058 bp); vertical bars denote different delineation methods used in the formulation of the primary molecular species hypotheses: ASAP, ABGD, and bPTP; (COI) refers to COI delimitation in all three methods). Asterisks indicate the maximal (1.00/100) posterior probability/bootstrap value. Echiniscus succineus was used as an outgroup. Scale bar represents substitutions per position.

Figure 6

Habitus of V. viridianus (SEM): (A) dorsal view; (B) frontal view; (C) rear view. Scale bars = 50 μm.

Figure 7

Anterior body portion of V. viridianus (SEM): (A) dorsal view; (B) head. Scale bars = 20 μm.

Figure 8

Cuticle of V. viridianus (SEM): (A) epicuticular granules and micropores; (B) lateral portion of disrupted epicuticle; (C,D) close up of intracuticular pillars. Scale bars in μm.

Figure 9

Leg morphology of V. viridianus (PCM). Arrows indicate pedal platelets in central leg portions, white asterisks indicate distinctly demarcated, central oval areas in pedal platelets, and black asterisks indicate pulvini in proximal leg portions. Scale bars = 20 μm.

Figure 10

Morphological details of V. viridianus (PCM): (A–C) conoid primary clava; (D) dactyloid primary clava; (E) typical for most echiniscids, tubby Echiniscus-like primary clava; (F) subcephalic plates; (G) claws I; (H) claws II; (I) claws III. Scale bars = 10 μm.

Figure 11

Leg structures of V. viridianus (SEM): (A) claws I; (B) claws II; (C) claws III; (D) claws IV; (E) leg morphology. Arrows indicate pedal platelets in central leg portions, white asterisks indicate distinctly demarcated, central oval areas in pedal platelets, and black asterisks indicate pulvini in proximal leg portions. Scale bars in μm.

Figure 12

Holotype of V. celatus sp. nov. (PCM, female, dorsal view). Scale bar = 20 μm.

Figure 13

Type specimens of V. celatus sp. nov. (PCM): (A) allotype (male, dorsal view); (B) paratype (female, dorsolateral view). Arrowheads indicate rudimentary papillae on legs II–III. Scale bars = 20 μm.

Figure 14

Morphological details of V. celatus sp. nov. (PCM): (A) subcephalic plates (holotype); (B) claws III (paratype, juvenile); (C) claws IV (holotype). Scale bars = 10 μm.

Figure 15

Habitats harbouring Viridiscus populations in Andreaea mosses growing on boulders by Lake Harris, Tuscaloosa, Alabama: (A) an overall view of the rock substrate; (B) close up of the moss growing on the vertical surfaces of the boulder (arrow indicates the moss matt); (C) close up of the pleurocarpous moss and leaf structure when dry; (D) moss leaves seen under a stereomicroscope (arrows indicate animals on moss leaves in the tun state).