Echiniscus C. A. S. Schultze, 1840

Dey, Pritam K., Gąsiorek, Piotr & Michalczyk, Łukasz, 2024, Convergent evolution of dark, ultraviolet-absorbing cuticular pigmentation in a new Afro-Oriental Echiniscus brunus species complex (Heterotardigrada: Echiniscidae), Zoological Journal of the Linnean Society 200 (1), pp. 34-59 : 40

Genus: Echiniscus C. A. S. Schultze, 1840

The Echiniscus brunus complex

Diagnosis: Unappendaged Echiniscus spp. of caramel colour in live individuals (e.g. Figs 3 , 5 ; compare also with Figs 4 , 6 ) caused by an interplay of orange pigments in the body cavity and dark brown pigmentation of the dorsal and pedal plates, and pulvini on legs. In specimens preserved in Hoyer’s medium, i.e. in which orange pigments dissolve asser several days, specimens are brown owing to the cuticular plate pigmentation ( Figs 5 , 9 , 10 , 12 , 14 ). The dark cuticular pigmentation is UV absorbent ( Fig. 7 ). All dorsal and (well-developed) pedal plates are with pores and without intracuticular pillars (sculpture of the E. spinulosus type; Figs 5 , 9 , 10 , 12 , 14 ). A median anterior suture has approximately from one-third to one-half of the scapular plate; it is visible in PCM and BFM as a bright incision and as a stripe of mostly smooth cuticle in SEM ( Figs 3E , 5 , 7B , 9A–C , 10A, E, F , 11A , 12A , 13A , 14A ). In some specimens, a crescent-shaped thickening on the caudal plate is present, beưer visible in PCM ( Figs 5 , 10A, E , 12A , 14A ), but also detectable in SEM ( Figs 11A, B , 13A, B ) and UVM ( Fig. 7B ). A pair of subcephalic ventral plates is present ( Figs 10C , 12C , 14C ).

Differential diagnosis: Because of the combination of distinct coloration (caramel/brown) and the lack of trunk appendages, species of the new complex are easily differentiable from all other Echiniscus spp. However, because of the presence of the dark cuticular pigmentation and the lack of trunk appendages, species of the E. brunus complex may be mistaken for Viridiscus at first sight. Thus, for accurate phenotypic identification, a careful examination of cuticular pigmentation (brown in the new complex vs. dark green to almost black in Viridiscus ), dorsal plate endocuticle (homogeneous and solid in the new complex vs. sponge layer in Viridiscus ) and scapular plate morphology (with a conspicuous anterior suture the new complex vs. no suture observed in Viridiscus ) is needed.

Composition: Echiniscus brunus ( India) and two undescribed candidate species, E. aff. brunus sp. can. 1 and ( India) E. aff. brunus sp. can. 2 ( Tanzania), genetically distinct but without a sufficient number of specimens to warrant a proper characterization of their morphological intraspecific variability and formal species descriptions.

Figures

Figure 3. An STM (top row) and BFM (boưom row) comparison of in vivo specimens of: A, D, Viridiscus viridianus; B, E, Echiniscus brunus sp. nov; C, F, E. cf.succineus. All specimens were imaged under the same microscope and camera seưings. The same individuals were photographed using both methods, and they are also shown in Figures 4–7. Scale bar: 50 μm (all photographs are at the same scale).

Figure 4. A BFM (top row) and PCM (boưom row) comparison of a specimen of Viridiscus viridianus mounted in Hoyer’s medium at 1 min, 20 h and 20 days asser mounting.Note the orange pigmentation dissolving as time passes (arrowheads indicate the remnants of the orange pigment). The same individual is shown in Figures 3 and 7. Scale bar: 50 μm (all photographs are at the same scale).

Figure 5. A BFM (top row) and PCM (boưom row) comparison of a specimen of Echiniscus brunus sp. nov. (paratype) mounted in Hoyer’s medium at 1 min, 20 h and 20 days asser mounting. Note the orange pigmentation dissolving as time passes (arrowheads indicate the remnants of the orange pigment). The same individual is shown in Figures 3 and 7. Scale bar: 50 μm (all photographs are at the same scale).

Figure 6. A BFM (top row) and PCM (boưom row) comparison of a specimen of E.cf.succineus mounted in Hoyer’s medium at 1 min, 20 h and 20 days asser mounting.Note the orange pigmentation dissolving as time passes (arrowheads indicate the remnants of the orange pigment). The same individual is shown in Figures 3 and 7. Scale bar: 50 μm (all photographs are at the same scale).

Figure 7. A UVM comparison of nine limno-terrestrial echiniscid species mounted in Hoyer’s medium, demonstrating the convergent dark, UV-absorbing cuticular pigmentation in Viridiscus and the Echiniscus brunus complex. A, Viridiscus viridianus. B, Echiniscus brunus sp. nov. C, Echiniscus cf.succineus. D, Echiniscus ehrenbergi. E, Acanthechiniscus victor. F, Barbaria bigranulata. G, Cornechiniscus madagascariensis. H, Hypechiniscus exarmatus. I, Mopsechiniscus granulosus. All specimens were imaged under the same microscope and camera seưings. Individuals in A–C are the same as those shown in Figures 3–6. Scale bar:50 μm (all photographs are at the same scale).

Figure 9. A PCM comparison of scapular plates of species with (A–F) and without (G–I) dark cuticular pigmentation mounted in Hoyer’s medium. A, Echiniscus brunus sp. nov. (paratype). B, Echiniscus aff.brunus sp. can. 1 from India. C, Echiniscus aff.brunus sp. can.2 from Tanzania. D, Viridiscus viridissimus. E, Viridiscus perviridis. F, Echiniscus ehrenbergi (paratype). G, Echiniscus testudo. H, Acanthechiniscus victor. I, Cornechiniscus madagascariensis. Arrowheads indicate the anterior suture. Scale bars: 20 μm.

Figure 10. Echiniscus brunus sp. nov. seen in PCM. A, dorsolateral view (holotype); the inset shows claws I (paratype). B, cuticular pores, lateral view of intersegmental plates (holotype). C, paired subcephalic plates (holotype).D, pedal plate II (holotype). E, larva, dorsolateral view (paratype); the inset shows claws III. F, armour malformation: median plate 2 fused with the paired segmental plate II (paratype). The flat arrowhead indicates sparsely arranged circular pores, whereas indented arrowheads indicate tightly arranged polygonal pores. The black indented arrowhead with white borders indicates the armour malformation.Scale bars are in micrometres.

Figure 11. Echiniscus brunus sp. nov. seen in SEM. A, dorsal view. B, lateral view.C, cuticular pores in intersegmental plates. D, pedal plate I. The flat arrowhead indicates sparsely arranged circular pores, whereas the indented arrowhead indicates tightly arranged polygonal pores. Scale bars are in micrometres.

Figure 12. Echiniscus aff. brunus sp. can. 1 from India seen in PCM. A, dorsolateral view; the inset shows claws I. B, cuticular pores in intersegmental plates. C, paired subcephalic plates. D, pedal plate II. The flat arrowhead indicates sparsely arranged circular pores. Scale bars are in micrometres.

Figure 13. Echiniscus brunus sp. can. 1 from India seen in SEM. A, dorsal view. B, lateral view.C, cuticular pores in intersegmental plates. D, pedal plate I. The flat arrowhead indicates sparsely arranged circular pores.Scale bars are in micrometres.

Figure 14. Echiniscus aff. brunus sp. can. 2 from Tanzania seen in PCM. A, dorsal view; the inset shows claws III. B, cuticular pores in intersegmental plates. C, paired subcephalic plates. D, pedal plate II. Scale bars are in micrometres.