Convergent evolution of dark, ultraviolet-absorbing cuticular pigmentation in a new Afro-Oriental Echiniscus brunus species complex (Heterotardigrada: Echiniscidae)

Abstract

Abstract Green, brown and black pigments are uncommon in the otherwise typically yellow to orange Echiniscidae. Viridiscus, a genus currently represented by a handful of species, in which cuticular coloration varies from a light green through dark green to almost black, has been an exception. Here, we uncover a new echiniscid lineage from the primeval subtropical and tropical rainforests of India and Tanzania, comprising three species with a caramel/brown body. Given that both Viridiscus and the new species complex lack dorsolateral trunk appendages and share dark cuticular pigmentation, they may be mistaken for each other. However, a closer inspection of the dorsal cuticular sculpture and molecular phylogenetic analyses indicate that the new complex belongs within the Echiniscus spinulosus morphogroup. Thus, the presence of dark pigments in two indirectly related lineages is evidence for the convergence of dark cuticular coloration in limno-terrestrial heterotardigrades. We detected three species in the new complex: Echiniscus brunus sp. nov., from the Western Ghats (Tamil Nadu, India), and two other candidate species, Echiniscus aff. brunus sp. can. 1 and Echiniscus aff. brunus sp. can. 2, from India and Tanzania, respectively. We refrain from describing the two latter species formally because they exhibit little or no morphological differences, which is yet another clear case of the crucial role of DNA barcoding in an accurate estimation of tardigrade species richness.