Turning to the dark side: Evolutionary history and mole adcular species delimitation of a troglomorphic lineage of armoured harvestman (Opiliones: Stygnopsidae)

From a biological point of view, caves are one of the most exciting environments on Earth, considered as evolutionary laboratories due to the adaptive traits (troglomorphisms) usually exhibited by the fauna that inhabit them. Among Opiliones, the family Stygnopsidae contains cave-inhabiting members who exhibit some degree of troglomorphic characters, such as Minisge gen.n., a lineage formed by two new troglomorphic species from the Huautla Cave System, Oaxaca, Mexico, one of the deepest and most complex cave systems in the World. One of the new species inhabits the middle depths (~ 400 to ~ 600 m), whereas the other one is considerably shallower (~ 20 to ~ 200 m). Using the barcoding gene (CO1), we tested the morphology-based species delimitation hypothesis using genetic distances, likelihood-based and bayesian-based methods (ABGD, GMYC and bPTP), which give different results with respect to morphology. The shallower species exhibits considerable gene flow among the various caves sampled and the genetic data support our morphology-based conclusions- whereas the deeper species shows less gene flow among some of the caves in the system, and the genetic data contradict our morphology-based conclusion that there is only one species involved. However, the genetic differences among the populations sampled vary primarily in the third codon position and represent synonymous mutations. Finally, the two species of Minisge gen.n. probably diverged 3.9 Mya according to a time-calibrated phylogeny, but at this time it is not possible to determine when they colonized the cave environment, although we favour the hypothesis that each species of Minisge colonized the caves independently: the deeper inhabitant, which exhibits a greater degree of troglomorphisms, first- and subsequently the shallower inhabitant, which exhibits a lesser degree of troglomorphisms.