Opportunities and challenges presented by cryptic bryophyte species.

Abstract

Cryptic bryophyte species exhibit a decoupling in the degree of morphological and molecular divergence, as a result of different processes, from recent divergence to stasis. Here a body of cryptic species literature comprising 110 papers published between 2000 and end 2018 is reviewed. Most studies of cryptic species focused on northern hemispheric taxa, but we do not yet have sufficient studies to assess whether a geographic bias in the distribution of cryptic species exists, and we don’t know how many cryptic bryophyte species there might be globally. Fully two-thirds of all studies on cryptic bryophyte species rested their claims of morphological crypsis on previous taxonomic investigations, without revision of morphology to confirm cryptic species status. There is more than one kind of morphological crypsis, and while quantification of morphological patterns can contribute to our understanding of crypsis this is a widely neglected component. The usage of ‘cryptic species’ as an etymological tool to flag instances where traditional species concepts are deficient devalues the term, and a distinction between genuine crypsis and business as usual revision of species circumscription should be re-established and maintained. Hybridisation is possibly an under-appreciated contributor to cryptic species, but inference of hybridization has been limited by study design. Opportunities exist in the application of geometric morphometric methods and next generation sequencing technologies to overcome intrinsic limitations in traditional morphological and molecular data sources.