The multilevel organismal diversity approach deciphers difficult to distinguish nudibranch species complex

Abstract

Species identification is a key procedure for broad-scoped ecological, phylogeographic and evolutionary studies. However, to perform a taxonomic study in the molecular era is a complicated task that has many pitfalls. In the present study we use particular examples of common but difficult to distinguish European species within the genus of Polycera (Nudibranchia, Mollusca) to discuss the general issues of the “cryptic species” problem that has broad biological and interdisciplinary importance and can significantly impede ecological, evolutionary, and other biodiversity-related research. The largest dataset of molecular and morphological information for European nudibranchs ever applied encompasses a wide geographical area and shapes a robust framework in this study. Four species are recognized in the species complex, including a new one. It is shown that a lack of appropriate taxonomic analysis led recently to considerable errors in species identity assessment of this complex. Chromatic polymorphism for each species is mapped in a periodic-like framework and combined with statistical analysis of the diagnostic features that considerably facilitates identification of particular species in the complex for biologists and practitioners. The present study evidently shows that “cryptic” and “non-cryptic” components are present within the same species. Therefore, this species complex is well suited for the exploring and testing of general biological problems. One of the main conclusions of this study is that division of biological diversity into “cryptic” and “non-cryptic” components is counterproductive. We propose that the central biological phenomenon of a species can instead be universally designated as multilevel organismal diversity thereby provide a practical set of methods for its investigation.

Polycera norvegica is a valid species, and a plea for good taxonomic practices – a reply to Korshunova et al., 2021

“…In their recent paper, Korshunova et al . (2021) discuss the problem of recognizing cryptic species and the practical measures required to identify and name these taxa through the integration of morphological, molecular phylogenetic, phylogeographic and ecological data with statistical analyses, a methodology which they named ‘The multilevel organismal diversity approach’. They used as the model group, the European species of the nudibranch genus Polycera Cuvier, 1816 with a focus on the Polycera quadrilineata – Polycera faeroensis species groups.…”

“…We here show that the arguments used by Korshunova et al . (2021) to synonymize these two species are unfounded. Their emendation, which is both arbitrary and unjustified, is here reverted in the interests of scientific integrity and taxonomic stability.…”

“…Korshunova et al . (2021) have explicitly stated that whereas Sørensen et al . (2020) did not investigate the numerous synonyms of P. quadrilineata but only referred to the list of names available in MolluscaBase (2020), they have ‘carefully investigated all existing available synonyms of P. quadrilineata (Figures 1, 2, 3, 4, 5, 6, 7, Table 1, Supplementary information Figure S1)’.…”

“…This is a misleading statement because it suggests that all the various nominal species names currently considered to be synonyms of P. quadrilineata are discussed by Korshunova et al . (2021) when in fact their figures and tables illustrate only the new material studied by the authors. There are no comparative figures or discussion or reference to original descriptions and illustrations of the various synonyms, which the authors seem to equally assume are the ones listed in MolluscaBase (2020), but this is not explicitly stated.…”

“…The only synonyms of P. quadrilineata discussed by Korshunova et al . (2021) out of the previous 11 available (10 names after Korshunova et al ., 2021) (see MolluscaBase, 2021) are Polycera ornata d'Orbigny, 1837 and Thecacera capitata .…”