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

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.

Automated summary

Species identification is essential for ecological, phylogeographic, and evolutionary studies, but conducting taxonomic studies in the molecular era can be challenging. This study focuses on common, difficult-to-distinguish European species within the genus Polycera to address the broader issues of cryptic species and their impact on biodiversity research. By analyzing a large dataset encompassing morphological and molecular data, the study identifies four species within a complex, highlighting the importance of accurate taxonomic analysis. The study reveals the presence of both cryptic and non-cryptic components within the same species, suggesting the need for a more comprehensive understanding of organismal diversity.