Speciation of European bullheads (Cottus spp.): The next problem for taxonomy of northern freshwater fishes.

High morphological and ecological diversity displayed by European bullheads led Freyhof et al. (2005) to describe 15 nominal species. However, the basis for declaring these nominal species is contentious due to a lack of rigorous statistical analysis of morphological variations among populations, limitations in the inferences made from previous generations of molecular studies, a dearth of next-generation sequencing tools for European bullheads and an underestimation of the complexity of evolutionary processes. Here, we argue that the current evidence is insufficient for accurate speciation of European bullhead into the nominal species proposed by Freyhof et al. (2005). We suggest that future studies incorporate analytical approaches such as chromosome-level reference genomes, integration of multiple biological data sources into multispecies coalescent models and rigorous statistical analysis of the full range of morphological characters, both meristic and mensural. Furthermore, we highlight the necessity of incorporating a standardised conceptual framework while considering bullhead speciation, as well as some practical suggestions of what such a process would look like.